Do Galaxy Morphologies Really Affect the Efficiency of Star Formation During the Phase of Galaxy Transition?

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Published 2019 April 2 • © 2019. The American Astronomical Society. All rights reserved.
, , Citation Shuhei Koyama et al 2019 ApJ 874 142 DOI 10.3847/1538-4357/ab0e75

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skoyama@cosmos.phys.sci.ehime-u.ac.jp

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1 Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan skoyama@cosmos.phys.sci.ehime-u.ac.jp

2 Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA

3 Department of Astronomical Science, Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

4 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

5 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan

6 Department of Space and Astronautical Science, Graduate University for Advanced Studies (SOKENDAI), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan

7 Astronomical Institute, Tohoku University, 63 Aramaki, Aoba-ku, Sendai 980-8578, Japan

8 Institute of Astronomy & Astrophysics, Academia Sinica, Taipei 10617, Taiwan

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  1. Received 2018 December 30
  2. Revised 2019 March 7
  3. Accepted 2019 March 8
  4. Published

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Keywords

galaxies: evolution; galaxies: ISM; galaxies: star formation

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0004-637X/874/2/142

Abstract

Recent simulations predict that the presence of the stellar bulge suppress the efficiency of star formation (SF) in early-type galaxies, and this "morphological quenching" scenario is supported by many observations. In this study, we discuss the net effect of galaxy morphologies on the star formation efficiency (SFE) during the phase of galaxy transition, on the basis of our CO(J = 1 − 0) observations of 28 local "green valley" galaxies with the Nobeyama 45 m Radio Telescope. We observed 13 disk-dominated and 15 bulge-dominated green valley galaxies at fixed stellar mass (M*) and star formation rate (SFR), supplemented by 1 disk- and 6 bulge-dominated galaxies satisfying the same criteria from the xCOLD GASS survey. By using a total of 35 green valley galaxies, we reveal that the distributions of molecular gas mass, molecular gas fraction, and SFE of green valley galaxies do not change with their morphologies, suggesting little impact of galaxy morphologies on their SFE, and interestingly, this result is also valid for normal star-forming galaxies on the SF main sequence selected from the xCOLD GASS galaxies. On the other hand, we find that ∼20% of the bulge-dominated green valley galaxies do not show significant CO emission line, showing high SFEs for their M* and SFR. These molecular gas deficient sources that are identified only in the bulge-dominated green valley galaxies may represent an important population during the quenching phase under the influence of the stellar bulge, but our results suggest that the presence of the stellar bulge does not decrease the efficiency of ongoing SF, in contrast to the prediction of the morphological quenching scenario.

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1. Introduction

Over the past decade, it has been established that star-forming galaxies (mostly with disk-dominated morphologies) exhibit a tight positive correlation between star formation rate (SFR) and stellar mass (M*) across their environment—i.e., the so-called star-forming main sequence (MS; Daddi et al. 2007; Elbaz et al. 2007; Noeske et al. 2007; Peng et al. 2010; Koyama et al. 2013), while red or passive galaxies (mostly with bulge-dominated morphologies) are distributed far below the MS by ∼1.5–2 dex on the SFR–M* diagram. It is believed that galaxies located on the MS eventually evolve into the passive galaxy population by quenching their star formation (SF), accompanying morphological transformation (e.g., Wuyts et al. 2011). The key questions are what triggers this transition event and how galaxies migrate from the MS to the passive galaxy sequence.

An interesting scenario proposed for the SF quenching mechanism is the so-called "morphological quenching" scenario (Martig et al. 2009), where the galaxy morphologies themselves control their SF acitivity. Martig et al. (2009) predicted that gas disks can be stabilized against SF when they are embedded in a stellar spheroid, resulting in a lower SF efficiency in early-type galaxies. A recent observational campaign of nearby early-type galaxies (the ATLAS3D project; Cappellari et al. 2011) and their molecular gas follow-up observations (e.g., Young et al. 2011; Martig et al. 2013; Davis et al. 2014) have shown that star-forming regions in early-type and late-type galaxies follow a different SF law (Kennicutt–Schmidt law; Schmidt 1959; Kennicutt 1998) in the sense that SF in early-type galaxies is less efficient by a factor of ∼2× than SF in late-type galaxies, consistent with the morphological quenching scenario. However, many of these earlier works attempted to compare typical early- and late-type galaxies, and consequently, early-type galaxies were usually selected from the passive galaxy population, while late-type galaxies were selected from the star-forming population in the MS. A concern here is that the two populations are in very different evolutionary stages of galaxy formation, and that it may not be possible to assess the real effects of galaxy morphologies on the SF quenching. In other words, the claim that early-type galaxies have a lower SF efficiency than late-type galaxies does not necessarily indicate that they are really quenched by the morphological quenching process.

A potential approach to directly assess the real impact of galaxy morphologies on the SF efficiency is to compare early- and late-type galaxies at the same evolutionary stage. In this context, the green valley galaxies (Salim 2014) are of great interest because they are expected to be in the SF quenching phase (Bell et al. 2004; Faber et al. 2007; Martin et al. 2007). Green valley galaxies have originally been defined as an intermediate population located in between the red sequence and the blue cloud in the color–magnitude diagram (CMD). However, this definition may misidentify galaxies in the SF quenching because the red colors of galaxies have a finite limit, which could lead to an artificial bimodality in the CMD (Schiminovich et al. 2007; Salim 2014). Therefore, more recent studies tend to exploit a modern definition of the green valley in the SFR–M* diagram; i.e., green valley galaxies are located below the MS by ∼0.5–1.5 dex on the SFR–M* diagram, reflecting that their SF activity is lower than that of MS galaxies. As demonstrated by recent extensive 12CO(J = 1 − 0) (hereafter CO) observations, the molecular gas mass fraction (${f}_{{{\rm{H}}}_{2}}={M}_{{{\rm{H}}}_{2}}/{M}_{* }$) and star formation efficiency ($\mathrm{SFE}\,=\mathrm{SFR}/{M}_{{{\rm{H}}}_{2}}$) both change across the MS (Saintonge et al. 2012, 2016; Genzel et al. 2015), and accordingly, green valley galaxies are known to have intermediate levels of ${f}_{{{\rm{H}}}_{2}}$ and SFE on average. A recent study based on the spatially resolved observations of three green valley galaxies with ALMA and MaNGA by Lin et al. (2017) has also confirmed this trend.

It is known that the majority of green valley galaxies have larger bulge and smaller disk components than normal MS galaxies in general (Belfiore et al. 2018; Bremer et al. 2018), but at the same time, it is also true that there still remains a wide variety of morphologies in the green valley galaxy population, which ranges from late- to early-type galaxies (Schawinski et al. 2014). Schawinski et al. (2014) proposed that these green valley galaxies with early- and late-type morphologies are in different quenching processes. It is therefore expected that these "disk+green" and "bulge+green" galaxies are short-lived (hence rare), but extremely important targets to understand the effect of morphologies during the phase of galaxy transition events. They are equally quenched (judging from their location on the SFR–M* diagram), but are in the very different stage in terms of their morphological transformation. Disk+green galaxies are in the transitional phase and have not started their morphological change. Bulge+green galaxies are also in the transitional phase, but have already completed their morphological transformation. It is therefore desirable to directly compare the molecular gas properties in these two distinct populations at fixed SFR and M* to assess the real effect of galaxy morphologies on the SF quenching process.

In this paper, we present our new CO observations of nearby green valley galaxies with disk- and bulge-dominated morphologies using the Nobeyama 45 m radio telescope (NRO 45 m). By combining our NRO 45 m data and other CO data taken from the literature, we directly measure their SFE and test if morphological transformation (i.e., the growth of stellar bulge) can really reduce the efficiency of SF within the galaxies. This paper is organized as follows. In Section 2 we describe our sample selection and our new CO observations with the NRO 45 m telescope, as well as our method and supplementary data from the xCOLD GASS survey. We present our main results on the morphological dependence of the molecular gas properties in Section 3, and the discussion is given in Section 4. Finally, our conclusions are presented in Section 5. Throughout this paper, we assume the ΛCDM universe with H0 = 70 km s−1 Mpc−1, Ωm = 0.3 and ΩΛ = 0.7, and Kroupa (2001) initial mass function (IMF).

2. Data and Method

2.1. Sample Selection

The goal of this paper is to study the morphological dependence of SFE of galaxies in the phase of galaxy transition at fixed M* and SFR. To achieve this goal, we perform new CO observations of galaxies selected from a small window in the SFR–M* plane. We start from the Sloan Digital Sky Survey (SDSS) DR7 spectroscopic catalog (York et al. 2000; Abazajian et al. 2009). M* and SFR are computed by groups at the Max Plank Institute for Astrophysics and at Johns Hopkins University (MPA/JHU; Kauffmann et al. 2003; Brinchmann et al. 2004), where the SFRs for star-forming galaxies are based either on model fitting to the continuum + emission lines, or on Hα emission line. Therefore, the associated timescale of SF activity should be short (∼10 Myr), which is relevant for the investigation of SF activity within the galaxies in the phase of transition. We then select the target galaxies for our CO observation with the NRO 45 m telescope following the procedure as described below.

We restrict the redshift range to 0.025 < z < 0.05. The lower limit (z > 0.025) is applied to cover the main part of galaxies by a single beam size of the NRO 45 m telescope (∼18 arcsec at 110 GHz, which corresponds to 9.2 kpc at z = 0.025). The upper limit (z < 0.05) is also applied to detect their CO line with a reasonable observational time of the NRO 45 m telescope.

To select galaxies in the galaxy transition phase, we select galaxies from a small window in the M–sSFR plane (10.8 < log(M*/M⊙) < 11.2 and $-11\lt \mathrm{log}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})$ < −10.5), as shown in the color shades of the left panel in Figure 1.

Figure 1.

Figure 1. (Left) Stellar mass vs. specific SFRs diagram for all SDSS galaxies (gray contours), while the blue and red symbols show the selected disk- and bulge-dominated green valley galaxies (disk+green and bulge+green, respectively), where the filled and open circles show galaxies from our NRO 45 m observations and the xCOLD GASS survey, respectively. The yellow and cyan shaded regions show the M* and sSFR range applied to select our NRO 45 m CO observation target galaxies ($10.8\lt \mathrm{log}({M}_{* }/{M}_{\odot })\lt 11.2$ and $-11\lt \mathrm{log}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})\lt -10.5$). (Right) Black histogram: arbitrary scaled distribution of the C-index for the entire SDSS sample with 0.025 < z < 0.05, $10.8\lt \mathrm{log}({M}_{* }/{M}_{\odot })\lt 11.2$ and −11 < sSFR/yr−1 < −10.5. The blue and red histograms show the distribution of disk-dominated and bulge-dominated green valley galaxies targeted by our NRO 45 m observations, respectively.

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Active galactic nuclei (AGNs) may enhance the central luminosity of galaxies and affect their morphological classification. It is also expected that the presence of AGNs may affect the SF activity in the galaxies (AGN feedback). We therefore remove AGNs from our sample based on the SDSS spectral subclass (AGN and/or BROADLINE).9 We note that we cross-match our final sample to the Swift/BAT 105-month catalog (Oh et al. 2018), and confirm that there is no X-ray source in our sample.

We compute the concentration index (C-index; Morgan 1958), which is defined by R90petro,r/R50petro,r, where R90petro,r and R50petro,r are the radius containing 90% and 50% of the Petrosian flux for SDSS r-band photometric data, respectively. The C-index is known to be strongly correlated with the dominance of the bulge component in galaxies (Shimasaku et al. 2001; Strateva et al. 2001), and we use the C-index as an indicator of galaxy morphologies in this study. The black histogram in the right panel of Figure 1 shows the (arbitrarily scaled) distribution of the C-index for all SDSS galaxies with 0.025 < z < 0.05, 10.8 < log(M*/M⊙) < 11.2, and $-11\,\lt \mathrm{log}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})\,\lt -10.5;$ i.e., the same criteria that we applied to select our NRO 45 m targets, as described above. This plot demonstrates that green valley galaxies have a wide variety of morphologies even when we fix stellar mass and SFR.

By visually inspecting the individual galaxies with a high or low C-index, and by considering their target visibility from the NRO 45 m telescope, we select 13 disk-dominated green valley galaxies with a C-index <2.2 (hereafter disk+green sample; see the blue histogram in the right panel of Figure 1) and 15 bulge-dominated green valley galaxies with a C-index >2.8 (hereafter bulge+green sample; see the red histogram in the right panel of Figure 1). We present the SDSS optical color-composite image of individual galaxies in Figure 2, which shows that this procedure allows us to successfully select green valley galaxies with distinct morphologies. We note that in Section 3.1, we also check the confidence in the C-index as a morphological indicator by comparing it to the stellar mass surface density within 1 kpc. The blue and red filled circles in the left panel of Figure 1 show the distributions of our disk+green and bulge+green galaxies in the sSFR–M* plane. In this figure, we also show the supplementary data from the xCOLD GASS survey with open circles (see Section 2.3 for details). The properties of our NRO 45 m target galaxies are summarized in Table 1.

Figure 2.

Figure 2.  CO spectra obtained by our NRO 45 m observations and the SDSS cutout three-color images (g, r, i) for the disk+green (top panels) and bulge+green galaxies (bottom panels). The CO flux of each galaxy is calculated by integrating the flux within the velocity range that is colored in yellow. The white circles in the SDSS images show the beam size of the NRO 45 m telescope (18''); its center indicates the SDSS fiber position. The physical properties of all the target galaxies are summarized in Table 1.

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Table 1.  Summary of the Physical Quantities of Our NRO 45 m Green Valley Galaxy Sample

ID Morphology C-index R.A. Decl. z log SFRMPA/JHU log SFRWISE log M* ICO log LCO log ${M}_{{{\rm{H}}}_{2}}$
      (hms) (dms)   (M⊙ yr−1) (M⊙ yr−1) (M⊙) (K km s−1) (K km s−1pc2) (M⊙)
d1 disk 1.80 08 04 46.760 +10 46 41.83 0.035 $-{0.06}_{-0.34}^{+0.39}$ 0.81 ± 0.14 ${10.87}_{-0.09}^{+0.10}$ 7.78 ± 2.13 9.17 ± 0.12 9.81 ± 0.12
d2 disk 1.90 13 50 32.134 +03 11 38.96 0.032 ${0.24}_{-0.45}^{+0.39}$ 0.43 ± 0.14 ${10.84}_{-0.10}^{+0.09}$ 4.72 ± 0.87 8.88 ± 0.08 9.52 ± 0.08
d3 disk 1.92 17 11 47.781 +28 19 27.27 0.042 ${0.06}_{-0.56}^{+0.42}$ 0.24 ± 0.14 ${10.81}_{-0.09}^{+0.10}$ 1.75 ± 0.74 8.68 ± 0.18 9.31 ± 0.18
d4 disk 1.95 13 13 13.457 +33 59 04.00 0.035 ${0.18}_{-0.39}^{+0.38}$ 0.60 ± 0.14 ${10.96}_{-0.10}^{+0.10}$ 6.92 ± 1.18 9.12 ± 0.07 9.75 ± 0.07
d5 disk 1.95 10 25 17.750 +17 08 21.09 0.045 ${0.03}_{-0.34}^{+0.36}$ 0.18 ± 0.14 ${10.87}_{-0.10}^{+0.09}$ 2.26 ± 0.60 8.83 ± 0.12 9.47 ± 0.12
d6 disk 2.04 13 22 40.387 +32 53 22.76 0.036 ${0.27}_{-1.32}^{+0.80}$ 0.58 ± 0.14 ${10.88}_{-0.12}^{+0.13}$ 7.47 ± 1.53 9.17 ± 0.09 9.80 ± 0.09
d7 disk 2.07 15 06 34.472 +05 13 25.12 0.035 ${0.27}_{-0.32}^{+0.35}$ 0.50 ± 0.14 ${10.84}_{-0.09}^{+0.10}$ 7.30 ± 1.31 9.13 ± 0.08 9.76 ± 0.08
d8 disk 2.09 11 45 17.564 +26 46 02.63 0.030 ${0.30}_{-0.35}^{+0.35}$ 0.45 ± 0.14 ${10.94}_{-0.09}^{+0.09}$ 4.51 ± 0.64 8.81 ± 0.06 9.44 ± 0.06
d9 disk 2.09 12 26 32.710 +16 50 40.66 0.045 ${0.36}_{-1.08}^{+0.53}$ 0.75 ± 0.14 ${10.90}_{-0.09}^{+0.10}$ 5.20 ± 0.95 9.20 ± 0.08 9.84 ± 0.08
d10 disk 2.11 08 43 06.123 +25 32 20.62 0.044 ${0.21}_{-0.57}^{+0.44}$ 0.24 ± 0.14 ${10.83}_{-0.09}^{+0.10}$ 2.52 ± 0.33 8.87 ± 0.06 9.51 ± 0.06
d11 disk 2.12 09 48 54.495 +24 52 29.04 0.043 ${0.16}_{-0.29}^{+0.36}$ 0.56 ± 0.14 ${11.00}_{-0.11}^{+0.12}$ 4.08 ± 1.36 9.07 ± 0.15 9.70 ± 0.15
d12 disk 2.15 12 52 44.495 +59 15 56.47 0.043 ${0.22}_{-0.33}^{+0.31}$ 0.09 ± 0.14 ${11.09}_{-0.09}^{+0.09}$ 1.48 ± 0.68 8.62 ± 0.20 9.25 ± 0.20
d13 disk 2.16 15 37 03.843 +25 33 55.25 0.035 ${0.18}_{-0.31}^{+0.32}$ 0.24 ± 0.14 ${10.92}_{-0.09}^{+0.10}$ 2.90 ± 1.10 8.73 ± 0.16 9.36 ± 0.16
e1 bulge 2.84 10 44 54.128 +27 28 08.74 0.044 ${0.40}_{-0.43}^{+0.39}$ 0.36 ± 0.14 ${10.95}_{-0.09}^{+0.10}$ 3.64 ± 1.18 9.04 ± 0.14 9.67 ± 0.14
e2 bulge 2.96 13 01 07.398 +54 47 57.42 0.031 ${0.40}_{-1.08}^{+0.51}$ 0.67 ± 0.14 ${11.01}_{-0.10}^{+0.10}$ 9.55 ± 2.94 9.16 ± 0.13 9.80 ± 0.13
e3 bulge 2.99 12 30 55.791 +51 16 56.59 0.043 ${0.01}_{-1.28}^{+0.59}$ −0.11 ± 0.14 ${10.86}_{-0.09}^{+0.09}$ <1.93 <8.74 <9.37
e4 bulge 3.02 14 38 56.819 +28 21 11.76 0.044 ${0.32}_{-0.27}^{+0.34}$ 0.00 ± 0.14 ${10.88}_{-0.09}^{+0.10}$ <2.89 <8.94 <9.57
e5 bulge 3.02 08 38 45.730 +25 14 11.27 0.044 ${0.05}_{-0.48}^{+0.41}$ <-0.51 ${10.84}_{-0.09}^{+0.10}$ <3.53 <9.01 <9.64
e6 bulge 3.04 08 16 44.864 +27 35 30.76 0.040 ${0.31}_{-0.39}^{+0.38}$ 0.21 ± 0.14 ${10.89}_{-0.09}^{+0.10}$ 3.21 ± 1.27 8.89 ± 0.17 9.53 ± 0.17
e7 bulge 3.07 13 34 09.413 +13 16 50.94 0.044 ${0.43}_{-0.33}^{+0.34}$ 0.53 ± 0.14 ${11.01}_{-0.09}^{+0.10}$ 6.85 ± 1.27 9.31 ± 0.08 9.94 ± 0.08
e8 bulge 3.07 17 02 37.467 +24 52 10.02 0.049 ${0.14}_{-1.24}^{+0.65}$ 0.19 ± 0.14 ${11.11}_{-0.09}^{+0.10}$ 1.68 ± 0.76 8.78 ± 0.20 9.42 ± 0.20
e9 bulge 3.10 11 16 32.347 +29 16 33.46 0.046 ${0.37}_{-0.25}^{+0.32}$ <-0.22 ${10.94}_{-0.09}^{+0.10}$ 1.25 ± 0.76 8.61 ± 0.26 9.24 ± 0.26
e10 bulge 3.13 10 22 19.386 +36 34 58.90 0.026 ${0.06}_{-0.63}^{+0.41}$ 0.47 ± 0.14 ${10.95}_{-0.09}^{+0.10}$ 13.04 ± 2.05 9.13 ± 0.07 9.77 ± 0.07
e11 bulge 3.28 11 16 23.197 +12 00 55.32 0.046 ${0.13}_{-1.15}^{+0.55}$ <-0.16 ${11.05}_{-0.09}^{+0.09}$ <2.89 <8.98 <9.61
e12 bulge 3.31 16 26 30.965 +25 53 40.53 0.050 ${0.16}_{-0.59}^{+0.38}$ 0.50 ± 0.14 ${10.84}_{-0.09}^{+0.10}$ 5.03 ± 0.91 9.28 ± 0.08 9.91 ± 0.08
e13 bulge 3.32 14 52 32.147 +17 03 46.03 0.045 ${0.11}_{-0.95}^{+0.47}$ 0.11 ± 0.14 ${11.06}_{-0.10}^{+0.09}$ 2.33 ± 0.57 8.85 ± 0.11 9.48 ± 0.11
e14 bulge 3.33 15 37 26.092 +21 44 37.68 0.041 ${0.12}_{-0.71}^{+0.40}$ 0.31 ± 0.14 ${10.93}_{-0.10}^{+0.09}$ 2.78 ± 0.55 8.84 ± 0.09 9.48 ± 0.09
e15 bulge 3.34 14 33 12.953 +52 57 47.56 0.047 ${0.12}_{-0.36}^{+0.36}$ −0.09 ± 0.14 ${10.86}_{-0.09}^{+0.10}$ <3.21 <9.04 <9.67
$\langle e3,4,5,11,15\rangle $ bulge 3.13 ⋯ ⋯ 0.045 0.13 −0.09 10.90 <1.26 <8.59 <9.22

Note. log SFRWISE, ICO, LCO and ${M}_{{{\rm{H}}}_{2}}$ with "<" denote the upper limits.

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2.2. Observation and Data Reduction

We performed CO observations of the 13 disk+green and 15 bulge+green galaxies from 2017 December to 2018 February semester using the NRO 45 m telescope (CG171004: Y. Koyama et al.). The CO emission line (rest-frame 115.27 GHz) is shifted to 109.78–112.46 GHz according to the redshift distribution of our sample (0.025 < z < 0.05). We used a multi-beam 100 GHz band solid-state imaging spectrometer receiver (FOREST; Minamidani et al. 2016) and a copy of a part of the FX-type correlator for the Atacama Compact Array (SAM45; Kamazaki et al. 2012). We performed single-point observations with two beams (ON-ON mode; Nakajima et al. 2013). A typical on-source integration time is two hours for each galaxy. The 13CO(J = 1 − 0) line of IRC+10216 was observed daily as a standard source for flux calibration. The flux calibration was performed by the chopper wheel method and the scaling factor (f ) defined as a flux ratio of the standard source and the reference data listed on the NRO website.10 The main-beam temperature (Tmb) is calculated by

Equation (1)

where ${T}_{a}^{* }$ is the antenna temperature. The main-beam efficiency (ηmb) during the semester is 0.43 at 110 GHz, according to the NRO website.11

We performed data reduction by following the procedure described in Koyama et al. (2017) using the NEWSTAR software developed by NRO based on the Astronomical Image Processing System package. We only used the data with wind velocities of <5 m s−1, pointing accuracies with <5'', system noise temperature (Tsys) with <300 K, and data with an rms noise temperature of Trms < 0. 045 K in the Tmb scale at a velocity resolution of 200 km s−1 to exclude bad baseline spectra. We subtracted baselines by linear fitting and combined the spectra for both beams and polarizations. Finally, we performed spectral binning to a resolution of 40 km s−1. The observed CO spectra are shown in Figure 2. We then calculated the integrated intensity ICO with

Equation (2)

The Trms at the velocity resolution of Δv = 40 km s−1 is 1.2–4.0 mK (Tmb) for our targets. We detected the CO emission line with the signal (peak temperature) -to-noise (rms) ratio (S/N) of >3 from 23 galaxies out of 28 observed galaxies.

Finally, we calculated the CO luminosity (${L}_{\mathrm{CO}}^{{\prime} }$) of individual galaxies using the following equation:

Equation (3)

where Ωb is the beam solid angle of θmb (=18''), and DL is the luminosity distance of each target galaxy. The physical quantities of our target galaxies derived from our NRO 45 m observations are summarized in Table 1. We note that some of the galaxies are slightly larger in optical size than the NRO beam size (white circles in the SDSS images), particularly for the disk+green galaxies. However, we do not apply aperture correction because the molecular gas component is reported to be concentrated on the central part of galaxies (e.g., Young & Scoville 1991), and therefore the effect of CO flux loss due to this aperture effect is expected to be negligibly small. For the five galaxies without a CO line detection, we assigned the 3σ upper limits of CO luminosities assuming an FWHM of 200 km s−1 (estimated from the mean CO line width of the CO-detected galaxies).

We also performed a spectral stacking analysis to estimate the mean CO luminosity of the five CO-undetected bulge+green galaxies (e3, 4, 5, 11, and 15). The stacking analysis employed the weighted mean stacking for the spectra with

Equation (4)

where w denotes 1/rms2 for each spectrum. We show the stacked spectrum in Figure 3. Although Trms is properly reduced by the stacking analysis (=0.89 mK), the CO line is still undetectable at an S/N > 3 level. We therefore assign their (average) 3σ upper limit from the noise level of this stacking analysis. We also report the mean properties of the CO-undetected bulge+green galaxies from this stacking analysis in the bottom line of Table 1.

Figure 3.

Figure 3. Stacked CO spectrum for the five CO-undetected bulge+green galaxies (ID = e3, 4, 5, 11, and 15; see Table 1).

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2.3. xCOLD GASS

In this study, we also use the CO data of local galaxies, which are publicly available from the xCOLD GASS survey (Saintonge et al. 2017). The xCOLD GASS is an extended version of the COLD GASS survey (Saintonge et al. 2011a, 2011b, 2012) that was performed with the IRAM 30 m telescope, and it provides the most extensive CO data sets of 532 local galaxies in a redshift range of 0.01 < z < 0.05.

We measured their C-index by applying the same criteria as we used to select our NRO 45 m targets (see Section 2.1), and then we selected 1 disk+green galaxy and 6 bulge+green galaxies from the xCOLD GASS sample (see the open circles in the left panel of Figure 1). It is expected that only a few of the xCOLD GASS sample satisfy our criteria because the xCOLD GASS survey is originally designed to cover a wide range in the M*–SFR plane, while our focus is to study galaxies at fixed M* and SFR.

2.4. The Final Sample

By combining our NRO 45 m sample and the xCOLD GASS sample, our final sample includes 14 disk+green and 21 bulge+green galaxies (35 galaxies in total). We calculate the molecular gas mass (${M}_{{{\rm{H}}}_{2}}$) of individual galaxies with ${M}_{{{\rm{H}}}_{2}}={\alpha }_{\mathrm{CO}}{L}_{\mathrm{CO}}^{{\prime} }$, where αCO is the CO-to-H2 conversion factor. We adopt the Galactic value of αCO = 4.3 ${M}_{\odot }{({\rm{K}}\mathrm{km}{{\rm{s}}}^{-1}{\mathrm{pc}}^{2})}^{-1}$, which includes the contribution of heavy elements (mainly from helium), as is commonly used in studies of star-forming galaxies in the local universe (Bolatto et al. 2013). We note that the αCO value is reported to depend on the gas-phase metallicity of galaxies (e.g., Genzel et al. 2012; Bolatto et al. 2013). Unfortunately, it is not possible to measure the metallicities for many of our green valley galaxies because their emission lines are weak, but by comparing the average metallicities for 5 of 14 disk+green galaxies (9.13 ± 0.02) and for 3 of 21 bulge+green galaxies (9.01 ± 0.14) listed in the MPA-JHU catalog (Tremonti et al. 2004), we conclude that the difference in αCO that is due to the metallicity effect between the disk+green and bulge+green samples must be negligible.

Figure 4 shows the M*, SFR, and redshift distributions of our final bulge+green and disk+green galaxies. As intended, there is no systematic difference between the disk+green and bulge+green galaxies in the distribution of M*, SFR, and redshift. We emphasize again that the aim of this paper is to assess the real effect of galaxy morphologies on SF quenching (SF efficiency) at fixed M* and SFR by eliminating any bias from the analyses.

Figure 4.

Figure 4. Distributions of SFR (left), M* (middle), and redshift (right) for the disk+green (blue) and bulge+green (red) galaxies. The plots demonstrate that there are no systematic differences between the disk+green and bulge+green galaxies in the distributions of these physical parameters.

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3. Results

3.1. The Distribution of ${M}_{{{\rm{H}}}_{2}}$, ${f}_{{{\rm{H}}}_{2}}$ and SFE

We compare the molecular gas properties of green valley galaxies with different morphological types as a first step. In Figure 5 we show the distribution of ${M}_{{{\rm{H}}}_{2}}$, ${f}_{{{\rm{H}}}_{2}}$, and SFE ($=\mathrm{SFR}/{M}_{{{\rm{H}}}_{2}}$) for our disk+green and bulge+green galaxies. The mean properties of CO-detected galaxies with each morphological type are summarized in Table 2. We perform the two-sample Kolmogorov–Smirnov (KS) test between disk+green and bulge+green galaxies. The derived p-values (i.e., the probability that the two samples are drawn from the same parent population) are 0.98 for ${M}_{{{\rm{H}}}_{2}}$, 0.72 for ${f}_{{{\rm{H}}}_{2}}$ and 0.45 for SFE, suggesting that there is no clear morphological dependence of ${M}_{{{\rm{H}}}_{2}}$, ${f}_{{{\rm{H}}}_{2}}$ and SFE at least for the CO-detected galaxies.

Figure 5.

Figure 5. Distributions of ${M}_{{{\rm{H}}}_{2}}$ (left), ${f}_{{{\rm{H}}}_{2}}$ (middle), and SFE (right) for the disk+green (blue) and bulge+green (red) galaxies. The color-coding is the same as in Figure 4. The red arrows show the upper and lower limits for the CO-undetected bulge+green galaxies, which are not included in the histograms.

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Table 2.  Means and Standard Deviations of ${M}_{{{\rm{H}}}_{2}}$, ${f}_{{{\rm{H}}}_{2}}$ and SFE for the CO-detected Disk+green and Bulge+green Galaxies

Morphology log ${M}_{{{\rm{H}}}_{2}}$ (M⊙) log ${f}_{{{\rm{H}}}_{2}}$ log SFE (yr−1)
disk-dominated 9.59 ± 0.20 −1.34 ± 0.23 −9.36 ± 0.24
bulge-dominated 9.54 ± 0.25 −1.41 ± 0.25 −9.35 ± 0.22

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As we reported in Section 2.2, there are five CO-undetected galaxies in our bulge+green galaxies. Because we did not detect their CO lines even in the stacked spectrum (see Section 2.2 and Figure 3), the CO-undetected galaxies might be deficient in molecular gas (in spite of their similar levels of SFRs). This result suggests that these galaxies have significantly higher SFEs than the other galaxies.

In Figure 6 we plot the SFE against their C-index to more quantitatively examine the morphological dependence of the SFE. It can be seen that there is no significant difference between the CO-detected disk+green (blue diamond) and bulge+green galaxies (red diamond), consistent with the results shown in Figure 5, while the result from the stacking analysis for the CO-undetected galaxies (shown with the black arrow) seems to show a significantly higher SFE. This is an opposite trend to that predicted by the morphological quenching scenario, and this population might represent an interesting population to understand the morphological impacts on SF quenching. We discuss this issue in more detail in Section 4.3.

Figure 6.

Figure 6. Relation between C-index and SFE at fixed M* and SFR. For comparison, we also plot two xCOLD GASS sources with intermediate morphology (2.2 < C-index < 2.8) in the same M*–sSFR window ($10.8\lt \mathrm{log}({M}_{* }/{M}_{\odot })\lt 11.2$ and $-11\lt \mathrm{log}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})\lt -10.5$). The gray arrows show the upper limits for the individual CO-undetected sources, while the black arrow indicates the upper limit from their stacking analysis. The blue and red diamonds show the mean SFE for the CO-detected disk+green and bulge+green galaxiess, respectively, where the error bars for the vertical and horizontal axis represent the standard error of the mean SFE and the range of the C-index for each subsample, respectively.

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We note that the C-index is sensitive to the difference of the bulge-to-total stellar mass ratio of galaxies and not their bulge mass, while some studies suggest that the passive galaxy fraction strongly depends on the bulge mass of galaxies and not on their bulge-to-total stellar mass ratio (Bluck et al. 2014). We therefore test the effects of a different morphological indicator by computing the stellar mass surface density measured within the central 1 kpc (${M}_{* ,1\mathrm{kpc}}$) as another morphological indicator, which better represents the bulge mass of individual galaxies. We estimate ${M}_{* ,1\mathrm{kpc}}$ by using the stellar mass measured in the SDSS 3'' fiber (corresponding to ∼1.5–3.0 kpc at z = 0.025–0.05) listed in MPA/JHU catalog and by simply scaling it to 1 kpc area. In Figure 7 we show the relation between C-index and ${M}_{* ,1\mathrm{kpc}}$, and the color code indicates the difference in stellar mass. It can be seen that there is a tight positive correlation between C-index and ${M}_{* ,1\mathrm{kpc}}$ at fixed stellar mass. Because our sample is selected from a small stellar mass range ($10.8\lt \mathrm{log}({M}_{* }/{M}_{\odot })\lt 11.2$), this plot demonstrates that the C-index is well correlated with ${M}_{* ,1\mathrm{kpc}}$ as long as we consider the sample used in this study. In Figure 8 we plot the SFE as a function of ${M}_{* ,1\mathrm{kpc}}$; the red and blue symbols indicate the bulge+green and disk+green galaxies classified according to the C-index, which confirms our claim that there is no significant correlation between the SFE and galaxy morphologies. This plot also demonstrates that the disk+green and bulge+green galaxies are also well separated by ${M}_{* ,1\mathrm{kpc}}$. We therefore conclude that our results are not affected by the use of different morphological indicators.

Figure 7.

Figure 7. Relation between C-index and ${M}_{* ,1\mathrm{kpc}}$ for all the SDSS galaxies with 0.025 < z < 0.05. The different color contours mean the different M* range; i.e., M* increases from purple to red contours as shown in the figure. There is a tight positive correlation between C-index and ${M}_{* ,1\mathrm{kpc}}$ at fixed stellar mass. Because our sample is selected from a small stellar mass range ($10.8\lt \mathrm{log}({M}_{* }/{M}_{\odot })\lt 11.2$), which corresponds to the yellow and red contours, the C-index is well correlated with ${M}_{* ,1\mathrm{kpc}}$.

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Figure 8.

Figure 8. SFE as a function of ${M}_{* ,1\mathrm{kpc}}$. The disk+green and bulge+green galaxies, defined according to the C-index in this work, are shown by blue and red symbols.

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3.2. Morphological Dependence of the ${M}_{{{\rm{H}}}_{2}}$–SFR Relation

It is well known that there is a positive correlation between ${M}_{{{\rm{H}}}_{2}}$ and SFR—this is the fundamental correlation describing the SF (e.g., Kennicutt 1998). We investigate the distribution of disk+green and bulge+green galaxies in the ${M}_{{{\rm{H}}}_{2}}$–SFR plane.

The left panel of Figure 9 shows the ${M}_{{{\rm{H}}}_{2}}$–SFR relation for our green valley galaxy samples with different morphologies. A visual inspection suggests that the CO-detected disk+green (blue circles) and bulge+green (red circles) galaxies follow the same relation, or at least that the two samples are distributed in the same regions in this diagram. To statistically compare the distribution between the disk+green and bulge+green galaxies in the ${M}_{{{\rm{H}}}_{2}}$–SFR plane, we first fit all the xCOLD GASS galaxies with $10.8\lt \mathrm{log}{M}_{* }/{M}_{\odot }\lt 11.2$ (shown with gray circles in the diagram) by the linear regression form using the bisector method. The best-fitting line is derived as

Equation (5)

which is indicated by the broken line in the left panel of Figure 9. We then measure the distance to this best-fit line in the orthogonal direction for each galaxy and perform the two-sample KS test between the disk+green and bulge+green galaxies to test if there is any difference in zero-points and/or dispersion between the correlations for disk+green and bulge+green galaxies. The cumulative distribution functions of the distance to the best-fit line are shown in the right panel of Figure 9. The derived p-value is 0.50, indicating that it is unlikely that the CO-detected disk+green and bulge+green galaxies are drawn from different parent populations.

Figure 9.

Figure 9. (Left) Relation between ${M}_{{{\rm{H}}}_{2}}$ and SFR for disk+green (blue circles) and bulge-green (red circles) galaxies. The red arrows show the upper limits of ${M}_{{{\rm{H}}}_{2}}$ for each CO-undetected bulge+green galaxies, and the black arrow shows the upper limit from their stacking analysis. The dashed line indicates the best-fit relation determined by using the high-mass sample ($10.8\lt \mathrm{log}\,{M}_{* }/{M}_{\odot }\lt 11.2$) of xCOLD GASS galaxies (gray circles). (Right) CDFs of the distance in the orthogonal direction to the best-fit line for disk- and bulge-dominated galaxies. The p-value derived from two-sample KS test is 0.50, suggesting there is no significant difference between the distribution of the two samples in this diagram. We note that the CO-undetected galaxies are not included in this plot, but it is unlikely that these CO-undetected sources affect our conclusion (see Section 3.2 for details).

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Finally, we test the effect of the CO-undetected galaxies in our sample on our results. For this purpose, we include the CO-undetected galaxies into the KS test assuming two extreme cases. The first approach is to use the upper limit values of ${f}_{{{\rm{H}}}_{2}}$ for CO-undetected galaxies, and in this case, we obtain p = 0.54 from the KS test. The second approach is to assume their ${f}_{{{\rm{H}}}_{2}}$ to be 1 dex smaller than the minimum ${f}_{{{\rm{H}}}_{2}}$ for the CO-detected galaxies (∼0.1%), and we derive p = 0.25 in this case. In these ways, we suggest that the result is unchanged even if we include the CO-undetected galaxies in the KS test. We therefore conclude that the majority of green valley galaxies (at fixed stellar mass and SFR) have the same SFE, independent of their galaxy morphologies. We note that some fraction of the bulge-dominated galaxies have different properties from other galaxies (as represented by the CO-undetected sources identified in our bulge+green galaxies), and they are an interesting population against the morphological quenching scenario, although the percentage of these exceptional sources is small (∼23%) at this moment.

4. Discussion

4.1. Morphological Dependence of the SFE for MS Galaxies

We tried to test the real effect of galaxy morphologies on SF quenching by comparing the SFE of green valley galaxies at fixed M* and sSFR with distinct morphologies of disk+green and bulge+green galaxies. Our observations demonstrate that the SFE distribution of green valley galaxies does not significantly change with their galaxy morphologies (although there exist a few outliers with a higher SFE in the bulge+green sample). In other words, we do not see the systematic decrease of SFE in bulge-dominated galaxies as reported by previous studies, at least when we study green valley galaxies at fixed SFR and M*.

An interesting examination to be performed as a next step is to study whether our results are universal for galaxies with different SF levels (e.g., MS galaxies). To this end, we use all the CO data of the star-forming MS galaxies in the xCOLD GASS sample located within ±0.6 dex (corresponding to 2σ) from the local MS relation defined by Elbaz et al. (2007):

Equation (6)

In Figure 10 we plot all the galaxies including the MS and green valley galaxies with bulge- and disk-dominated morphologies on the M*–SFE plane. Here, the MS galaxies are classified as bulge-dominated with a C-index >2.8 and disk-dominated with a C-index <2.2, as with the green valley galaxies. We also perform the KS test of SFE distributions for bulge- and disk-dominated galaxies on the MS galaxies, and derive p = 0.96, demonstrating that there is no morphological difference in SFE for the MS galaxies. This is consistent with our results on the green valley galaxies, and further in agreement with Saintonge et al. (2017), who report that the molecular gas depletion timescale for galaxies on and above the MS does not strongly depend on stellar mass surface density. Although Saintonge et al. (2017) showed a positive correlation between stellar mass surface density and molecular gas depletion timescale using all the xCOLD GASS galaxies, our results suggest that the trend may be primarily driven by the general trend between the morphologies and the evolutionary stages of galaxies, as we pointed out in Section 1. It is also evident from Figure 10 that there is a significant drop in average SFEs from the MS to the green valley phase for both bulge- and disk-dominated galaxies. Our results suggest that physical mechanisms triggering SF quenching equally decrease the SFE for both disk- and bulge-dominated galaxies.

Figure 10.

Figure 10. Distribution of galaxies in the M*–SFE plane for the disk-dominated MS (disk+MS, cyan), disk-dominated green valley (disk+green, blue), bulge-dominated MS (bulge+MS, orange), and bulge-dominated green valley galaxies (bulge+green, red). Diamonds and their error bars show the mean and standard deviation for M* and SFE, where the MS galaxies are divided into three M* bins ($\mathrm{log}({M}_{* }/{M}_{\odot })$ = 9.0–9.5, 9.5–10.0, and 10.0–10.5). As for the left panel in Figure 9, the red and black arrows show the lower limits of the SFE for CO-undetected bulge+green galaxies and their stacking result, respectively.

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4.2. Interpretation of CO-undetected Sources

As shown in the previous sections, we identified some outliers showing exceptionally high SFEs in our bulge+green sample. In this section, we try to discuss the nature and origin of these outliers. We first investigate the effects of uncertainties in SFR. We have used the SFRs computed by the MPA/JHU group mainly based on the optical emission lines (Brinchmann et al. 2004). However, they might have large uncertainties in SFR because their measurements are primarily based on the spectra observed with SDSS 3'' fiber. The typical galaxy sizes of our sample are ∼15'' (see Figure 2), meaning that the aperture correction is very large, which may affect the robustness of our results. Furthermore, SFRs of galaxies without strong emission lines (hence low SFRs) are derived assuming a stellar-mass dependent dust attenuation obtained from all the SDSS star-forming galaxies. It is expected that SFRs derived with this approach would also have large uncertainties. Some of our target green valley galaxies do not show significant optical emission lines to properly estimate their dust attenuation level. It is therefore important to verify our results through some independent SFR measurements.

We attempt to use the infrared luminosity (LIR) as an alternative SFR tracer. The LIR is estimated from the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) 22 μm (W4 band) photometry provided by Salim et al. (2016), where they derive LIR of individual galaxies by fitting the WISE 22 μm (W4 band) luminosities to the luminosity-dependent IR templates of Chary & Elbaz (2001; see Salim et al. 2016 for more details on the method and robustness of their LIR measurements). We note that we use the W4 magnitude measured in the elliptical aperture (w4gmag) in the ALLWISE source catalog for extended sources with etx_flg > 0,12 while we use the profile-fitting photometry (w4mpro) for galaxies with etx_flg = 0. We then derive the IR-based SFRs by using the conversion equation established by Kennicutt (1998) adjusted to the Kroupa (2001) IMF:

Equation (7)

The SFRs derived from LIR (SFRWISE) for the individual sources are also listed in Table 1.

In the left panel of Figure 11, we compare the SFRs from the MPA/JHU catalog and those from WISE data, where the gray contours show the distribution of all SDSS galaxies. The circles and arrows show our samples detected and undetected in WISE, where the open symbols indicate the CO-undetected galaxies. It can be seen that the two SFRs are correlated with each other (see the contours in the left panel of Figure 11), but this plot demonstrates that SFRWISE tends to be higher than SFRMPA/JHU (typically by ∼0.2–0.3 dex). Indeed, most of our samples also show SFRWISE > SFRMPA/JHU, as shown in the same figure. In the right panel of Figure 11, we plot SFRWISE against ${M}_{{{\rm{H}}}_{2}}$. This is the same plot as in the left panel of Figure 9, but here we use SFRWISE instead of SFRMPA/JHU. We fit the data points in the same way as in Figure 9, and obtained the best-fit relation of

Equation (8)

We also perform the KS test following the procedure of Section 3.2 and obtain p = 0.04. The p-value is lower than that derived when we use SFRMPA/JHU, and it is possible that the disk+green and bulge+green samples are drawn from different parent population. However, our result still suggests that even if two samples are different (<0.1 dex), it is significantly smaller than the SFE offset values between late- and early-type galaxies (a factor of ∼2; i.e., ∼0.3 dex) reported by previous studies (Martig et al. 2013; Davis et al. 2014), as mentioned in Section 1. We also compare the SFE distributions, redefined by SFRWISE, between disk+green and bulge+green samples, and confirm that there is no significant difference (p ∼ 0.7). After all, an important message from the result of this analysis is that our conclusions on the morphological independence of SFEs of the green valley galaxies are not strongly affected by the choice of SFR tracers.

Figure 11.

Figure 11. (Left) Comparison between the SFRs from the MPA/JHU catalog and those from WISE 22 μm photometry for our sample. The red and blue symbols indicate the bulge+green and disk+green galaxies, respectively. The circles and arrows show the SFRWISE for 22 μm detected sources and the 3σ upper limits of SFRWISE for 22 μm undetected sources, respectively. The open symbols indicate the CO-undetected sources. (Right) Relation between ${M}_{{{\rm{H}}}_{2}}$ and the IR-based SFRs derived with WISE 22 μm photometry. The meanings of the symbols are the same as in Figure 9. The black arrow shows the result of our stacking analysis for the CO-undetected sources. We note that we derive their average SFRWISE by creating a stacked WISE W4 image of these CO-undetected sources.

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In the left panel of Figure 11, we find that the CO-undetected galaxies (open symbols) have a lower SFRWISE than SFRMPA/JHU. These CO-undetected sources are shown with the red leftward arrows in the right panel of Figure 11 (upper limits for their ${M}_{{{\rm{H}}}_{2}}$), and in this case, most of the CO-undetected galaxies do not contradict the best-fit line of the ${M}_{{{\rm{H}}}_{2}}$–SFRWISE relation. We caution that these CO-undetected sources might be simply low SFR galaxies rather than the molecular gas deficient sources, as we claimed in the previous sections. Nevertheless, we stress that the result from our stacking analysis for the CO-undetected sources (shown with the black leftward arrow in the right panel of Figure 11) shows a clear offset from the best-fit line, meaning a significantly higher SFE. Therefore, we still suggest that these CO-undetected galaxies in our bulge+green sample have significantly higher SFEs on average. They may represent the galaxy population during the SF quenching process due to the presence of bulge component.

The reason for their molecular gas deficiencies (and unusually high SFEs) is unclear with the available data set, but we speculate that the fraction of dense molecular gas, which is known to be more tightly correlated with SF than the gas traced by CO line (e.g., Gao & Solomon 2004), could be increased in these CO-undetected bulge+green galaxies. Observations of such dense cold molecular gas (e.g., HCN) of the green valley galaxies is our important future work. The molecular gas deficiency may also be explained if the galaxies lose their molecular gas on a shorter timescale than that of their SF. However, the change in dense gas fraction in the bulge+green sample would be a more realistic scenario because it is difficult to decrease the bulk of their molecular gas on a very short timescale considering the typical timescale of SF (107–8 yr) (e.g., Egusa et al. 2009). Our results suggest that at least a non-negligible fraction (∼20%) of bulge-dominated green valley galaxies show a sign of molecular gas deficiency (hence unusually high SFE), but it is clear that we need a larger sample to more quantitatively understand the fraction and the nature of this interesting population, and to firmly conclude that this population appears only in bulge-dominated galaxies.

4.3. Comparison with Previous Studies

The morphological quenching scenario proposed by Martig et al. (2009) has been supported by many recent studies with CO observations. For example, Saintonge et al. (2011b) reported that the gas depletion time τdep (=${M}_{{{\rm{H}}}_{2}}$/SFR) increases with C-index of galaxies. Martig et al. (2013) and Davis et al. (2014) also investigated the relationship between ${{\rm{\Sigma }}}_{{\rm{H}}{\rm{I}}+{{\rm{H}}}_{2}}$ and ${{\rm{\Sigma }}}_{\mathrm{SFR}}$ for local early-type galaxies, and suggested that SF in early-type galaxies are less efficient by a factor of ∼2 than those in late-type galaxies. More recently, Colombo et al. (2018) showed that the spatially resolved gas depletion time ${\tau }_{\mathrm{dep}}^{\mathrm{mol}}(={{\rm{\Sigma }}}_{\mathrm{gas}}/{{\rm{\Sigma }}}_{\mathrm{SFR}})$ of elliptical galaxies tends to be longer than that of galaxies with spiral galaxies. However, it should be noted that these earlier works supporting the morphological quenching scenario attempted to compare typical early- and late-type galaxies. Early-type galaxies are usually selected from passive galaxy population on the red sequence, while late-type galaxies are selected from the star-forming population in the blue cloud (as mentioned in Section 1).

In this paper, we reported that there is no significant difference in the SFE for bulge- and disk-dominated green valley galaxies (as well as MS galaxies). We note, however, that our results do not necessarily suggest that galaxy morphologies do not affect the SF quenching process at all. As mentioned in Section 4.2, we find some CO-undetected galaxies with unusually high SFEs in our bulge+green sample, which can be identified as outliers in the ${M}_{{{\rm{H}}}_{2}}$–SFR diagram. These molecular gas deficient galaxies with bulge-dominated morphologies may represent the galaxy population during the SF quenching process due to the presence of bulge component.

Schawinski et al. (2014) suggested that the green valley galaxies with bulge-dominated morphologies are quenched on a shorter timescale than those with late-type morphologies. Interestingly, Fabello et al. (2011), who studied the morphological dependence of atomic gas contents in galaxies, claimed that early-type galaxies tend to have a smaller amount of atomic gas than the late-type galaxies at fixed M* and SFR, and this contradicts the morphological quenching scenario. As mentioned in Martig et al. (2013), the atomic gas component is distributed in the extended disks of galaxies, and it would not be an ideal tool to discuss the SF quenching process. Nevertheless, our CO observations also revealed that there is no significant difference in the average SFE for the bulge- and disk-dominated galaxies at fixed M* and SFR, and that 20% of the bulge+green galaxies have exceptionally higher SFE and a lower molecular gas fraction than the other bulge+green galaxies, consistent with the trend reported by Fabello et al. (2011). Overall, we suggest that the presence of the bulge component in galaxies should affect the molecular gas, in the sense that galaxies with a prominent bulge tend to rapidly remove and/or consume their molecular gas content (hence showing apparently higher SFEs), in contrast to the prediction of the morphological quenching scenario.

Finally, we comment that our results are based on the spatially integrated stellar mass, molecular gas mass, and SFR information, while it is very important to perform a systematic study of spatially resolved molecular gas properties in green valley galaxies using interferometric data (e.g., ALMA or NOEMA), in order to directly assess the morphological impacts on the individual star-forming regions within the galaxies. Lin et al. (2017) recently performed a pioneering work of the spatially resolved SF and molecular gas properties of three local green valley galaxies with ALMA and MaNGA data, and they reported that both ${f}_{{{\rm{H}}}_{2}}$ and SFE play a role in lowering the sSFR in the disk component of green valley galaxies. It is important to extend this survey to galaxies with various morphological types to reveal the real morphological impacts on SF quenching process inside the galaxies.

5. Conclusion

We present our new CO observations of the green valley galaxies with different morphologies ("disk+green" and "bulge+green" samples) at fixed M* and SFR using the NRO 45 m telescope, to investigate the real effect of galaxy morphologies on their SFE as well as the correlation between ${M}_{{{\rm{H}}}_{2}}$ and SFR. Our findings are summarized below.

  • 1.  
    The distribution of ${M}_{{{\rm{H}}}_{2}}$, ${f}_{{{\rm{H}}}_{2}}$ and SFE for the CO-detected green valley galaxies ($10.8\lt \mathrm{log}({M}_{* }/{M}_{\odot })\lt 11.2$ and $-11\lt \mathrm{log}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})\lt -10.5$) are independent of their morphologies. We also found that both CO-detected disk-dominated and bulge-dominated green valley galaxies follow the same correlation between ${M}_{{{\rm{H}}}_{2}}$ and SFR. There is no evidence for a decline in the SFE of galaxies with early-type morphologies.
  • 2.  
    We did not detect the CO line from ∼20% (5 out of 22) of the bulge+green galaxies. Even in the stacked spectrum of CO-undetected galaxies, the emission line was not detected. These CO-undetected galaxies show exceptionally high SFEs for their M* and SFR, in contrast to the prediction from the morphological quenching scenario. Although we confirmed that our main results on the morphological independence of the averaged SFEs of galaxies (at fixed M* and SFR) are not changed by these CO-undetected sources, these molecular gas deficient sources that are identified only in the bulge+green sample might be an important population during the phase of SF quenching due to the presence of stellar bulge in galaxies.
  • 3.  
    Using the CO data from the xCOLD GASS survey, we found that our results on the morphological independence of SFE is also valid for normal star-forming galaxy population on the MS; i.e., at a fixed M*, the average SFE of galaxies on the star-forming MS does not vary with their morphologies. This is in agreement with Saintonge et al. (2017), who used the stellar mass surface density as a morphological indicator instead of the C-index. We confirmed that there is a significant decrease in mean SFEs from the MS to the green valley phase, but the level of this decline is the same for both bulge- and disk-dominated galaxies. This result suggests that the SF quenching mechanism equally affects the SFE of galaxies, regardless of their morphological properties.

We emphasize again that our target galaxies are carefully selected from a small window in the SFR–M* diagram, while many other studies discussing the morphological difference in SFE compare early-type galaxies selected from passive (red-sequence) galaxy population and late-type galaxies selected from star-forming (blue-cloud) galaxies. Although there still remains a possibility that the SF quenching timescale is different between the disk- and bulge-dominated galaxies, our results suggest that the galaxy morphology is not a primary factor to control the SFE of galaxies as long as we focus on galaxies at fixed M* and SFR.

We thank the anonymous referee for careful reading and useful comments that helped us to improve our paper.

This research is based on SDSS-III and on observations at the Nobeyama 45 m radio telescope.

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III website is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.

The Nobeyama 45 m radio telescope is operated by the Nobeyama Radio Observatory, a branch of the National Astronomical Observatory of Japan.

This work was financially supported in part by a Grant-in-Aid for the Scientific Research (No. 26800107; 18K13588) by the Japanese Ministry of Education, Culture, Sports and Science.

Facility: NRO 45 m. -

Software: NEWSTAR.

Footnotes

10.3847/1538-4357/ab0e75