Thursday September 20th, 2012

Star-formation laws in extreme starbursts

S. Garcia-Burillo (1), A. Usero (1), A. Alonso-Herrero (2) ((1) OAN-Spain, (2) IFCA-UC-Spain)
(Submitted on 19 Sep 2012)

The observational study of star-formation laws is paramount to disentangling the physical processes at work on local and global scales in galaxies. To this aim we have expanded the sample of extreme starbursts, represented by local LIRGs and ULIRGs, with high-quality data obtained in the 1-0 line of HCN. The analysis of the new data shows that the star-formation efficiency of the dense molecular gas, derived from the FIR/HCN luminosity ratio, is a factor 3-4 higher in extreme starbursts compared to normal galaxies. We find a duality in the Kennicutt-Schmidt laws that is enhanced if we account for the different conversion factor for HCN (alpha_HCN) in extreme starbursts and correct for the unobscured star-formation rate in normal galaxies. We find that it is possible to fit the observed differences in the FIR/HCN ratios between normal galaxies and LIRGs/ULIRGs with a common constant star-formation rate per free-fall time (SFR_ff) if we assume that HCN densities are ~1-2 orders of magnitude higher in LIRGs/ULIRGs, and provided that SFR_ ff~0.005-0.01 and/or if alpha_HCN is a factor of a few lower than our favored values.

Comments: 4 pages, 3 figures, Proceedings IAU symposium No. 292: Molecular gas, dust and star formation in galaxies, ed. by Tony Wong and Juergen Ott
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1209.4305 [astro-ph.CO]
(or arXiv:1209.4305v1 [astro-ph.CO] for this version)

Quenching star formation at intermediate redshifts: downsizing of the mass flux density in the green valley

Thiago S. Gonçalves, D. Christopher Martin, Karín Menéndez-Delmestre, Ted Wyder, Anton Koekemoer
(Submitted on 18 Sep 2012)

The bimodality in galaxy properties has been observed at low and high redshift, with a clear distinction between star-forming galaxies in the blue cloud and passively evolving objects in the red sequence; the absence of galaxies with intermediate properties indicates that the quenching of star formation and subsequent transition between populations must happen rapidly. In this paper, we present a study of over 100 transiting galaxies in the so-called "green valley" at intermediate redshifts (z ~ 0.8). By using very deep spectroscopy with the DEIMOS instrument at the Keck telescope we are able to infer the star formation histories of these objects and measure the stellar mass flux density transiting from the blue cloud to the red sequence when the universe was half its current age. Our results indicate that the process happened more rapidly and for more massive galaxies in the past, suggesting a top-down scenario in which the massive end of the red sequence is forming first. This represent another aspect of downsizing, with the mass flux density moving towards smaller galaxies in recent times.

Comments: 13 pages; accepted for publication in ApJ
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1209.4084 [astro-ph.CO]
(or arXiv:1209.4084v1 [astro-ph.CO] for this version)
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