A New Diagnostic Diagram of Ionization Sources for High-redshift Emission Line Galaxies

We start from the main galaxy sample (Strauss et al. 2002) of the Sloan Digitial Sky Survey Data Release 7 (SDSS DR7) (Abazajian et al. 2009). The sample is complete in r-band Petrosian magnitude between 15 and 17.77 over 9380 deg². We limit the redshift range to z < 0.33, and there are 835,410 spectroscopic galaxies. To properly measure the emission lines, we use the scheme developed in Hao et al. (2005) to subtract stellar absorptions. They used several hundreds of SDSS low-redshift pure absorption galaxies to construct the principal component analysis (PCA) eigenspectra, and used the first eight eigenspectra to fit the continuum. An A-star template is added to represent a young stellar population. A power law is also added when fitting AGN spectra. The continuum-subtracted line emissions are left for refined line fitting. We use a Gaussian function to fit the lines [O ii] λ3727, [O i] λ6300, Hβ, [O iii] λ5007, [N ii] λ6548, Hα, [N ii] λ6583, and [S ii] λλ6717, 6731 (hereafter [O ii], [O i], Hβ, [O iii], [N ii] λ6548, Hα, [N ii] λ6583, and [S ii]) respectively. σ (line width, 1/2.35 FWHM in km s⁻¹) of the Gaussian profile for [O iii] λ5007 is denoted as σ_{[O iii]}. The line ratio [N ii] λ6583/[N ii] λ6548 is fixed as 3 and their profile and center are tied to be the same. In addition, we fit Hα and Hβ a second time, adding one broad Gaussian to account for possible broad Hα and Hβ lines. The lower limit of σ of the broad component is 400 km s⁻¹. The typical FWHM of the broad Hα component of Type 1 AGNs is larger than 1200 km s⁻¹ (Hao et al. 2005). We regard the broad Hα component as being prominent if an F-test suggests that the improvement is significant at the 3σ level. The intrinsic velocity dispersion σ_int of the emission lines is obtained by subtracting the instrument resolution of ∼56 km s⁻¹ using ${\sigma }_{\mathrm{int}}^{2}={\sigma }_{\mathrm{obs}}^{2}-{\sigma }_{\mathrm{Instrument}}^{2}$. The errors of σ and the line strength are obtained by the MPFIT package, which only includes the fitting errors (Markwardt 2009). We perform a simple test to check how well we can measure the line width. We add a Gaussian to a continuum with a given EW. σ of the Gaussian is the combination of the intrinsic width of the emission line, 10^1.8 = 63.1 km s⁻¹ (typical star-forming galaxy), and the instrumental resolution of 56 km s⁻¹. Random errors are added according to signal-to-noise ratio (S/N) = 3, 5, 7, 10. We fit the emission line using the MPFIT package, and measure the error of σ by comparing the measured value with the input one. The simulation is run 500 times. At EW = 3 (typical value for a star-forming galaxy with −0.5 < log[O iii]/Hβ < 1) and S/N = 3, 5, 7, 10, the errors in emission line width, σ, are 0.19 dex, 0.08 dex, 0.06 dex, and 0.04 dex. For the worst case—EW = 3, S/N = 3—the error in σ is 0.19 dex. For sources with higher [O iii]/Hβ and higher emission line width, the measurements are more reliable.

Our intermediate-redshift galaxy sample is based on observations from the DEEP2 Galaxy Redshift Survey (hereafter DEEP2; Davis et al. 2003; Newman et al. 2013).³ The DEEP2 survey has a limiting magnitude of R_AB = 24.1, and it covers 3.2 deg², spanning four separate fields on the sky. The spectra span a wavelength range of 6500–9100 Å at a spectral resolution of R ∼ 5000. The DEEP2 DR4 includes 52,989 galaxies. For our study, we limit the redshift range to be 0.32 < z < 0.82 to ensure the detection of Hβ and [O iii] in the DEEP2 wavelength coverage. The sample size is thus cut to 12,739 galaxies. The spectra are obtained with the DEIMOS spectrograph (Faber et al. 2003) at the Keck Observatory and reduced with the pipeline⁴ developed by the DEEP2 team at the University of California Berkeley. All the DEEP2 footprints are observed by Chandra Advanced CCD Imaging Spectrometer (ACIS-I) with total exposures across all four XDEEP2 fields ranging from ∼10 ks to 1.1 Ms (Nandra et al. 2005; Laird et al. 2009; Goulding et al. 2012). The intermediate-redshift data are used for calibration of the new KEx diagnostic diagram at z < 1.

Our KEx diagram uses only the narrow Hβ component and [O iii] emission lines so it is straightforward to use Type 1 AGN narrow-line components to calibrate our KEx diagram. We first use a sample of z < 0.3 Type 1 AGNs from the main galaxy sample of SDSS DR7 for testing. These sources have a broad Hα component with significance greater than 3σ as described in Section 2.1. The line width of broad Hβ is almost the same as that of Hα, with FWHM greater than 1200 km s⁻¹ (σ ∼ 510 km s⁻¹). σ of the narrow Hβ line is typically 200 km s⁻¹, and rarely exceeds 300 km s⁻¹. Usually the difference in width between broad and narrow lines is big enough for a reliable decomposition according to our simple simulation. If the narrow and broad components have similar width, some broad Hβ flux might go into narrow Hβ due to unreliable decomposition. This decreases [O iii]/Hβ, and makes this source more likely to be classified as an SFG. So it is possible that some Type 1 AGNs are misclassified as SFGs. This is very rare because a narrow broad Hβ line indicates a low-mass black hole, which is very rarely detected in the SDSS survey (Dong et al. 2012), and thus will not influence our result. The typical density in an AGN broad-line region is at least 10⁸ cm⁻³ (e.g., Netzer 2008), while the critical density of [O iii] λ5007 is 7 × 10⁵ cm⁻³ (e.g., Osterbrock 1989). Thus [O iii] does not have a broad component in Type 1 AGNs, and we do not need to decompose the [O iii] emission line into broad and narrow components.

We apply an S/N threshold of 3 to Hβ and [O iii] narrow lines, and plot these z < 0.3 Type 1 AGNs described in Section 2.3 in the KEx diagram in Figure 4(a). The [O iii]/Hβ lines only include the narrow component of Hβ. These sources (4624) reside mostly in the KEx-AGN and KEx-composite regions, while only 414 (9%) of them are in the KEx-SFG region.

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We further plot a sample of intermediate-z Type 1 AGNs selected from the QSO catalog of SDSS DR4 with 0 < z < 0.8 on the KEx diagram. The sources in this sample have small contamination from host galaxies in their optical light, and their properties and data reduction are described in Dong et al. (2011). Detailed analysis of narrow-line properties, especially the [O iii] line, can be found in Zhang et al. (2011, 2013a, 2013b). The bolometric luminosity range of these sources is 10⁴⁴–10⁴⁷ erg s⁻¹. In the left panel of Figure 4, we plot this sample on the KEx diagram in orange. Almost all (96%) of the sources lie in the KEx-AGN region, and a small fraction of the Type 1 sources (4%) lie in the KEx-composite region. Only 13 (0.3%) Type 1 sources are classified as KEx-SFGs. According to the unification model, if these Type 1 sources are viewed edge-on, almost all of them would be correctly classified as AGNs.

One may notice that some Type 1 AGNs lie outside the low-z locus. Some sources have higher [O iii]/Hβ line ratio and some have larger σ_{[O iii]}. These could be partly understood by the orientation effect. The Type 1 AGNs are found to have higher ionization state than Type 2 AGNs (Veilleux 1991; Schmitt et al. 2003a, 2003b); this is because high-ionization lines arise from regions closer to the nuclei and thus are more likely to be blocked when viewed edge-on. The inclination effect may play a role in the greater width of [O iii] emission here. The [O iii] emission line is known to show a blue-wing asymmetric profile (Heckman et al. 1981; Zhang et al. 2011; Bae & Woo 2014, 2016; Woo et al. 2016) and this is believed to be due to outflows from narrow-line regions. When the outflows are viewed in a face-on orientation, we would see a larger overall outflow velocity, which would lead to a larger line width.

Our intermediate-redshift galaxy sample is based on observations from the DEEP2 Galaxy Redshift Survey. Most of the galaxies in DEEP2 surveys are star-forming galaxies, as indicated by X-ray studies (Nandra et al. 2005; Laird et al. 2009; Goulding et al. 2012). It is interesting to check whether the X-ray and the KEx classifications are consistent and what causes the differences. An X-ray luminosity threshold of L_2–10keV > 10⁴² erg s⁻¹ is adopted. There are no star-forming galaxies with X-ray luminosity higher than this value in the local universe (Fabbiano 1989; Bauer et al. 2004; Colbert et al. 2004). However, weak AGNs with L_{2–10 keV} < 10⁴² erg s⁻¹ do exist even though they are more difficult to differentiate from star-forming or starburst galaxies without additional information. For DEEP2 X-ray data, the sensitivity of the shallowest data could ensure the detection of luminous X-ray sources (L_{2–10 keV} > 10⁴² erg s⁻¹).

[O iii]/Hβ is plotted versus σ_{[O iii]} for DEEP2 emission line galaxies as gray triangles in the right panel of Figure 4. We use the method described in Section 3 for spectral fitting. We apply an S/N cut of 3 to Hβ and [O iii] λ5007. 7866 sources are selected according to these criteria. We convert the hard 2–8 keV X-ray flux to rest-frame 2–10 keV luminosities (L_X(2–10 keV)) by assuming a power-law spectrum with photon index (γ = 1.8). The sources with L_X(2–10 keV) > 10⁴² erg s⁻¹ are classified as X-ray AGNs, and sources with L_X(2–10 keV) < 10⁴² erg s⁻¹ are classified as star-forming galaxies. The X-ray sources are plotted as pink and purple triangles in the right panel of Figure 4. We can see that most of the X-ray AGNs/SFGs are consistently classified as optical AGNs/SFGs. Out of the 93 X-ray AGNs, 48 (52%) are classified as KEx-AGN, 18 (19%) are KEx-composite, and 27 (29%) are KEx-SFGs.

Out of the 83 X-ray SFGs in our sample, 19 are KEx-AGN, 14 are KEx-composite, and 49 (59%) are KEx-SFGs. In Juneau et al. (2011), for the X-ray starbursts, 50% (8/16) are classified as MEx-SFGs, while 19% (3/16) are in the intermediate region, and the remaining 31% (5/16) reside in the AGN region. The two results are consistent.

On the other hand, we notice that some sources are optically classified as SFGs but have very powerful X-ray emission, indicating that they harbor active nuclei. 29% of X-ray AGNs are classified as star-forming galaxies on the KEx diagram. In Juneau et al. (2011), 20% of their X-ray AGNs are classified as MEx-intermediate and 15% are MEx-SFGs. Considering that the "intermediate" region is mixed with the star-forming region on the MEx diagram, our result is consistent with theirs. Yan et al. (2011) found that 25% of their X-ray AGNs reside in the star-forming region of their optical classification diagram, which replaces [N ii]/Hα in the BPT diagram with rest-frame U – B color. This is consistent with our result too. Castelló-Mor et al. (2012) studied the sources that have L_X(2–10 keV) > 10⁴² erg s⁻¹ but are classified as SFGs on the BPT diagram. These sources have large thickness parameter (T = F_X/F_{[O iii]}), large X-ray to optical flux ratio (X/O > 0.1), broad Hβ line width, steep X-ray spectra, and they display soft excess. These mismatches illustrate that neither X-ray nor optical classification is complete. Different classification schemes are complementary to each other. Judging from Figure 4, the evolution in σ_{[O iii]} and [O iii]/Hβ is not large so we do not need to shift the dividing line at z < 1.

Compared to the widely used BPT diagram, the KEx diagram uses σ_{[O iii]} instead of the [N ii]/Hα ratio as the horizontal axis to diagnose the ionizing source and physical properties of emission line galaxies. We showed that it gives very consistent results with the BPT diagram. Here we discuss the physical reasons behind the diagram and why it works. σ_{[O iii]} in principle traces the motion of the gas. To first order, this motion is determined by the gravitational potential of the galaxy. The emission line width correlates well with the stellar velocity dispersion in almost all types of emission line galaxies (e.g., Nelson 2000; Wang & Lu 2001; Greene & Ho 2005; Bian et al. 2006; Dumas et al. 2007; Komossa & Xu 2007, 2008; Chen et al. 2009; Ho 2009). In AGNs, several other sources of line broadening may be at work in addition to the stellar kinematics (Greene & Ho 2005). So the basic principle behind the KEx diagram is the different kinematics of emitting gas in AGNs and star-forming galaxies.

The boundary between star-forming galaxies and AGNs on the KEx diagram indicates that there is a maximum σ_{[O iii]} for star-forming galaxies. In local star-forming galaxies, [O iii] comes from the H ii region, which is more tightly correlated with the kinematics of the disk. It was found that the width of narrow emission lines of star-forming galaxies could efficiently trace the maximum rotation velocity of a galaxy (Rix et al. 1997; Mallén-Ornelas et al. 1999; Weiner et al. 2006; Mocz et al. 2012). The rotation speed is directly linked to the luminosity of the galaxy through the Tully–Fisher relation (Tully & Fisher 1977). The most luminous spiral galaxies also rotate fastest. A reasonable hypothesis is that the galaxies near the maximum σ_{[O iii]} boundary in the KEx diagram are the most luminous ones. We plot [O iii]/Hβ versus R-band absolute magnitude for all the star-forming galaxies in our sample in Figure 6. The star-forming galaxies cluster to the bottom left of the diagram, and show a clear boundary. We draw a magenta boundary curve to define the maximum luminosity a star-forming galaxy can reach at a given [O iii]/Hβ. Using the Tully–Fisher relation obtained for SDSS galaxies by Mocz et al. (2012), we convert the luminosity in the curve to the velocity dispersion of ionized gas (σ_gas), and obtain a curve of [O iii]/Hβ versus σ_gas, which is shown as the dashed line in Figure 2(a). We can see that the dashed line encompasses the regions where SFGs and composites are located, separating Type 2 AGNs from SFGs and composites. This illustrates that SFGs and composites are consistent with the TFR prediction, while AGNs are located outside the locus.

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We plot the distribution of Type 2 AGNs (red dots) on diagrams of [O iii]/Hβ versus M_R (absolute Petrosian magnitude) and [O iii]/Hβ versus M_* (stellar mass) (MEx diagram, Juneau et al. 2011) in Figure 6. The magenta lines in the right panel are the MEx dividing lines. The sources above the upper curve are MEx-AGN and the region between the solid upper curve and dashed line is the MEx-composite region. The stellar mass is drawn from the MPA-JHU catalog. Type 2 AGNs occupy the bright and massive end of Figure 6. This is consistent with previous findings that Type 2 AGNs reside exclusively in massive, luminous galaxies (e.g., Kauffmann et al. 2003). 4% (246/5860) of Sy2s are on the MEx-SF side, while 3% (186/5860) are on the KEx-SF side. On the plot of [O iii]/Hβ versus M_R, 7.7% (450/5960) of Sy2s lie on the SFG side of the dividing curve shown in the left panel of Figure 6. The fraction of BPT-classified Type 2 AGNs that are misclassified as SFGs is higher on these two diagrams than on the KEx diagram.

There is an additional enhancement in σ_{[O iii]} at fixed luminosity or stellar mass for AGNs relative to SFGs. This enhancement helps AGNs separate further from SFGs on the KEx diagram. In Figure 7, we plot the median σ_gas derived from five emission lines against M_R and stellar mass in the left and right panels. We focus on σ_{[O iii]} (blue line) first and discuss other emission lines later. The errors are calculated using the bootstrap method. We can see that at a given luminosity that covers both AGNs and SFGs, σ_{[O iii]} is on average 0.15 dex higher for Type 2 AGNs than for SFGs. And at a given stellar mass, Type 2 AGNs are 0.12 dex higher than SFGs. These offsets are critical to the clean separation between Type 2 AGNs and star-forming galaxies on the KEx diagram. Other emission lines gave similar results.

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What gives rise to the difference in σ_{[O iii]} for AGNs and SFGs? One main reason for it is that the emission lines in Type 2 AGNs are produced by gas in the narrow-line region that extends into the bulge, while the emission lines in SFGs are produced by gas in the H ii region in the disk. The disk and bulge of a galaxy have different matter distributions and kinematics. It is shown in Catinella et al. (2012) that at a given luminosity or baryon mass, disk-dominated galaxies show 0.1 dex smaller stellar velocity dispersion (σ_*, measured from the SDSS 3'' fiber) than bulge-dominated galaxies. At a given mass, bulge-dominated galaxies, which are more concentrated, show a higher σ_* than disk-dominated galaxies. Besides the difference in host galaxies, several other physical reasons may be behind the fact that Type 2 AGNs have broader emission lines. Greene & Ho (2005) found that the excess [O iii] line width relative to the kinematics of stellar or lower-ionization lines is about 30%–40% (0.11–0.15 dex), with small variations depending on AGN luminosity, AGN Eddington ratio, star formation rate, etc. When considering only the core of [O iii], the excess in [O iii] line width goes away (Greene & Ho 2005; Komossa et al. 2008). This supports the idea that the [O iii] core is produced by ionized gas in the bulge while another source of broadening related to AGNs (such as possibly winds or outflows from an accretion disk) is at work. A radio jet may play a part in broadening the emission line too (Mullaney et al. 2013).

The differences in σ_gas and σ_* between AGNs and SFGs are more pronounced against M_R than against M*. In particular, σ_* is significantly higher in AGN hosts at a fixed M_R but not so different at a fixed stellar mass. This means that AGN hosts have higher mass-to-light ratios, which can be interpreted as having more important bulge components. More massive bulges host more massive black holes, and therefore may be detectable as Type 2 AGNs down to lower Eddington ratios. Conversely, lower mass AGNs are only identified as Type 2 AGNs for comparatively higher Eddington ratios, and their excess [O iii] line width could be more pronounced relative to other lines than for higher mass AGNs if the Eddington ratio is the factor driving additional broadening (e.g., Greene & Ho 2005; Ho 2008).

In summary, the demarcation line between SFGs/composites and AGNs on the KEx diagram is defined by the Tully–Fisher relation of the most luminous and massive galaxies. The AGNs reside in luminous and massive galaxies and at a given luminosity/stellar mass, their σ_{[O iii]} are 0.15/0.12 dex higher than SFGs. These effects make the KEx diagram an efficient classification tool for emission line galaxies.

Tresse et al. (1996) and Rola et al. (1997) proposed to use EW([O ii]), EW([O iii]), and EW(Hβ) for galaxy classification at high redshift. Stasińska et al. (2006) studied using [O ii] for galaxy classification, and proposed a method that uses D_n(4000), EW([O ii]), and EW([Ne iii]) (DEW diagram) to select pure AGNs with z < 1.3 using only the optical spectrum. However, different types of galaxies overlap with each other severely on these classification diagrams.

Trouille et al. (2011) proposed to use g − z, [Ne iii], and [O ii] to clearly separate AGNs from SFGs. This method is very efficient in separating different types of galaxies, but the [Ne iii] emission line is weak even in AGNs, thus this diagram requires spectra of galaxies with high S/N for reliable classification. This is particularly hard for high-redshift objects, which are usually faint.

Our KEx diagram has a similar logic to the color–excitation (CEx) and mass–excitation (MEx) diagrams proposed by Yan et al. (2011) and Juneau et al. (2011). The CEx diagram makes use of the fact that AGNs reside locally in red or green galaxies. But this is likely to be invalid at higher redshift (Trump et al. 2011b). The MEx diagram and the one using rest-frame H-band magnitude (Weiner et al. 2006) are based on the fact that AGNs are harbored by massive galaxies. This is more robust at high redshift considering the AGN downsizing effect. Trump et al. (2011b) tested the validity of these two diagrams at z ∼ 1.5 and found that the MEx remains effective at z > 1 but CEx needs a new calibration. Coil et al. (2015) also find that the MEx diagram fails to identify all of the AGNs identified in the BPT diagram, and the CEx diagram is substantially contaminated at z ∼ 2.3. As discussed in Section 3.2, the KEx diagram can separate AGNs and SFGs because AGNs reside in massive galaxies and have σ_{[O iii]} 0.12 dex higher than star-forming galaxies of similar stellar mass as an enhancement. This enhancement makes the KEx diagram more efficient at separating AGNs from SFGs. One advantage of the KEx is that it requires only a spectrum that covers a small spectral range to obtain all required quantities.

It is found that the [O iii]/Hβ ratio in SFGs gets higher at z ∼ 1–2 (Shapley et al. 2005; Erb et al. 2006a; Brinchmann et al. 2008; Liu et al. 2008; Hainline et al. 2009; Wright et al. 2010; Trump et al. 2011a; Shirazi et al. 2014). The explanations for this trend include: high-redshift galaxies have elevated ionization parameter U (Brinchmann et al. 2008; Shirazi et al. 2014), higher electron densities and temperatures (Liu et al. 2008), AGNs or shocks (Groves et al. 2006; Wright et al. 2010). It is interesting to check how [O iii]/Hβ evolves with redshift at given σ_{[O iii]} in the KEx diagram.

In the right panel of Figure 4, we plot the median [O iii]/Hβ at given σ_{[O iii]} for the SDSS galaxies at z < 0.33 and the DEEP2 galaxies on the KEx diagram. The blue, dark green, green, yellow, orange, and red lines are median [O iii]/Hβ at given σ_{[O iii]} for KEx-SFGs at z ∼ 0.1, 0.3 < z < 0.4, 0.4 < z < 0.45, 0.5 < z < 0.6, 0.6 < z < 0.7, and 0.7 < z < 0.8 respectively. The σ values are corrected for instrumental broadening. We can see in Figure 4 that the [O iii]/Hβ versus σ_{[O iii]} relation does not evolve from z ∼ 0.3 to z ∼ 0.8, but these galaxies have on average 0.2 dex higher [O iii]/Hβ than the local galaxies at given σ_{[O iii]}. One possible reason for this apparent evolution is that SDSS has a fixed aperture of 3'' that acquires the light from the center of the galaxy, while DEEP2 spectra are obtained through long slits, which enable them to include light from the outskirts of the galaxy. The outskirts have lower metallicity and larger rotation speed than the center. This would shift the [O iii]/Hβ versus σ_{[O iii]} relation to higher [O iii]/Hβ and higher σ_{[O iii]}. A better constraint on the systematics is need to test the evolution in the [O iii]/Hβ versus σ_{[O iii]} relation from z = 0–0.8.

The narrow-line regions of high-redshift galaxies have lower metallicity than local AGNs (Kewley et al. 2013b), even local AGNs with the same host stellar mass (Coil et al. 2015). The lower metallicity would elevate [O iii]/Hβ (Groves et al. 2006; Kewley et al. 2013a, 2013b; Feltre et al. 2016; Hirschmann et al. 2017). Considering that [O iii]/Hβ of SFGs would increase in high-redshift galaxies, SFGs and AGNs will still be separated well in terms of [O iii]/Hβ on the KEx diagram as shown in Figure 5.

In the KEx diagram, we use the width of the [O iii] emission line as a diagnostic. In order to check if different lines have different widths, in Figure 7 we plot the median value of σ against R-band absolute magnitude and stellar mass for different narrow lines using galaxies from SDSS DR7. We plot the stellar velocity dispersion (σ_*) as a red line for reference. The PCA continuum fitting cannot give the stellar velocity directly. We use σ_* stored in the SDSS spectrum file header. 32 giant K and G stars in M67 are used as stellar templates. These stellar templates are convolved with the velocity dispersion to fit the rest-frame wavelength range 4000–7000 Å by minimizing χ². The final estimation is the mean value of the estimates given by the "Fourier-fitting" and "Direct-fitting" methods. We found that SFGs show similar σ for all emission lines. The Type 2 AGNs, however, show some systematics in line width. The [O iii] line is broader than other low-ionization lines and recombination lines. This is expected, because the [O iii] emitting region is more concentrated due to its high ionization potential (Veilleux 1991; Trump et al. 2011b). Unexpectedly, Hβ shows the smallest line width, and the discrepancy is largest for high luminosity and high stellar mass. One possible cause of this discrepancy is that the Balmer absorption fitting is not perfect. The incorrect absorption fitting affects the resulting emission line flux and profile. Groves et al. (2012) found a significant discrepancy in Hβ when different versions of the continuum-fitting models were used. Hα, which should arise from the same emitting region as Hβ, does not follow the behavior of Hβ but shows the same trend as low-ionization lines because the absorption correction is much milder.

When comparing the line width of emission lines and σ_*, we found that the difference between σ_* in SFGs and Type 2 AGNs of similar stellar mass is very small. At high stellar masses, galaxies tend to have bulges, and the kinematics of a bulge would increase the measured σ_*. However, if the ionized gas in SFGs still comes from the disk, the velocity dispersion will not be affected by the bulge. Also, one can expect σ_gas < σ_* because the gas is more dissipative than the stars and can slow down dynamically (Ho 2009). The excess velocity width of emission lines in AGN hosts relative to SFGs can be due to a combination of bulge kinematics, narrow-line region kinematics, and wind/outflows. For AGNs of low to intermediate luminosity (on average), the broadening from outflows may not be as strong as for more luminous quasars (e.g., Greene & Ho 2005). A final discrimination of the relative importance of the mechanisms for emission line broadening may require spatially resolved kinematics (e.g., Davies et al. 2016).