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Register a new userThe Relation between Stellar and Dynamical Surface Densities in the Central Regions of Disk Galaxies
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Federico Lelli
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We use the SPARC (Spitzer Photometry & Accurate Rotation Curves) database to study the relation between the central surface density of stars Sstar and dynamical mass Sdyn in 135 disk galaxies (S0 to dIrr). We find that Sdyn correlates tightly with Sstar over 4 dex. This central density relation can be described by a double power law. High surface brightness galaxies are consistent with a 1:1 relation, suggesting that they are self-gravitating and baryon dominated in the inner parts. Low surface brightness galaxies systematically deviate from the 1:1 line, indicating that the dark matter contribution progressively increases but remains tightly coupled to the stellar one. The observed scatter is small (~0.2 dex) and largely driven by observational uncertainties. The residuals show no correlations with other galaxy properties like stellar mass, size, or gas fraction.
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We derive the stellar-to-halo mass relation (SHMR), namely $f_starpropto M_star/M_{rm h}$ versus $M_star$ and $M_{rm h}$, for early-type galaxies from their near-IR luminosities (for $M_star$) and the position-velocity distributions of their globular cluster systems (for $M_{rm h}$). Our individual estimates of $M_{rm h}$ are based on fitting a dynamical model with a distribution function expressed in terms of action-angle variables and imposing a prior on $M_{rm h}$ from the concentration-mass relation in the standard $Lambda$CDM cosmology. We find that the SHMR for early-type galaxies declines with mass beyond a peak at $M_starsim 5times 10^{10}M_odot$ and $M_{rm h}sim 10^{12}M_odot$ (near the mass of the Milky Way). This result is consistent with the standard SHMR derived by abundance matching for the general population of galaxies, and with previous, less robust derivations of the SHMR for early types. However, it contrasts sharply with the monotonically rising SHMR for late types derived from extended HI rotation curves and the same $Lambda$CDM prior on $M_{rm h}$ as we adopt for early types. The SHMR for massive galaxies varies more or less continuously, from rising to falling, with decreasing disc fraction and decreasing Hubble type. We also show that the different SHMRs for late and early types are consistent with the similar scaling relations between their stellar velocities and masses (Tully-Fisher and Faber-Jackson relations). Differences in the relations between the stellar and halo virial velocities account for the similarity of the scaling relations. We argue that all these empirical findings are natural consequences of a picture in which galactic discs are built mainly by smooth and gradual inflow, regulated by feedback from young stars, while galactic spheroids are built by a cooperation between merging, black-hole fuelling, and feedback from AGNs.
We use the Cosmic Assembly Deep Near-infrared Extragalactic Legacy Survey (CANDELS) data to study the relationship between quenching and the stellar mass surface density within the central radius of 1 kpc ($Sigma_1$) of low-mass galaxies (stellar mass $M_* lesssim 10^{9.5} M_odot$) at $0.5 leq z < 1.5$. Our sample is mass complete down to $sim 10^9 M_odot$ at $0.5 leq z < 1.0$. We compare the mean $Sigma_1$ of star-forming galaxies (SFGs) and quenched galaxies (QGs) at the same redshift and $M_*$. We find that low-mass QGs have higher $Sigma_1$ than low-mass SFGs, similar to galaxies above $10^{10} M_odot$. The difference of $Sigma_1$ between QGs and SFGs increases slightly with $M_*$ at $M_* lesssim 10^{10} M_odot$ and decreases with $M_*$ at $M_* gtrsim 10^{10} M_odot$. The turnover mass is consistent with the mass where quenching mechanisms transition from internal to environmental quenching. At $0.5 leq z < 1.0$, we find that the $Sigma_1$ of galaxies increases by about 0.25 dex in the green valley (i.e., the transitioning region from star forming to fully quenched), regardless of their $M_*$. Using the observed specific star formation rate (sSFR) gradient in the literature as a constraint, we estimate that the quenching timescale (i.e., time spent in the transition) of low-mass galaxies is a few ($sim4$) Gyrs at $0.5 leq z < 1.0$. The mechanisms responsible for quenching need to gradually quench star formation in an outside-in way, i.e., preferentially ceasing star formation in outskirts of galaxies while maintaining their central star formation to increase $Sigma_1$. An interesting and intriguing result is the similarity of the growth of $Sigma_1$ in the green valley between low-mass and massive galaxies, which suggests that the role of internal processes in quenching low-mass galaxies is a question worthy of further investigation.
We study the stellar populations of the brightest group galaxies (BGGs) in groups with different dynamical states, using GAMA survey data. We use two independent, luminosity dependent indicators to probe the relaxedness of their groups; the magnitude gap between the two most luminous galaxies ($Delta M_{12}$), and offset between BGG and the luminosity center ($D_{offset}$) of the group. Combined, these two indicators were previously found useful for identifying relaxed and unrelaxed groups. We find that the BGGs of unrelaxed groups have significantly bluer NUV-r colours than in relaxed groups. This is also true at the fixed sersic index. We find the bluer colours cannot be explained away by differing dust fraction, suggesting there are real differences in their stellar populations. SFRs derived from SED-fitting tend to be higher in unrelaxed systems. This is in part because of a greater fraction of BGGs with non-elliptical morphology, but also because unrelaxed systems have larger numbers of mergers, some of which may bring fuel for star formation. The SED-fitted stellar metallicities of BGGs in unrelaxed systems also tend to be higher by around 0.05 dex, perhaps because their building blocks were more massive. We find that the $Delta M_{12}$ parameter is the most important parameter behind the observed differences in the relaxed/unrelaxed groups, in contrast with the previous study of Trevisan et al. (2017). We also find that groups selected to be unrelaxed using our criteria tend to have higher velocity offsets between the BGG and their group.
For many massive compact galaxies, their dynamical masses ($M_mathrm{dyn} propto sigma^2 r_mathrm{e}$) are lower than their stellar masses ($M_star$). We analyse the unphysical mass discrepancy $M_star / M_mathrm{dyn} > 1$ on a stellar-mass-selected sample of early-type galaxies ($M_star gtrsim 10^{11} mathrm{M_odot}$) at redshifts $z sim 0.2$ to $z sim 1.1$. We build stacked spectra for bins of redshift, size and stellar mass, obtain velocity dispersions, and infer dynamical masses using the virial relation $M_mathrm{dyn} equiv K sigma_mathrm{e}^2 r_mathrm{e} / G$ with $K = 5.0$; this assumes homology between our galaxies and nearby massive ellipticals. Our sample is completed using literature data, including individual objects up to $z sim 2.5$ and a large local reference sample from the Sloan Digital Sky Survey (SDSS). We find that, at all redshifts, the discrepancy between $M_star$ and $M_mathrm{dyn}$ grows as galaxies depart from the present-day relation between stellar mass and size: the more compact a galaxy, the larger its $M_star / M_mathrm{dyn}$. Current uncertainties in stellar masses cannot account for values of $M_star / M_mathrm{dyn}$ above 1. Our results suggest that the homology hypothesis contained in the $M_mathrm{dyn}$ formula above breaks down for compact galaxies. We provide an approximation to the virial coefficient $K sim 6.0 left[ r_mathrm{e} / (3.185 mathrm{kpc}) right]^{-0.81} left[ M_star / (10^{11} mathrm{M_odot}) right]^{0.45}$, which solves the mass discrepancy problem. A rough approximation to the dynamical mass is given by $M_mathrm{dyn} sim left[ sigma_mathrm{e} / (200 mathrm{km s^{-1}}) right]^{3.6} left[ r_mathrm{e} / (3 mathrm{kpc}) right]^{0.35} 2.1 times 10^{11} mathrm{M_odot}$.
Previous studies of fueling black holes (BHs) in galactic nuclei have argued (on scales ~0.01-1000pc) accretion is dynamical with inflow rates $dot{M}simeta,M_{rm gas}/t_{rm dyn}$ in terms of gas mass $M_{rm gas}$, dynamical time $t_{rm dyn}$, and some $eta$. But these models generally neglected expulsion of gas by stellar feedback, or considered extremely high densities where expulsion is inefficient. Studies of star formation, however, have shown on sub-kpc scales the expulsion efficiency $f_{rm wind}=M_{rm ejected}/M_{rm total}$ scales with the gravitational acceleration as $(1-f_{rm wind})/f_{rm wind}simbar{a}_{rm grav}/langledot{p}/m_{ast}ranglesim Sigma_{rm eff}/Sigma_{rm crit}$ where $bar{a}_{rm grav}equiv G,M_{rm tot}(<r)/r^{2}$ and $langledot{p}/m_{ast}rangle$ is the momentum injection rate from young stars. Adopting this as the simplest correction for stellar feedback, $eta rightarrow eta,(1-f_{rm wind})$, we show this provides a more accurate description of simulations with stellar feedback at low densities. This has immediate consequences, predicting e.g. the slope and normalization of the $M-sigma$ and $M-M_{rm bulge}$ relation, $L_{rm AGN}-$SFR relations, and explanations for outliers in compact Es. Most strikingly, because star formation simulations show expulsion is efficient ($f_{rm wind}sim1$) below total-mass surface density $M_{rm tot}/pi,r^{2}<Sigma_{rm crit}sim3times10^{9},M_{odot},{rm kpc^{-2}}$ (where $Sigma_{rm crit}=langledot{p}/m_{ast}rangle/(pi,G)$), BH mass is predicted to specifically trace host galaxy properties above a critical surface brightness $Sigma_{rm crit}$ (B-band $mu_{rm B}^{rm crit}sim 19,{rm mag,arcsec^{-2}}$). This naturally explains why BH masses preferentially reflect bulge properties or central surface-densities ($Sigma_{1,{rm kpc}}$), not total galaxy properties.
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