Ism Masses and the Star Formation Law at Z = 1 to 6: Alma Observations of Dust Continuum in 145 Galaxies in the Cosmos Survey Field

Abstract

ALMA Cycle 2 observations of the long wavelength dust emission in 145 star-forming galaxies are used to probe the evolution of star-forming ISM. We also develop the physical basis and empirical calibration (with 72 low-z and z ∼2 galaxies) for using the dust continuum as a quantitative probe of interstellar medium (ISM) masses. The galaxies with highest star formation rates (SFRs) at < z > = 2.2 and 4.4 have gas masses up to 100 times that of the Milky Way and gas mass fractions reaching 50 to 80%, i.e. gas masses 1 -4 × their stellar masses. We find a single high-z star formation law:-an approximately linear dependence on the ISM mass and an increased star formation efficiency per unit gas mass at higher redshift. Galaxies above the Main Sequence (MS) have larger gas masses but are converting their ISM into stars on a timescale only slightly shorter than those on the MS -thus these 'starbursts' are largely the result of having greatly increased gas masses rather than and increased efficiency for converting gas to stars. At z > 1, the entire population of star-forming galaxies has ∼ 2 -5 times shorter gas depletion times than low-z galaxies. These shorter depletion times indicate a different mode of star formation in the early universe -most likely dynamically driven by compressive, high-dispersion gas motions -a natural consequence of the high gas accretion rates.

The Alma Spectroscopic Survey in the Hubble Ultra Deep Field: Molecular Gas Reservoirs in High-Redshift Galaxies

“…However, this approach relies on the assumption of a dust-to-gas mass ratio (DGR), which typically depends on the gas metallicity (Bolatto et al 2013;Sandstrom et al 2013;Genzel et al 2015). Recent ALMA results report substantially lower values of M gas than what are typically obtained in CObased studies (Scoville et al 2014(Scoville et al , 2016.…”

“…The tight relation observed between CO(1-0) luminosity and the restframe 850 μm monochromatic luminosity (see Figure 1 in Scoville et al 2016) suggests that a simple conversion factor can be used to derive gas masses from monochromatic dust continuum observations. By setting the dust temperature to = T 25 dust K (following Scoville et al 2014), we derive from Equation (12) in their paper M ISM from our 1 mm flux densities as follows:…”

“…A similar approach was presented by Scoville et al (2014Scoville et al ( , 2016. This calibration is tuned on a set of relatively massive0.2 4 10 11 [( -)…”

“…We stress that there is a factor >2 scatter in the Table 4 Gas Mass Estimates Based on the Dust Continuum , assuming a modified blackbody template for the dust emission with β=1.6 and = T 25 dust K. (5) Gas mass based on the 1 mm flux density, derived following Equation (2) (Groves et al 2015). (6) Gas mass based on the 1 mm flux density, derived following Equation (3) (Scoville et al 2014(Scoville et al , 2016. (7) Gas mass derived from the dust-mass estimate resulting from MAGPHYS SED fitting, assuming a dust-to-gas ratio DGR=1/100.…”

Dust Properties of C ii Detected z ∼ 5.5 Galaxies: New HST/WFC3 Near-IR Observations

“…For example, studies of emission line ratios of high-redshift (z>4) galaxies by Spitzer suggest that early galaxies grow at a fast pace, with high specific star formation rates (SFRs) and stellar mass assembly timescales of only a couple of hundred Myr (de Barros et al 2014;Faisst et al 2016a;Jiang et al 2016). A substantially higher gas fraction as well as changes in the ionization parameter of stellar radiation indicated by the [O III]/[O II]line ratio are also observed, indicative of a different H 2 region configuration (Steidel et al 2014;Genzel et al 2015;Silverman et al 2015;Faisst 2016;Masters et al 2016;Scoville et al 2016).…”