Monday, 1/7/2013

Molecular theme today, one on the CO(1-0)/CO(3-2) ratio of z~0.3 galaxies (used as proxy of gas excitation) and a second on doing spatially-resolved Kennicutt-Schmidt law work at intermed redshift (z ~ 1.3 here), using the PV diagram of the CO and OII emission. So separation is based on velocity, not just actual spatial resolution, kinda cool. But should OII be used as a star formation rate indicator?

Title: The EGNoG Survey: Gas Excitation in Normal Galaxies at z~0.3
Authors: Bauermeister, Amber; Blitz, Leo; Bolatto, Alberto D.; Bureau, Martin; Teuben, Peter J.; Wong, Tony; Wright, Melvyn C. H.
Publication: eprint arXiv:1301.0631
Publication Date: 01/2013
Origin: ARXIV
Keywords: Astrophysics – Cosmology and Extragalactic Astrophysics
Comment: Accepted for publication in the Astrophysical Journal, to appear January 2013; 18 pages, 10 figures, 6 tables
Bibliographic Code: 2013arXiv1301.0631B

Abstract

As observations of molecular gas in galaxies are pushed to lower star formation rate galaxies at higher redshifts, it is becoming increasingly important to understand the conditions of the gas in these systems to properly infer their molecular gas content. The rotational transitions of the carbon monoxide (CO) molecule provide an excellent probe of the gas excitation conditions in these galaxies. In this paper we present the results from the gas excitation sample of the Evolution of molecular Gas in Normal Galaxies (EGNoG) survey at the Combined Array for Research in Millimeter-wave Astronomy (CARMA). This subset of the full EGNoG sample consists of four galaxies at z~0.3 with star formation rates of 40-65 M_Sun yr^-1 and stellar masses of ~2×10^11 M_Sun. Using the 3 mm and 1 mm bands at CARMA, we observe both the CO(1-0) and CO(3-2) transitions in these four galaxies in order to probe the excitation of the molecular gas. We report robust detections of both lines in three galaxies (and an upper limit on the fourth), with an average line ratio, r_31 = L’_CO(3-2) / L’_CO(1-0), of 0.46 \pm 0.07 (with systematic errors \lesssim 40%), which implies sub-thermal excitation of the CO(3-2) line. We conclude that the excitation of the gas in these massive, highly star-forming galaxies is consistent with normal star-forming galaxies such as local spirals, not starbursting systems like local ultra-luminous infrared galaxies. Since the EGNoG gas excitation sample galaxies are selected from the main sequence of star-forming galaxies, we suggest that this result is applicable to studies of main sequence galaxies at intermediate and high redshifts, supporting the assumptions made in studies that find molecular gas fractions in star forming galaxies at z~1-2 to be an order of magnitude larger than what is observed locally.

Title: Towards a resolved Kennicutt-Schmidt law at high redshift
Authors: Freundlich, J.; Combes, F.; Tacconi, L. J.; Cooper, M. C.; Genzel, R.; Neri, R.; Bolatto, A.; Bournaud, F.; Burkert, A.; Cox, P.; Davis, M.;Förster Schreiber, N. M.; Garcia-Burillo, S.; Gracia-Carpio, J.; Lutz, D.; Naab, T.; Newman, S.; Sternberg, A.; Weiner, B.
Publication: eprint arXiv:1301.0628
Publication Date: 01/2013
Origin: ARXIV
Keywords: Astrophysics – Cosmology and Extragalactic Astrophysics
Comment: 6 pages, 4 figures, 2 tables
Bibliographic Code: 2013arXiv1301.0628F

Abstract

Massive galaxies in the distant Universe form stars at much higher rates than today. Although direct resolution of the star forming regions of these galaxies is still a challenge, recent molecular gas observations at the IRAM Plateau de Bure interferometer enable us to study the star formation efficiency at sub-galactic scales around redshift z = 1.2. We present a method to obtain the gas and star formation rate (SFR) surface densities of ensembles of clumps composing galaxies at this redshift, even though the corresponding scales are not resolved. This method is based on the identification of these structures in position-velocity diagrams corresponding to slices within the galaxies. We use unique IRAM observations of the CO(3-2) rotational line and DEEP2 spectra of four massive star forming distant galaxies – EGS13003805, EGS13004291, EGS12007881 and EGS13019128 in the AEGIS terminology – to determine the gas and SFR surface densities of the identifiable ensembles of clumps that constitute them. The integrated CO line luminosity is assumed to be directly proportional to the total gas mass, and the SFR is deduced from the [OII] recombination line. We identify the ensembles of clumps with the angular resolution available in both CO and [OII] spectroscopy, i.e. 1-1.5". SFR and gas surface densities are averaged in areas of this size, which is also the thickness of the DEEP2 slits and of the extracted IRAM slices, and we derive a spatially resolved Kennicutt-Schmidt (KS) relation at a scale of ~ 8 kpc. The data points globally follow a power law of exponent N=1 corresponding to a depletion time of 1.0 Gyr, but with a large scatter, which means that the depletion time varies from point to point within the galaxies. We find a depletion time of 1.4 +/- 0.9 Gyr.

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