VALES

II. The physical conditions of interstellar gas in normal star-forming galaxies up to z = 0.2 revealed by ALMA

¹ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
e-mail: thomas.hughes@uv.cl
² Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁵ Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
⁶ School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
⁷ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
⁸ CAPES Foundation, Ministry of Education of Brazil, 70040-020 Brasília/DF, Brazil
⁹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
¹⁰ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹¹ Instituto de Astronomía, Universidad Nacional Autonoma de México, A.P. 70-264, 04510 México, D.F., México
¹² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹³ CAS Key Laboratory for Researches in Galaxies and Cosmology, Center for Astrophysics, Department of Astronomy, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, 230026 Anhui, PR China

Received: 26 August 2016
Accepted: 17 November 2016

Abstract

We use new Band 3 CO(1–0) observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to study the physical conditions in the interstellar gas of a sample of 27 dusty main-sequence star-forming galaxies at 0.03 < z < 0.2 present in the Valparaíso ALMA Line Emission Survey (VALES). The sample is drawn from far-IR bright galaxies over ~160 deg² in the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS), which is covered by high-quality ancillary data including Herschel [Cii] 158 μm spectroscopy and far-infrared (FIR) photometry. The [Cii] and CO(1–0) lines are both detected at >5σ in 26 sources. We find an average [Cii] to CO(1–0) luminosity ratio of 3500 ± 1200 for our sample that is consistent with previous studies. Using the [Cii], CO(1–0) and FIR measurements as diagnostics of the physical conditions of the interstellar medium, we compare these observations to the predictions of a photodissociation region (PDR) model to determine the gas density, surface temperature, pressure, and the strength of the incident far-ultraviolet (FUV) radiation field, G₀, normalised to the Habing Field. The majority of our sample exhibit hydrogen densities of 4 < log n/ cm³ < 5.5 and experience an incident FUV radiation field with strengths of 2 < log G₀ < 3 when adopting standard adjustments. A comparison to galaxy samples at different redshifts indicates that the average strength of the FUV radiation field appears constant up to redshift z ~ 6.4, yet the neutral gas density increases as a function of redshift by a factor of ~100 from z = 0 to z = 0.2 that persists regardless of various adjustments to our observable quantities. Whilst this evolution could provide an explanation for the observed evolution of the star formation rate density with cosmic time, the result most likely arises from a combination of observational biases when using different suites of emission lines as diagnostic tracers of PDR gas.