Radial distribution of near-UV flux in disc galaxies in the range 0 ⪉ z ⪉ 1

Abstract

Context: In the last 8 Gyr of cosmic time the global star formation rate (SFR) has declined by roughly an order of magnitude, and most of this activity has been progressively confined to ever less massive galaxies (“downsizing”). Previous studies show that the main drivers of the decline in the global SFR are the spiral galaxies. However, we still lack a detailed understanding of how the star formation (SF) has changed as a function of position within the discs. Such studies will help to build a more accurate picture of how stellar discs are formed and grow.
Aims: The goal of this paper is to quantify the changes in the SF distribution within the disc galaxies in the last 8 Gyr. As a proxy for the SF radial profile, we use the near-UV surface brightness distributions, suitably allowing for extinction.
Methods: We compare the effective radii (R_eff) and concentration of the flux distribution in the rest-frame near-UV (NUV-GALEX band) for a sample of 270 galaxies in the range 0<z<1. This radial distribution is compared to that measured in the rest-frame B-band, which traces older stellar populations. The analysis is performed using deep, high-resolution, multi-band images from GALEX, SDSS, and HST/ACS-GOODS-South.
Results: The relation R_eff(NUV)-M_star suffers a moderate change between z∼1 and z∼0, at a fixed stellar mass of 10¹⁰ M_⊙ galaxies increase their effective radii by a factor 1.18 ± 0.06. The ratio R_eff(NUV)/R_eff(B) has increased by 10% in the same period. Median profiles in NUV show signs of truncation at R R_eff, and median colour profiles (NUV-B) show a minimum (a “bluest” point) also around R 1-1.5R_eff. The surface brightness of the discs (at R_eff) has decreased by 80% in NUV, and by 60% in B since z∼1. The distributions of NUV flux are more compact at z∼1 than nowadays, in terms of the fraction of flux enclosed in a specific radius (in kpc). The central brightness in B and NUV has increased with respect to the brightness level of the discs, both in B and NUV, perhaps indicating a continuous accumulation of the bulge structure since z∼1, or that they have been left exposed because of the significant decline in the star-forming activity in the discs.
Conclusions: Our results indicate that the SF surface density has decreased dramatically in discs since z∼1, and this decline has been more intense in the central parts (⪉R_eff) of the galaxies. In addition, our data suggest that the bulges/pseudo-bulges have grown in surface brightness with regard to the discs since z∼1.