Monday 7/23/2012

Galaxy Pairs in the Sloan Digital Sky Survey – V. Tracing changes in star formation rate and metallicity out to separations of 80 kpc

Jillian M. Scudder, Sara L. Ellison, Paul Torrey, David R. Patton, J. Trevor Mendel
(Submitted on 19 Jul 2012)

We present a sample of 1899 galaxies with a close companion taken from the SDSS DR7. The galaxy pairs are selected to have velocity differences < 300 km/s, projected separations (rp) < 80 kpc/h, mass ratios between 0.1 and 10, and robust measurements of star formation rates and gas-phase metallicities. We match the galaxies in total stellar mass, redshift, and local density to a set of 10 control galaxies per pair galaxy. For each pair galaxy we can therefore calculate the statistical change in star formation rate (SFR) and metallicity associated with the interaction process. Relative to the control sample, we find that galaxies in pairs show typical SFR enhancements that are, on average, 60% higher than the control sample at rp < 30 kpc/h. In addition, the pairs demonstrate more modest SFR enhancements of ~30% out to at least 80 kpc/h (the widest separations in our sample). Galaxies in both major and minor mergers show significant SFR enhancements at all rp, although the strongest starbursts (with SFR enhancements of a factor of ~10) appear to be found only in the major mergers. For the first time, we are also able to trace the metallicity changes in galaxy pairs as a function of projected separation. The metallicity is generally diluted in galaxy pairs by ~0.02 dex, with an average metallicity decrement of -0.03 dex at the smallest separations. The SFR and metallicity trends with projected separation are interpreted through a comparison with theoretical models. These simulations indicate that the peak in SFR enhancements at small separations is due to systems near the end of the merger process. The extended plateau in SFR enhancements out to at least 80 kpc/h is dominated by galaxies that have made a pericentric passage and are now experiencing triggered star formation on their trajectory towards apogalacticon, or on a subsequent close approach. (Abridged)

Comments: Accepted for publication in MNRAS. 18 pages, 13 figures
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1207.4791v1 [astro-ph.CO]

Spitzer and Herschel-based SEDs of 24um-bright z~0.3-3.0 starbursts and obscured quasars

A.Sajina (Tufts University), L. Yan (IPAC/Caltech), D. Fadda (NASA Herschel Science Center/Caltech), K. Dasyra (Observatoire de Paris), M. Huynh (U. of Western Australia)
(Submitted on 20 Jul 2012)

In this paper, we characterize the infrared spectral energy distributions (SEDs) of mid-IR selected z~0.3-3.0 and L_IR~10^11-10^13Lsun galaxies, and study how their SEDs differ from those of local and high-z analogs. Our mid-IR flux-limited sample of 191 sources is unique in size, and spectral coverage, including Spitzer mid-IR spectroscopy. Here we add Herschel photometry at 250um, 350um, and 500um, which allows us to obtain accurate total IR luminosities, as well as constrain the relative contributions of AGN and starbursts to those luminosities. Our sample constitutes ~23% AGN (i.e. where the AGN contributes >50% of L_IR), ~30% starbursts (where AGN contributes <20% of L_IR and the mid-IR spectra are starburst-like); and ~47% composites (which show both significant AGN and starburst activity). The AGN-dominated sources divide into ones that show a strong silicate 9.7um absorption feature, implying highly obscured systems, and ones that do not. The high-tau_9.7 sources are half of our z>1.2 AGN, but show SEDs that are extremely rare among local AGN. The SEDs of our z~2 starburst-dominated ULIRGs are much closer to those of local LIRGs than ULIRGs. This is consistent with our earlier finding that, unlike local ULIRGs, our high-z starbursts are typically only in the early stages of a merger. The SEDs of the composite sources are most similar to the local archetypal warm ULIRG, Mrk231. In summary, our results show that there is strong evolution in the SEDs between local and z~2 IR-luminous galaxies, as well as that there is a wide range of SEDs among high redshift IR-luminous sources. The publicly-available SED templates we derive from our sample will be particularly useful for infrared population synthesis models, as well as in the interpretation of other mid-IR high-z galaxies in particular those detected by the recent all sky WISE survey.

Comments: 50 pages, 15 figures, ApJ in press
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1207.4963v1 [astro-ph.CO]
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