Many investigations of star formation rates (SFRs) in galaxies have explored details of dust obscuration, with a number of recent analyses suggesting that obscuration appears to increase in systems with high rates of star formation. To date these analyses have been primarily based on nearby (z < 0.03) or UV selected samples. Using 1.4 GHz imaging and optical spectroscopic data from the Phoenix Deep Survey, the SFR-dependent obscuration is explored. The use of a radio selected sample shows that previous studies exploring SFR-dependent obscurations have been biased against obscured galaxies. The observed relation between obscuration and SFR is found to be unsuitable to be used as an obscuration measure for individual galaxies. Nevertheless, it is shown to be successful as a first order correction for large samples of galaxies where no other measure of obscuration is available, out to intermediate redshifts (z ~ 0.8).Comment: 9 pages (including 5 encapsulated postscript figures), aastex, uses emulateapj5.sty. Accepted for publication in Ap

We present predictions for the structural and photometric properties of early‐type galaxies in the Lambda cold dark matter (ΛCDM) cosmology from the published semi‐analytical galaxy formation models of Baugh et al. and Bower et al. These calculations were made with the galform code, which tracks the evolution of the disc and bulge components of a galaxy, using a self‐consistent model to compute the scalelengths. The sizes of galactic discs are determined by the conservation of the angular momentum of cooling gas. The sizes of merger remnants are computed by applying the virial theorem and conserving the binding energy of the progenitors and their orbital energy. There are a number of important differences between the two galaxy formation models. To suppress the overproduction of bright galaxies, the Bower et al. model employs active galactic nuclei heating to stifle gas cooling, whereas the Baugh et al. model invokes a superwind which ejects cooled gas. Also, in the Baugh et al. model a top‐heavy stellar initial mass function is adopted in starbursts. We compare the model predictions with observational results derived from the Sloan Digital Sky Survey. The model enjoys a number of notable successes, such as giving reasonable reproductions of the local Faber–Jackson relation (velocity dispersion–luminosity), the velocity dispersion–age relation, and the Fundamental Plane relating the luminosity, velocity dispersion and effective radius of spheroids. These achievements are all the more remarkable when one bears in mind that none of the parameters has been adjusted to refine the model predictions. We study how the residuals around the Fundamental Plane relation depend on galaxy properties. We examine in detail the physical ingredients of the calculation of galaxy sizes in galform, showing which components have the most influence over our results. We also study the evolution of the scaling relations with redshift. However, in spite of the successes, there are some important disagreements between the predictions of the model and observations: the brightest model spheroids have effective radii smaller than observed and the zero‐point of the Fundamental Plane shows little or no evolution with redshift in the model.

The Phoenix Deep Survey is a multi-wavelength survey based on deep 1.4 GHz radio imaging, reaching well into the sub-100 µJy level. One of the aims of this survey is to characterize the sub-mJy radio population, exploring its nature and evolution. In this paper we present the catalog and results of the spectroscopic observations aimed at characterizing the optically "bright" (R 21.5 mag) counterparts of faint radio sources. Out of 371 sources with redshift determination, 21% have absorption lines only, 11% show AGN signatures, 32% are star-forming galaxies, 34% show narrow emission lines that do not allow detailed spectral classification (due to poor signal-to-noise ratio and/or lack of diagnostic emission lines) and the remaining 2% are identified with stars. For the star-forming galaxies with a Balmer decrement measurement we find a median extinction of A Hα = 1.9 mag, higher than that of optically selected samples. This is a result of the radio selection, which is not biased against dusty systems. Using the available spectroscopic information, we estimate the radio luminosity function of starforming galaxies in two independent redshift bins at z ≈ 0.1 and 0.3 respectively. We find direct evidence for strong luminosity evolution of these systems consistent with L 1.4 GHz ∝ (1 + z) 2.7 .

We present predictions for the abundance and nature of extremely red objects (EROs) in the Λ cold dark matter model. EROs are red, massive galaxies observed at z≥ 1 and their numbers and properties pose a challenge to hierarchical galaxy formation models. We compare the predictions from two published models, one of which invokes a ‘superwind’ to regulate star formation in massive haloes and the other which suppresses gas cooling in haloes through ‘radio‐mode’ active galactic nucleus (AGN) feedback. The superwind model underestimates the number counts of EROs by an order of magnitude, whereas the radio‐mode AGN feedback model gives excellent agreement with the number counts and redshift distribution of EROs. In the AGN feedback model the ERO population is dominated by old, passively evolving galaxies, whereas observations favour an equal split between old galaxies and dusty starbursts. Also, the model predicts a more extended redshift distribution of passive galaxies than is observed. These comparisons suggest that star formation may be quenched too efficiently in this model.