Active Galactic Nuclei in Dusty Starbursts at z = 2: Feedback Still to Kick in

We fit the multiwavelength SEDs of the Herschel sources to derive their physical characterization. We used both the MAGPHYS (da Cunha et al. 2008), and the SED3FIT (Berta et al. 2013) SED-fitting codes, the latter accounting for an additional AGN component (dusty torus). MAGPHYS relies on the energy balance between the dust-absorbed stellar continuum and the reprocessed dust emission at infrared (IR) wavelengths. SED3FIT combines the emission from stars, dust heated by star formation, and a possible AGN-torus component from the library of Feltre et al. (2012). We fitted each observed SED by using the best available redshift (either spectroscopic or photometric) as input. The SFR was derived from the total IR (rest frame [8–1000] μm) luminosity taken from the best-fit galaxy SED (subtracted by the AGN luminosity, if present), assuming a Kennicutt (1998) conversion.

Out of 1790 Herschel detected sources at 1.5 < z < 2.5, we identified a sample of 164 SBs (see Figure 1). We limit our analysis to M_* > 10¹⁰ M_⊙ (to ensure an unbiased mass complete selection, see Laigle et al. 2016) reducing the SB sample to 152 objects.

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In order to quantify the relative incidence of a possible AGN component, we fitted each individual observed SED. The fit obtained with the AGN is preferred if the reduced χ² value of the best fit (at ≥99% confidence level, on the basis of a Fisher test) is significantly smaller than that obtained from the fit without the AGN (see Delvecchio et al. 2014). From our analysis, 35 out of 152 SBs (about 23%) show a significant AGN component. In the following, we will refer to these two classes as SBs with evidences of a nuclear activity (SBs-AGN) and SFR-dominated SBs (SBs-SFR), respectively. We have verified that out of 152 SBs in our starting sample, only eight are classified as X-ray AGN (Laigle et al. 2016), six of which are identified by our AGN classification. This check ensures that we recover the bulk (i.e., 75%) of the classical X-ray/AGN selection,¹⁸ extending the sample to include also the most obscured active sources (e.g., Bongiorno et al. 2012; Gruppioni et al. 2016). Moreover, because at 1.5 < z < 2.5 the IRAC bands span the rest frame near-IR where the galaxy stellar emission peaks, to ensure that the AGN classification is not contaminated we performed a further test. Thus, to verify if substantially old stellar populations could bias the AGN selection, we computed the mass-weighted ages of all SBs in our sample with and without the dusty torus component. In spite of a significant scatter (∼0.3 dex), no offset is observed in the measured average stellar ages of the systems, both for SBs-SFR and SBs-AGN (probing that the SED3FIT software correctly recovers, in a statistical sense, the near-IR light arising from the stellar populations and from the torus). We have also tested our AGN selection procedure against classical IRAC/Spitzer color–color diagnostics, as the one proposed by Donley et al. (2012), finding that ∼80% of the SBs-AGN fall into the AGN region, while just one of the SBs-SFR are misclassified as AGN (see Figure 2, left panel). A comparison with the average fraction of the luminosities coming from the AGN component at [5–40] μm in the SED modeling, confirms that sources lying above the dashed line in the left panel of Figure 2 have four times larger AGN contributions (25% versus 6%, in terms of the light fraction arising from the torus in the mid-IR) with respect to the sources below the same line. This indicates that the procedure adopted in this work to classify AGN preferentially selects sources dominated by a torus component at mid-IR wavelengths (as expected). The comparison of our AGN selection criterium with the color–color plot presented in Figure 2 (left panel) shows that our SBs-AGN sample is highly reliable¹⁹ (contamination ≤20%), but not necessarily complete (Donley et al. 2012).

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We searched for possible ALMA millimetric counterparts of the SBs in the public archive, adopting a search radius of 25. The final sample includes 33 sources with a clear detection over 3σ, mostly in band 7 at 870 μm (with four undetected sources that we do not include in this work), with few of them detected also in band 3 or 2. The measurement sets were calibrated running the data reduction scripts with the Common Astronomy Software Applications (CASA). Fluxes and associated errors were evaluated with CASA, by fitting the emissions with a bidimensional Gaussian, and are reported in Table 1. For the 33 ALMA detected sources, we included the sub-mm fluxes in the observed SED and we performed a second fitting run, as described in Section 2.1. The updated physical parameters are reported in Table 1.

Table 1.  Main Observational and Physical parameters of the SBs with an ALMA Continuum Detection (Redshifts between Parentheses are Spectroscopic).

ID	R.A.	Decl.	z	$\mathrm{log}{L}_{\mathrm{IR}}$	$\mathrm{log}{L}_{\mathrm{AGN}}$	$\mathrm{log}{M}_{* }$	$\mathrm{log}{M}_{\mathrm{gas}}$	AGN	F870μm	F1.3mm	F3.0mm
COSMOS15	(deg)	(deg)		(L⊙)	(erg s−1)	(M⊙)	(M⊙)		(mJy)	(mJy)	(mJy)
182648	150.64316	1.558194	1.6887	11.115	45.595	10.4616	9.840	yes	0.49 ± 0.10	⋯	⋯
221280	149.76807	1.617000	2.3220	11.185	43.945	10.4703	10.282	yes	1.11 ± 0.22	⋯	⋯
244448	150.01202	1.652130	(1.5180)	10.645	⋯	10.7505	10.472	no	2.01 ± 0.25	⋯	⋯
280968	149.79010	1.711870	1.7844	11.343	⋯	10.4035	10.384	no	1.33 ± 0.25	⋯	⋯
323041	149.81653	1.779770	2.0933	11.244	44.135	10.5122	10.018	yes	1.38 ± 0.46	⋯	⋯
349784	150.48938	1.821710	1.9693	11.100	⋯	10.8425	10.628	no	1.59 ± 0.34	⋯	⋯
386956	150.34194	1.880208	2.2493	11.346	⋯	10.6594	10.201	no	1.88 ± 0.30	⋯	⋯
505526	150.42101	2.068100	2.2684	10.666	45.245	11.0934	11.313	yes	11.93 ± 0.71	⋯	⋯
506667	150.72984	2.071170	2.4433	11.200	⋯	10.5955	10.590	no	2.23 ± 0.77	⋯	⋯
524710	149.76853	2.099614	2.1059	11.321	⋯	10.4187	10.214	no	1.72 ± 0.23	⋯	⋯
571598	150.61642	2.167971	1.5052	11.167	⋯	10.8600	10.792	no	5.39 ± 0.23	⋯	⋯
578926	150.40103	2.180390	(2.3341)	11.567	⋯	10.9570	10.617	no	2.05 ± 0.55	⋯	⋯
600601	150.13265	2.211946	1.9875	10.752	45.605	11.1349	11.081	yes	8.27 ± 0.44	2.46 ± 0.10	⋯
605409	149.76813	2.219876	(1.7766)	11.112	⋯	10.8708	10.585	no	3.78 ± 1.03	⋯	⋯
640026	150.03663	2.270976	1.7977	11.313	44.185	10.2632	10.349	yes	1.09 ± 0.27	⋯	⋯
642313	149.60419	2.275064	2.0069	11.254	⋯	10.6391	10.677	no	1.77 ± 0.31	⋯	⋯
651584	149.92196	2.289929	2.3341	11.600	⋯	10.8786	10.626	no	4.93 ± 0.34	⋯	⋯
734578	149.52823	2.413200	1.9641	11.765	⋯	10.5523	10.893	no	⋯	1.88 ± 0.23	⋯
745498	150.46551	2.429549	1.6332	10.731	⋯	10.7144	10.355	no	3.47 ± 0.34	⋯	⋯
747590	150.22447	2.433010	1.6351	10.903	⋯	10.6513	10.285	no	1.60 ± 0.24	⋯	⋯
752016	150.33683	2.439920	2.2682	11.585	⋯	10.7232	10.665	no	4.50 ± 0.31	⋯	⋯
754372	150.06907	2.444010	2.4355	11.439	45.595	11.0108	10.468	yes	4.86 ± 0.65	⋯	0.275 ± 0.065
769248	150.25528	2.466839	(2.2640)	10.903	⋯	10.5134	10.629	no	3.80 ± 0.42	⋯	⋯
794848	150.09341	2.507339	2.1990	11.365	45.975	10.7362	10.648	yes	3.09 ± 0.25	⋯	⋯
810228	150.11307	2.528020	2.0167	11.264	⋯	10.6265	10.687	no	⋯	⋯	0.137 ± 0.060
815012	150.60329	2.536536	(2.2872)	11.119	⋯	11.1034	10.692	no	6.70 ± 0.35	⋯	⋯
818426	150.72202	2.541904	2.2664	11.607	45.935	10.6405	10.807	yes	1.01 ± 0.30	⋯	⋯
842595	149.99796	2.578227	2.4200	11.693	44.205	10.7809	10.732	yes	1.97 ± 0.32	⋯	⋯
902320	150.03726	2.669600	(1.5990)	10.274	⋯	10.9826	11.162	no	6.41 ± 0.39	⋯	0.147 ± 0.057
917423	149.99218	2.693436	2.1284	11.330	⋯	10.7933	10.581	no	1.77 ± 0.34	⋯	⋯
917546	150.16165	2.691588	(1.9745)	10.937	44.785	10.4437	10.156	yes	1.39 ± 0.25	⋯	⋯
951838	150.26832	2.749270	2.0186	11.128	45.535	9.9814	10.076	yes	1.60 ± 0.24	⋯	⋯
980250	150.01611	2.792355	1.7598	10.533	⋯	11.1303	10.875	no	5.03 ± 0.25	⋯	⋯

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For comparative analyses, we use the reference sample compiled by Sargent et al. (2014) and Perna et al. (2018), including "typical" star-forming galaxies at z ≤ 3 with measurements of their CO luminosity. Local ULIRGs and high-z SBs with a CO detection are added. Detailed references for the various samples included are reported in Sargent et al. (2014) and Perna et al. (2018). We added the recent compilation of SBs by Silverman et al. (2015) and Silverman et al. (2018), at z ∼ 1.6.

Dust masses (M_dust) have been derived following the procedure described in Magdis et al. (2012, 2017) by fitting the SED in the IRAC-ALMA observed frame with the Draine & Li (2007) models. The total gas mass (M_gas, which incorporates both the molecular and atomic phases) can be inferred from the dust mass through the metallicity dependent gas-to-dust ratio (GDR; e.g., Eales et al. 2010; Magdis et al. 2012): M_gas = M_dust/GDR(Z).

For consistency with M_gas estimates of SBs in the literature, we adopt GDR ≈ 30, that based on Magdis et al. (2012) corresponds to a CO-to-M_gas α_co conversion factor of 0.8 M_⊙/(K km s⁻¹ pc²), typically used for strong SBs. For completeness we also infer M_gas estimates with GDR ≈ 95 that corresponds to solar gas-phase metallicity that could be considered as a lower limit (on metallicity, hence the upper limits on GDR and thus on M_gas) for dust-obscured SBs (e.g., Puglisi et al. 2017). These M_gas estimates are consistent (within 0.15 dex) with the average M_gas estimates derived based on the monochromatic flux densities in the R–J tail of the SED (one or more of 870, 1300, and 3000 μm in our case) and the recipe presented in Scoville et al. (2017).

We report in Figure 3 (left panel) the gas masses of the SBs as a function of their M_*, divided into SBs-AGN and SBs-SFR (blue and red filled stars, respectively). The SB population is dominated by gas-rich galaxies, with gas fractions (defined as f_gas = M_gas/(M_gas + M_*)) spanning the range 20%–70%. This is similar to the typical f_gas (∼50%) of the normal star-forming sources at similar z (open circles). Local star-forming galaxies (both MS and ULIRGs/SBs) are instead much gas poorer, with f_gas ∼10%, as expected on the basis of the observed gas fraction evolution with cosmic time (e.g., Magdis et al. 2012; Tacconi et al. 2018, and references therein).

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When looking at the separate gas fractions of SBs-AGN and SBs-SFR, we do not observe a significant difference, providing average values of f_gas = 43% ± 4% and f_gas = 42% ± 2% for the two classes, respectively.

To shed more light on our analysis in the most efficient star-forming sources, we study (right panel of Figure 3) the distribution of M_gas as a function of SFR, in order to compare the star formation efficiency (i.e., SFE = SFR/M_gas) of SBs-AGN and SBs-SFR. Our sample turns to be "SFR-selected" by construction, with SFR ≥150 M_⊙ yr⁻¹ due to the requirement of being Herschel selected (see Figure 1). We then observe that SBs lie on a contiguous sequence of increasing SFE that fills the gap between the two paradigmatic sequences of normal galaxies and ULIRG/SBs (dotted lines, as from Sargent et al. 2014), usually interpreted as the main loci of two extremely different SF modes. Our results support recent works suggesting the existence of a continuous increase in SFE with elevation from the MS, as opposed to a bimodal distribution (Silverman et al. 2018).

As mentioned in Section 1, we can compare our results with the expectations of the AGN-galaxy co-evolutionary scenario, that predicts a luminous IR phase of buried SMBH growth, co-existing with a SB (likely arising from a merger; Donley et al. 2018; Koss et al. 2018) before feedback phenomena deplete the cold molecular gas reservoir of the galaxy and an optically luminous QSO shines out (Hopkins et al. 2008). On one side, we have qualitatively observed that SBs including an AGN have on average more compact and nucleated UV-rest frame morphologies with respect to "inactive" SBs, suggesting that they are kept in a different dynamical evolutionary phase. This could correspond to the key transition when the late mergers trigger a high SFR, before the fully developed AGN phase. Simulations and observations, indeed, suggests a temporal delay between the peak of the SFR and the peak of the BHAR (Rodighiero et al. 2015; Bergvall et al. 2016; Lapi et al. 2016). On the other side, we did not find a significant reduction of gas fractions in the SBs-AGN hosts compared to "inactive" SBs-SFR. We argue that, if major mergers are the main triggering mechanism of obscured BHAR in SBs, feedback phenomena (producing large outflows from the central BH) are not efficient in removing significant amount of molecular gas in the host galaxies.

An alternative interpretation for the properties of the off-MS galaxies is provided by an in situ co-evolution scenario (Lapi et al. 2018), envisaging high-z galaxy evolution to be mainly ruled by the interplay between in situ processes like gas infall, compaction, star formation, accretion onto the hosted central SMBHs, and related feedback processes (with wet galaxy mergers having a minor role at z ≥ 1). In this framework z ∼ 2 sources with SFR ≥ a few 10² M_⊙ constitute the progenitor of local massive spheroids with stellar mass M_* ≥ 10¹¹ M_⊙. During their star-forming phase, lasting some 10⁸ yr, these galaxies feature a nearly constant SFR and a linearly increasing stellar mass. In the SFR–M_* diagram they follow an almost horizontal track (see the cyan tracks in Figure 1) while moving toward the galaxy MS locus; there they will have acquired most of their mass before being quenched by energy/momentum feedback from the SMBH.

Being in the early stages of their evolution, the SBs can host only a rather small SMBH, originating a bolometric AGN luminosity L_AGN that is weaker than that L_SFR associated to the SFR in the host. However, the BH mass is expected to grow exponentially generating a noticeable statistical variance in L_AGN at given SFR. As a consequence, in the SFR versus L_AGN plane, SBs are expected to populate a strip parallel to the L_AGN axis, and located to the left of the locus where L_SFR = L_AGN (see Figure 4 and Bianchini et al. 2018). The hosted AGNs are expected to be obscured and have a luminosity that is still not powerful enough to originate substantial feedback effects on the interstellar medium (ISM) of the host galaxy; thus, the SFR and the gas mass of the host are still not much affected. The compact morphologies of SBs-AGN (possibly linked to a forming bulge) could indicate that these sources are observed when the host stellar mass and the BH mass are reaching their maximum, just before the feedback gets into action and the BHAR and SFR gets quenched (Lapi et al. 2014); further size evolution of the stellar component may be induced by the feedback itself and by late-time mass addition from dry mergers (see Lapi et al. 2018).