The estimated stellar masses of galaxies are widely used to characterize how the galaxy population evolves over cosmic time. If stellar masses can be estimated in a robust manner, free from any bias, global diagnostics such as the stellar mass functi
on can be used to constrain the physics of galaxy formation. We explore how galaxy stellar masses, estimated by fitting broad-band spectral energy distributions (SEDs) with stellar population models, can be biased as a result of commonly adopted assumptions for the star-formation and chemical enrichment histories, recycled fractions and dust attenuation curves of galaxies. We apply the observational technique of broad-band SED fitting to model galaxy SEDs calculated by the theoretical galaxy formation model GALFORM, isolating the effect of each of these assumptions. We find that, averaged over the entire galaxy population, the common assumption of exponentially declining star-formation histories does not adversely affect stellar mass estimation. We show that fixing the metallicity in SED fitting or using sparsely sampled metallicity grids can introduce mass dependent systematics into stellar mass estimates. We find that the common assumption of a star-dust geometry corresponding to a uniform foreground dust screen can cause the stellar masses of dusty model galaxies to be significantly underestimated. Finally, we show that stellar mass functions recovered by applying SED fitting to model galaxies at high redshift can differ significantly in both shape and normalization from the intrinsic mass functions predicted by a given model. Given these differences, our methodology of using stellar masses estimated from model galaxy SEDs offers a new, self-consistent way to compare model predictions with observations.
Using GALFORM, a semi-analytical model of galaxy formation in the Lambda cold dark matter cosmology, we study the rest-frame ultraviolet (UV) colours of Lyman-break galaxies (LBGs) in the redshift range 2.5 < z < 10. As the impact of dust on UV lumin
osity can be dramatic, our model includes a self-consistent computation of dust attenuation based on a radiative transfer model. We find that intrinsically brighter galaxies suffer stronger dust attenuation than fainter ones, though the relation has a large scatter. The model predicts galaxies with UV colours consistent with the colour selection regions designed to select LBGs in observational surveys. We find that the drop-out technique that selects LBGs based on two rest-frame UV colours is robust and effective, selecting more than 70 per cent of UV bright galaxies at a given redshift. We investigate the impact on the predicted UV colours of varying selected model parameters. We find that the UV colours are most sensitive to the modelling of dust attenuation and in particular, to the extinction curve used in the radiative transfer calculation. If we assume a Milky Way dust extinction curve, the predicted UV continuum slopes are, in general, bluer than observed. However, we find that the opposite is true when using the Small Magellanic Cloud dust extinction curve. This demonstrates the strong dependence of UV colours on dust properties and highlights the inadequacy of using the UV continuum slope as a tracer of dust attenuation without any further knowledge of the galaxy inclination or dust characteristics in high redshift galaxies.
We predict the formation histories, properties and descendants of Lyman-break galaxies (LBGs) in the Lambda-CDM cosmology. In our model, which incorporates a top-heavy IMF in starbursts, we find that most LBGs are starbursts triggered by minor merger
s of galaxies. The duration of the LBG phase is predicted to be quite short, ~20-60 Myr. We investigate the distributions of stellar and halo masses and morphologies for bright (L_UV > L*_UV) and faint (L_UV > 0.1 L*_UV) LBGs at z=3, z=6 and z=10 (where we classify LBGs according to their rest-frame UV luminosities relative the observed characteristic luminosity L*_UV at z approx 3). Bright LBGs at z=3 are predicted to have median stellar masses ~ 1x10^9 Msun/h and host halo masses ~ 3x10^{11} Msun/h, and to be typically mildly disk-dominated in stellar mass. On the other hand, faint LBGs at z=10 are predicted to have median stellar masses of only ~ 1x10^7 Msun/h and host halo masses 2x10^{10} Msun/h, and to be generally bulge-dominated. Bright LBGs at z=3 evolve to present-day galaxies with median stellar mass ~ 5x^{10} Msun/h (comparable to the Milky Way), consisting of roughly equal numbers of disk- and bulge-dominated systems, and hosted by halos with median mass ~2x10^{13} Msun/h (corresponding to medium-size galaxy groups). The model predicts that 40% of Milky Way mass galaxies at the present-day have a bright LBG progenitor in the redshift range 3<z<4, while 95% have a faint LBG progenitor in the same redshift range, and 7% have a faint LBG progenitor at 10<z<11. With our multiwavelength model, we also investigate the overlap between the LBG population and that of submillimetre selected galaxies (SMGs); at z=3, only ~1% of bright LBGs are also predicted to also be bright SMGs (with an 850 mum flux in excess of 5 mJy).
Future galaxy surveys will map the galaxy distribution in the redshift interval $0.5<z<2$ using near-infrared cameras and spectrographs. The primary science goal of such surveys is to constrain the nature of the dark energy by measuring the large-sca
le structure of the Universe. This requires a tracer of the underlying dark matter which maximizes the useful volume of the survey. We investigate two potential survey selection methods: an emission line sample based on the ha line and a sample selected in the H-band. We present predictions for the abundance and clustering of such galaxies, using two publish
We use a model for the evolution of galaxies in the far-IR based on the LambdaCDM cosmology to make detailed predictions for upcoming cosmological surveys with the Herschel Space Observatory. We use the combined GALFORM semi-analytical galaxy formati
on model and GRASIL spectrophotometric code to compute galaxy SEDs including the reprocessing of radiation by dust. The model, which is the same as that in Baugh et al. (2005), assumes two different IMFs: a normal solar neighbourhood IMF for quiescent star formation in disks, and a very top-heavy IMF in starbursts triggered by galaxy mergers. We have shown previously that the top-heavy IMF appears necessary to explain the number counts and redshifts of faint sub-mm galaxies. In this paper, we present predictions for galaxy luminosity functions, number counts and redshift distributions in the Herschel imaging bands. We find that source confusion will be a serious problem in the deepest planned surveys. We also show predictions for physical properties such as star formation rates and stellar, gas and halo masses, together with fluxes at other wavelengths (from the far-UV to the radio) relevant for multi-wavelength follow-up observations. We investigate what fraction of the total IR emission from dust and of the high-mass star formation over the history of the Universe should be resolved by planned surveys with Herschel, and find a fraction ~30-50%, depending on confusion. Finally, we show that galaxies in Herschel surveys should be significantly clustered.
