ABSTRACT
We present a semi-analytical model of satellite galaxies, SatGen, which can generate large statistical samples of satellite populations for a host halo of desired mass, redshift, and ...assembly history. The model combines dark matter (DM) halo merger trees, empirical relations for the galaxy–halo connection, and analytical prescriptions for tidal effects, dynamical friction, and ram-pressure stripping. SatGen emulates cosmological zoom-in hydrosimulations in certain aspects. Satellites can reside in cored or cuspy DM subhaloes, depending on the halo response to baryonic physics that can be formulated from hydrosimulations and physical modelling. The subhalo profile and the stellar mass and size of a satellite evolve depending on its tidal mass-loss and initial structure. The host galaxy can include a baryonic disc and a stellar bulge, each described by a density profile that allows analytic satellite orbit integration. SatGen complements simulations by propagating the effect of halo response found in simulated field galaxies to satellites (not properly resolved in simulations) and outperforms simulations by sampling the halo-to-halo variance of satellite statistics and overcoming artificial disruption due to insufficient resolution. As a first application, we use the model to study satellites of Milky Way (MW)- and M31-sized hosts, making it emulate simulations of bursty star formation and of smooth star formation, respectively, and to experiment with a disc potential in the host halo. We find that our model reproduces the observed satellite statistics reasonably well. Different physical recipes make a difference in satellite abundance and spatial distribution at the 25 per cent level, not large enough to be distinguished by current observations given the halo-to-halo variance. The MW/M31 disc depletes satellites by ${\sim } 20{{\ \rm per\ cent}}$ and has a subtle effect of diversifying the internal structure of satellites, which is important for alleviating certain small-scale problems. We discuss the conditions for a massive satellite to survive in MW/M31.
We study the properties of giant clumps and their radial gradients in high-z disc galaxies using AMR cosmological simulations. Our sample consists of 770 snapshots in the redshift range z = 4-1 from ...29 galaxies that at z = 2 span the stellar mass range (0.2–3) × 1011 M⊙. Extended gas discs exist in 83 per cent of the snapshots. Clumps are identified by gas density in 3D and their stellar and dark matter components are considered thereafter. While most of the overdensities are diffuse and elongated, 91 per cent of their mass and 83 per cent of their star formation rate (SFR) are in compact round clumps. Nearly all galaxies have a central, massive bulge clump, while 70 per cent of the discs show off-centre clumps, 3–4 per galaxy. The fraction of clumpy discs peaks at intermediate disc masses. Clumps are divided based on dark matter content into in situ and ex situ originating from violent disc instability (VDI) and minor mergers, respectively. 60 per cent of the discs are in a VDI phase showing off-centre in situ clumps, which contribute 1–7 per cent of the disc mass and 5–45 per cent of its SFR. The in situ clumps constitute 75 per cent of the off-centre clumps in terms of number and SFR but only half the mass, each clump containing on average 1 per cent of the disc mass and 6 per cent of its SFR. They have young stellar ages, 100–400 Myr, and high specific SFR (sSFR), 1–10 Gyr−1. They exhibit gradients resulting from inward clump migration, where the inner clumps are somewhat more massive and older, with lower gas fraction and sSFR and higher metallicity. Similar observed gradients indicate that clumps survive outflows. The ex situ clumps have stellar ages 0.5–3 Gyr and sSFR ∼0.1–2 Gyr−1, and they exhibit weaker gradients. Massive clumps of old stars at large radii are likely ex situ mergers, though half of them share the disc rotation.
We study the evolution of the scaling relations that compare the effective density ( ) and core density ( kpc) to the stellar masses of star-forming galaxies (SFGs) and quiescent galaxies. These ...relations have been fully in place since and have exhibited almost constant slope and scatter since that time. For SFGs, the zero points in and decline by only . This fact plus the narrowness of the relations suggests that galaxies could evolve roughly along the scaling relations. Quiescent galaxies follow different scaling relations that are offset to higher densities at the same mass and redshift. Furthermore, the zero point of their core density has declined by only since , while the zero point of the effective density declines by . When galaxies quench, they move from the star-forming relations to the quiescent relations. This involves an increase in the core and effective densities, which suggests that SFGs could experience a phase of significant core growth relative to the average evolution along the structural relations. The distribution of massive galaxies relative to the SFR-M and the quiescent relations exhibits an L-shape that is independent of redshift. The knee of this relation consists of a subset of "compact" SFGs that are the most likely precursors of quiescent galaxies forming at later times. The compactness selection threshold in exhibits a small variation from z = 3 to 0.5, M kpc−2, allowing the most efficient identification of compact SFGs and quiescent galaxies at every redshift.
ABSTRACT
We provide prescriptions to evaluate the dynamical mass (Mdyn) of galaxies from kinematic measurements of stars or gas using analytic considerations and the VELA suite of cosmological ...zoom-in simulations at z = 1–5. We find that Jeans or hydrostatic equilibrium is approximately valid for galaxies of stellar masses above M⋆ ∼ 109.5 M⊙ out to 5 effective radii (Re). When both measurements of the rotation velocity vϕ and of the radial velocity dispersion σr are available, the dynamical mass $M_{\rm dyn} \!\simeq \! G^{-1} V_{\rm c}^2 r$, can be evaluated from the Jeans equation $V_{\rm c}^2= v_\phi ^2 + \alpha \sigma _{\rm r}^2$ assuming cylindrical symmetry and a constant, isotropic σr. For spheroids, α is inversely proportional to the Sérsic index n and α ≃ 2.5 within Re, stars for the simulated galaxies. The prediction for a self-gravitating exponential disc, α = 3.36(r/Re), is invalid in the simulations, where the dominant spheroid causes a weaker gradient from α ≃ 1 at Re, gas to 4 at 5Re, gas. The correction in α for the stars due to the gradient in σr(r) is roughly balanced by the effect of the aspherical potential, while the effect of anisotropy is negligible. When only the effective projected velocity dispersion σl is available, the dynamical mass can be evaluated as $M_{\rm dyn} = K G^{-1} R_{\rm e} \sigma _{\rm l}^2$, where the virial factor K is derived from α, given the inclination and vϕ/σr. We find that the standard value K = 5 is approximately valid only when averaged over inclinations and for compact and thick discs, as it ranges from 4.5 to above 10 between edge-on and face-on projections.
Studying giant star-forming clumps in distant galaxies is important to understand galaxy formation and evolution. At present, however, observers and theorists have not reached a consensus on whether ...the observed "clumps" in distant galaxies are the same phenomenon that is seen in simulations. In this paper, as a step to establish a benchmark of direct comparisons between observations and theories, we publish a sample of clumps constructed to represent the commonly observed "clumps" in the literature. This sample contains 3193 clumps detected from 1270 galaxies at 0.5 ≤ z < 3.0 . The clumps are detected from rest-frame UV images, as described in our previous paper. Their physical properties (e.g., rest-frame color, stellar mass ( M * ), star formation rate (SFR), age, and dust extinction) are measured by fitting the spectral energy distribution (SED) to synthetic stellar population models. We carefully test the procedures of measuring clump properties, especially the method of subtracting background fluxes from the diffuse component of galaxies. With our fiducial background subtraction, we find a radial clump U − V color variation, where clumps close to galactic centers are redder than those in outskirts. The slope of the color gradient (clump color as a function of their galactocentric distance scaled by the semimajor axis of galaxies) changes with redshift and M * of the host galaxies: at a fixed M * , the slope becomes steeper toward low redshift, and at a fixed redshift, it becomes slightly steeper with M * . Based on our SED fitting, this observed color gradient can be explained by a combination of a negative age gradient, a negative E(B − V) gradient, and a positive specific SFR gradient of the clumps. We also find that the color gradients of clumps are steeper than those of intra-clump regions. Correspondingly, the radial gradients of the derived physical properties of clumps are different from those of the diffuse component or intra-clump regions.
Disk galaxies at high redshift have been predicted to maintain high gas surface densities due to continuous feeding by intense cold streams leading to violent gravitational instability, transient ...features, and giant clumps. Gravitational torques between the perturbations drive angular momentum out and mass in, and the inflow provides the energy for keeping strong turbulence. We use analytic estimates of the inflow for a self-regulated unstable disk at a Toomre stability parameter Q ~ 1, and isolated galaxy simulations capable of resolving the nuclear inflow down to the central parsec. We predict an average inflow rate ~10 M yr--1 through the disk of a 1011 M galaxy, with conditions representative of z ~ 2 stream-fed disks. The inflow rate scales with disk mass and (1 + z)3/2. It includes clump migration and inflow of the smoother component, valid even if clumps disrupt. This inflow grows the bulge, while only a fraction of 10--3 of it needs to accrete onto a central black hole (BH), in order to obey the observed BH-bulge relation. A galaxy of 1011 M at z ~ 2 is expected to host a BH of ~108 M , accreting on average with moderate sub-Eddington luminosity L X ~ 1042-1043 erg s--1, accompanied by brighter episodes when dense clumps coalesce. We note that in rare massive galaxies at z ~ 6, the same process may feed ~109 M BH at the Eddington rate. High central gas column densities can severely obscure active galactic nuclei in high-redshift disks, possibly hindering their detection in deep X-ray surveys.
We use a semi-analytic model for disk galaxies to explore the origin of the time evolution and small scatter of the galaxy SFR sequence – the tight correlation between star formation rate (SFR) and ...stellar mass (Mstar). The steep decline of SFR from z∼ 2 to the present, at fixed Mstar, is a consequence of the following. First, disk galaxies are in a steady state with the SFR following the net (i.e. inflow minus outflow) gas accretion rate. The evolution of the SFR sequence is determined by evolution in the cosmological specific accretion rates, ∝ (1 +z)2.25, but is found to be independent of feedback. Although feedback determines the outflow rates, it shifts galaxies along the SFR sequence, leaving its zero-point invariant. Second, the conversion of accretion rate to SFR is materialized through gas density, not gas mass. Although the model SFR is an increasing function of both gas mass fraction and gas density, only the gas densities are predicted to evolve significantly with redshift. Third, star formation is fueled by molecular gas. Since the molecular gas fraction increases monotonically with increasing gas density, the model predicts strong evolution in the molecular gas fractions, increasing by an order of magnitude from z= 0 to z∼ 2. On the other hand, the model predicts that the effective surface density of atomic gas is , independent of redshift, stellar mass or feedback. Our model suggests that the scatter in the SFR sequence reflects variations in the gas accretion history, and thus is insensitive to stellar mass, redshift or feedback. The large scatter in halo spin contributes negligibly, because it scatters galaxies along the SFR sequence. An observational consequence of this is that the scatter in the SFR sequence is independent of the size (both stellar and gaseous) of galaxy disks.
We investigate the effects of radiation pressure from stars on the survival of the star-forming giant clumps in high-redshift massive disc galaxies, during the most active phase of galaxy formation. ...The clumps, typically of mass and radius , are formed in the turbulent gas-rich discs by violent gravitational instability and then migrate into a central bulge in ∼10 dynamical times. We show that the survival or disruption of these clumps under the influence of stellar feedback depends critically on the rate at which they form stars. If they convert a few per cent of their gas mass to stars per free-fall time, as observed for all local star-forming systems and implied by the Kennicutt–Schmidt law, they cannot be disrupted. Only if clumps convert most of their mass to stars in a few free-fall times can feedback produce significant gas expulsion. We consider whether such rapid star formation is likely in high-redshift giant clumps.
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