The star formation history of the Hubble sequence: spatially resolved colour distributions of intermediate-redshift galaxies in the Hubble Deep Field

Abstract

We analyse the spatially resolved colours of distant galaxies of known redshift in the Hubble Deep Field, using a new technique based on matching resolved four‐band colour data to the predictions of evolutionary synthesis models. Given some simplifying assumptions, we demonstrate how our technique is capable of probing the evolutionary history of high‐redshift systems, noting the specific advantage of observing galaxies at an epoch closer to the time of their formation. We quantify the relative age, dispersion in age, on‐going star formation rate and star formation history of distinct components. We explicitly test for the presence of dust and quantify its effect on our conclusions. To demonstrate the potential of the method, we study the spirals and ellipticals in the near‐complete sample of 32 I814<21.9 mag galaxies with z∼0.5 studied by Bouwens, Broadhurst & Silk. The dispersion of the internal colours of a sample of 0.4<z<1 early‐type field galaxies in the HDF indicates that ∼ 40 per cent (4/11) show evidence of star formation which must have occurred within the past third of their ages at the epoch of observation. This result contrasts with that derived for HST‐selected ellipticals in distant rich clusters, and is largely independent of assumptions with regard to metallicity. For a sample of well‐defined spirals, we similarly exploit the dispersion in colour to analyse the relative histories of bulge and disc stars, in order to resolve the current controversy regarding the ages of galactic bulges. Dust and metallicity gradients are ruled out as major contributors to the colour dispersions that we observe in these systems. The median ages of bulge stars are found to be significantly higher than those in galactic discs, and they exhibit markedly different star formation histories. This result is inconsistent with a secular growth of bulges from disc instabilities, but is consistent with gradual disc formation by accretion of gas on to bulges, as predicted by hierarchical theories. We extend our technique in order to discuss the star formation history of the entire Bouwens et al. sample in the context of earlier studies concerned with global star formation histories. Finally, we consider how to extend our method using near‐infrared data and to deeper samples.

The Origin of Blue Cores in Hubble Deep Field E/S0 Galaxies

“…A more conclusive test in this line will need a complete census of elliptical galaxies up to ; recent results from Im z p 1 et al (2000) are inconclusive but compatible with very mild density evolution. Similarly, using data set multicolor information, a population of E/S0 galaxies with colors significantly bluer than passive evolution has been reported (Kodama, Bower, & Bell 1999;Abraham et al 1999). In a recent detailed analysis, Menanteau, Abraham, & Ellis (2001), using resolved multicolor photometry of E/S0 galaxies in both Hubble Deep Field-North (HDF-N) and Hubble Deep Field-South (HDF-S), reported solid evidence for the continuous star formation in E/S0 galaxies.…”

Structure of Disk‐dominated Galaxies. II. Color Gradients and Stellar Population Models

“…However, based on the lack of r 1/4 shaped bulges found in a later study of bulge SB profiles of S0-Sbc galaxies using HST near-IR high resolution imaging, Balcells et al (2003) have revised their interpretation and conclude that these bulges could not have formed from violent relaxation in mergers, but rather are more consistent with secular formation processes. On the other hand, in an HST study of spatially resolved colors of high redshift (z $ 0:5) galaxies in the Hubble Deep Field, Abraham et al (1999) rule out metallicity gradients as major contributors to galaxy color distributions. They do find, in agreement with Peletier et al (1999), that large bulges are significantly older than their disks and therefore rule out secular evolution formation processes in favor of a gradual disk formation by accretion of gas.…”

Photometry and spectroscopy of GRB 060526: a detailed study of the afterglow and host galaxy of az = 3.2 gamma-ray burst

“…Cowie et al 1996) while massive galaxies prove to be rather unchanged throughout the history of the universe (e.g. Abraham et al 1999;Heavens et al 2004). One would therefore expect that GRB hosts should also have higher luminosities towards higher redshifts.…”