GOODS- HERSCHEL : GAS-TO-DUST MASS RATIOS AND CO-TO-H ₂ CONVERSION FACTORS IN NORMAL AND STARBURSTING GALAXIES AT HIGH- z

Abstract

We explore the gas-to-dust mass ratio (M gas /M d ) and the CO luminosityto-M gas conversion factor (α CO ) of two well studied galaxies in the GOODS-N field, that are expected to have different star forming modes, the starburst GN20 at z = 4.05 and the normal star-forming galaxy BzK-21000 at z = 1.52. Detailed sampling is available for their Rayleigh-Jeans emission via ground based mm interferometry (1.1 − 6.6 mm) along with Herschel PACS and SPIRE data that probe the peak of their infrared emission. Using the physically motivated Draine & Li (2007) models, as well as a modified black body function, we measure the dust mass (M dust ) of the sources and find (2.0 +0.7 −0.6 × 10 9 ) M ⊙ for GN20 and (8.6 +0.6 −0.9 × 10 8 ) M ⊙ for BzK-21000. The addition of mm data reduces the uncertainties of the derived M dust by a factor of ∼ 2, allowing the use of the local M gas /M d vs metallicity relation to place constraints on the α CO values of the -2two sources. For GN20 we derive a conversion factor of α CO < 1.0 M ⊙ pc −2 (K km s −1 ) −1 , consistent with that of local ULIRGs, while for BzK-21000 we find a considerably higher value, α CO ∼ 4.0 M ⊙ pc −2 (K km s −1 ) −1 , in agreement with an independent kinematic derivation reported previously. The implied star formation efficiency is ∼ 25 L ⊙ /M ⊙ for BzK-21000, a factor of ∼ 5 − 10 lower than that of GN20. The findings for these two sources support the existence of different disk-like and starburst star-formation modes in distant galaxies, although a larger sample is required to draw statistically robust results.

CO excitation of normal star-forming galaxies out toz= 1.5 as regulated by the properties of their interstellar medium

“…First, their gas fractions appear to be rising substantially at least from z = 0 to 2, which begins from about 5-10% at z = 0 to 40-50% at z = 2 for stellar masses of about a few 10 10 M . This result has been obtained both from investigating CO emission lines mainly from the IRAM Plateau de Bure interferometer (PdBI, Daddi et al 2008Daddi et al , 2010aTacconi et al 2010Tacconi et al , 2013Geach et al 2011) and was confirmed by gas-mass estimates obtained by converting dust masses (Magdis et al 2011(Magdis et al , 2012Santini et al 2014;Scoville et al 2014;Magnelli et al 2013;Genzel et al 2015). Their ISM properties change as well: most notably, their dust temperatures increase, which is reflected by the increase with redshift of the average intensity of the radiation field U (Magdis et al 2012;Magnelli et al 2013;Béthermin et al 2015;Genzel et al 2015).…”

“…; Liu et al, in prep. ), and fitted the SEDs using the models of Draine & Li (2007), following the approach in Magdis et al (2011Magdis et al ( , 2012. Results are shown in Fig.…”

“…For the spiral galaxies we fitted their IR SEDs using the models of Draine & Li (2007) to the photometry provided by the Kingfish project (Dale et al 2012), in analogy to the procedures used for the BzK galaxies (see Magdis et al 2011Magdis et al , 2012. We use here and in the following only objects for which U could be measured, which limits the sample to six objects in total.…”

Dusty star-forming galaxies at high redshift

“…Turning toward more 'normal' disk galaxies at high-z, Daddi et al (2010b) utilize dynamical arguments to infer α CO = 3.6±0.8 M pc −2 (K km s −1 ) −1 (i.e. only slightly less than the mean Milky Way value), while Magdis et al (2011) find a value α CO = 4.1 +3.3 −2.7 M pc −2 (K km s −1 ) −1 when considering dust-to-gas ratio based arguments.…”

HERUS: A CO ATLAS FROM SPIRE SPECTROSCOPY OF LOCAL ULIRGs

“…In the local universe, ULIRGs are invariably mergers hosting compact (<kpc) star-forming regions and active galactic nuclei (AGNs; e.g., Melnick & Mirabel 1990;Clements et al 1996; Rigopoulou et al 1999;Soifer et al 2000;Farrah et al 2001), but at higher redshifts ULIRGs may host more extended star-forming regions, and may be associated both with interacting and isolated galaxies (e.g., Pope et al 2006;Farrah et al 2008;Magdis et al 2011;Symeonidis et al 2013;Béthermin et al 2014). The highredshift ULIRGs thus lie on the galaxy mass-specific star formation rate density relation (the so-called "main sequence"), making them typical rather than extreme at these redshifts (e.g., Elbaz et al 2011).…”