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Register a new userEffects of self-consistent rest-ultraviolet colours in semi-empirical galaxy formation models
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Connecting the observed rest-ultraviolet (UV) luminosities of high-$z$ galaxies to their intrinsic luminosities (and thus star formation rates) requires correcting for the presence of dust. We bypass a common dust-correction approach that uses empirical relationships between infrared (IR) emission and UV colours, and instead augment a semi-empirical model for galaxy formation with a simple -- but self-consistent -- dust model and use it to jointly fit high-$z$ rest-UV luminosity functions (LFs) and colour-magnitude relations ($M_{mathrm{UV}}$-$beta$). In doing so, we find that UV colours evolve with redshift (at fixed UV magnitude), as suggested by observations, even in cases without underlying evolution in dust production, destruction, absorption, or geometry. The observed evolution in our model arises due to the reduction in the mean stellar age and rise in specific star formation rates with increasing $z$. The UV extinction, $A_{mathrm{UV}}$, evolves similarly with redshift, though we find a systematically shallower relation between $A_{mathrm{UV}}$ and $M_{mathrm{UV}}$ than that predicted by IRX-$beta$ relationships derived from $z sim 3$ galaxy samples. Finally, assuming that high $1600 r{A}$ transmission ($gtrsim 0.6$) is a reliable LAE indicator, modest scatter in the effective dust surface density of galaxies can explain the evolution both in $M_{mathrm{UV}}$-$beta$ and LAE fractions. These predictions are readily testable by deep surveys with the James Webb Space Telescope.
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Observational systematics complicate comparisons with theoretical models limiting understanding of galaxy evolution. In particular, different empirical determinations of the stellar mass function imply distinct mappings between the galaxy and halo masses, leading to diverse galaxy evolutionary tracks. Using our state-of-the-art STatistical sEmi-Empirical modeL, STEEL, we show fully self-consistent models capable of generating galaxy growth histories that simultaneously and closely agree with the latest data on satellite richness and star-formation rates at multiple redshifts and environments. Central galaxy histories are generated using the central halo mass tracks from state-of-the-art statistical dark matter accretion histories coupled to abundance matching routines. We show that too flat high-mass slopes in the input stellar-mass-halo-mass relations as predicted by previous works, imply non-physical stellar mass growth histories weaker than those implied by satellite accretion alone. Our best-fit models reproduce the satellite distributions at the largest masses and highest redshifts probed, the latest data on star formation rates and its bi-modality in the local Universe, and the correct fraction of ellipticals. Our results are important to predict robust and self-consistent stellar-mass-halo-mass relations and to generate reliable galaxy mock catalogues for the next generations of extra-galactic surveys such as Euclid and LSST.
It is now possible for hydrodynamical simulations to reproduce a representative galaxy population. Accordingly, it is timely to assess critically some of the assumptions of traditional semi-analytic galaxy formation models. We use the Eagle simulations to assess assumptions built into the Galform semi-analytic model, focussing on those relating to baryon cycling, angular momentum and feedback. We show that the assumption in Galform that newly formed stars have the same specific angular momentum as the total disc leads to a significant overestimate of the total stellar specific angular momentum of disc galaxies. In Eagle, stars form preferentially out of low specific angular momentum gas in the interstellar medium (ISM) due to the assumed gas density threshold for stars to form, leading to more realistic galaxy sizes. We find that stellar mass assembly is similar between Galform and Eagle but that the evolution of gas properties is different, with various indications that the rate of baryon cycling in Eagle is slower than is assumed in Galform. Finally, by matching individual galaxies between Eagle and Galform, we find that an artificial dependence of AGN feedback and gas infall rates on halo mass doubling events in Galform drives most of the scatter in stellar mass between individual objects. Put together our results suggest that the Galform semi-analytic model can be significantly improved in light of recent advances.
We investigate the dynamical evolution of galaxies in groups with different formation epochs. Galaxy groups have been selected to be in different dynamical states, namely dynamically old and dynamically young, which reflect their early and late formation times, respectively, based on their halo mass assembly. Brightest galaxies in dynamically young groups have suffered their last major galaxy merger typically $sim 2$ Gyr more recently than their counterparts in dynamically old groups. Furthermore, we study the evolution of velocity dispersion in these two classes and compare them with the analytic models of isolated halos. The velocity dispersion of dwarf galaxies in high mass, dynamically young groups increases slowly in time, while the analogous dispersion in dynamically old high-mass groups is constant. In contrast, the velocity dispersion of giant galaxies in low mass groups decreases rapidly at late times. This increasing velocity bias is caused by dynamical friction, and starts much earlier in the dynamically old groups. The recent {sc Radio-SAGE} model of galaxy formation suggests that radio luminosities of central galaxies, considered to be tracers of AGN activity, are enhanced in halos that assembled more recently, independent of the time since the last major merger.
We present the rest-frame UV wavelength dependence of the Petrosian-like half-light radius ($r_{50}$), and the concentration parameter for a sample of 198 star-forming galaxies at 0.5 < z < 1.5. We find a ~5% decrease in $r_{50}$ from 1500 AA to 3000 AA, with half-light radii at 3000 AA ranging from 0.6 kpc to 6 kpc. We also find a decrease in concentration of ~0.07 (1.9 < $C_{3000}$ < 3.9). The lack of a strong relationship between $r_{50}$ and wavelength is consistent with a model in which clumpy star formation is distributed over length scales comparable to the galaxys rest-frame optical light. While the wavelength dependence of $r_{50}$ is independent of size at all redshifts, concentration decreases more sharply in the far-UV (~1500 AA) for large galaxies at z ~ 1. This decrease in concentration is caused by a flattening of the inner ~20% of the light profile in disk-like galaxies, indicating that the central regions have different UV colors than the rest of the galaxy. We interpret this as a bulge component with older stellar populations and/or more dust. The size-dependent decrease in concentration is less dramatic at z ~ 2, suggesting that bulges are less dusty, younger, and/or less massive than the rest of the galaxy at higher redshifts.
We used the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) to observe the semi-forbidden CIII] emission in Green Pea galaxies at 0.13 < z < 0.3. We detect CIII] emission in 7/10 galaxies with CIII] equivalent widths that range from 2-10AA~. The observed CIII] emission line strengths are consistent with the predictions from photoionization models which incorporate the effects of binary stellar evolution with young stellar ages < 3-5 Myrs, and high ionization parameters (logU > -2). The hard ionizing radiation from young massive stars, and high nebular temperatures at low-metallicities can account for the observed high equivalent widths of CIII] and [OIII] emission lines. The Green Pea galaxies do not show a significant correlation between the Ly$alpha$ and CIII] equivalent widths, and the observed scatter is likely due to the variations in the optical depth of Ly$alpha$ to the neutral gas. Green Pea galaxies are likely to be density-bounded, and we examined the dependence of CIII] emission on the Lyman continuum optical depth. The potential LyC leaker galaxies in our sample have high CIII] equivalent widths that can only be reproduced by starburst ages as young as < 3 Myrs and harder ionizing spectra than the non-leakers. Among the galaxies with similar metallicities and ionization parameters, the CIII] equivalent width appears to be stronger for those with higher optical depth to LyC, as expected from the photoionization models. Further investigation of a larger sample of CIII]-emitters is necessary to calibrate the dependence of CIII] emission on the escape of LyC radiation, and to enable application of the CIII] diagnostics to galaxies in the reionization epoch.
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