The relationship between galaxy star formation rates (SFR) and stellar masses (M * ) is re-examined using a mass-selected sample of ∼62,000 star-forming galaxies at z ≤ 1.3 in the COSMOS 2-deg 2 field. Using new far-infrared photometry from Herschel-PACS and SPIRE and Spitzer-MIPS 24 µm, along with derived infrared luminosities from the NRK method based on galaxies' locations in the restframe color-color diagram (N U V − r) vs. (r − K), we are able to more accurately determine total SFRs for our complete sample. At all redshifts, the relationship between median SF R and M * follows a powerlaw at low stellar masses, and flattens to nearly constant SFR at high stellar masses. We describe a new parameterization that provides the best fit to the main sequence and characterizes the low mass power-law slope, turnover mass, and overall scaling. The turnover in the main sequence occurs at a characteristic mass of about M 0 ∼ 10 10 M at all redshifts. The low mass power-law slope ranges from 0.9-1.3 and the overall scaling rises in SFR as a function of (1 + z) 4.12±0.10 . A broken power-law fit below and above the turnover mass gives relationships of SF R ∝ M 0.88±0.06 * below the turnover mass and SF R ∝ M 0.27±0.04 * above the turnover mass. Galaxies more massive than M * 10 10 M have on average, a much lower specific star formation rate (sSFR) than would be expected by simply extrapolating the traditional linear fit to the main sequence found for less massive galaxies.

We present the mass-metallicity (MZ) and luminosity-metallicity (LZ) relations at z ∼ 0.8 from ∼ 1350 galaxies in the Deep Extragalactic Evolutionary Probe 2 (DEEP2) survey. We determine stellar masses by fitting the spectral energy distribution inferred from photometry with current stellar population synthesis models. This work raises the number of galaxies with metallicities at z∼ 0.8 by more than an order of magnitude. We investigate the evolution in the MZ and LZ relations in comparison with local MZ and LZ relations determined in a consistent manner using ∼ 21, 000 galaxies in the Sloan Digital Sky Survey. We show that high stellar mass galaxies (M ∼ 10 10.6 M ⊙ ) at z∼ 0.8 have attained the chemical enrichment seen in the local universe, while lower stellar mass galaxies (M ∼ 10 9.2 M ⊙ ) at z∼ 0.8 have lower metallicities (∆log(O/H) ∼ 0.15 dex) than galaxies at the same stellar mass in the local universe. We find that the LZ relation evolves in both metallicity and B-band luminosity between z∼ 0.8 and z ∼ 0, with the B-band luminosity evolving as a function of stellar mass. We emphasize that the B-band luminosity should not be used as a proxy for stellar mass in chemical evolution studies of star-forming galaxies. Our study shows that both the metallicity evolution and the B-band luminosity evolution for emission-line galaxies between the epochs are a function of stellar mass, consistent with the cosmic downsizing scenario of galaxy evolution. Subject headings:

We present metallicity gradients in 49 local field star-forming galaxies. We derive gasphase oxygen abundances using two widely adopted metallicity calibrations based on the [O III]/Hβ, [N II]/Hα and [N II]/[O II] line ratios. The two derived metallicity gradients are usually in good agreement within ±0.14 dex R −1 25 (R 25 is the B-band iso-photoal radius), but the metallicity gradients can differ significantly when the ionisation parameters change systematically with radius. We investigate the metallicity gradients as a function of stellar mass (8 < log(M * /M ⊙ ) < 11) and absolute B-band luminosity (−16 > M B > −22). When the metallicity gradients are expressed in dex kpc −1 , we show that galaxies with lower mass and luminosity, on average, have steeper metallicity gradients. When the metallicity gradients are expressed in dex R −1 25 , we find no correlation between the metallicity gradients, and stellar mass and luminosity. We provide a local benchmark metallicity gradient of field star-forming galaxies useful for comparison with studies at high redshifts. We investigate the origin of the local benchmark gradient using simple chemical evolution models and observed gas and stellar surface density profiles in nearby field spiral galaxies. Our models suggest that the local benchmark gradient is a direct result of the coevolution of gas and stellar disk under virtually closed-box chemical evolution when the stellar-to-gas mass ratio becomes high (≫ 0.3). These models imply low current mass accretion rates ( 0.3 × SFR), and low mass outflow rates ( 3 × SFR) in local field star-forming galaxies. Key words:2 The effective radius is the radius at which the integrated flux is half of the total one. Comparing to the disk scale-length for the classical exponential profile, Re = 1.67835R d .

We present the first systematic investigation into the metallicity gradients in galaxy close pairs. We determine the metallicity gradients for 8 galaxies in close pairs using H II region metallicities obtained with high signal-to-noise multi-slit observations with the Keck LRIS Spectrograph. We show that the metallicity gradients in close pairs are significantly shallower than gradients in isolated spiral galaxies such as the Milky Way, M83, and M101. These observations provide the first solid evidence that metallicity gradients in interacting galaxies are systematically different from metallicity gradients in isolated spiral galaxies. Our results suggest that there is a strong relationship between metallicity gradients and the gas dynamics in galaxy interactions and mergers.