The [C ii] emission as a molecular gas mass tracer in galaxies at low and high redshifts

Abstract

We present ALMA Band 9 observations of the [C II]158µ m emission for a sample of 10 main-sequence galaxies at redshift z∼2, with typical stellar masses (log M /M ∼10.0-10.9) and star formation rates (∼35-115 M yr −1 ). Given the strong and well understood evolution of the interstellar medium from the present to z = 2, we investigate the behaviour of the [C II] emission and empirically identify its primary driver. We detect [C II] from six galaxies (four secure, two tentative) and estimate ensemble averages including non detections. The [C II]-to-infrared luminosity ratio (L [C II] /L IR ) of our sample is similar to that of local main-sequence galaxies (∼ 2 × 10 −3 ), and ∼ 10 times higher than that of starbursts. The [C II] emission has an average spatial extent of 4 -7 kpc, consistent with the optical size. Complementing our sample with literature data, we find that the [C II] luminosity correlates with galaxies' molecular gas mass, with a mean absolute deviation of 0.2 dex and without evident systematics: the [C II]-to-H 2 conversion factor (α [C II] ∼ 30 M /L ) is largely independent of galaxies' depletion time, metallicity, and redshift. [C II] seems therefore a convenient tracer to estimate galaxies' molecular gas content regardless of their starburst or main-sequence nature, and extending to metal-poor galaxies at low-and high-redshifts. The dearth of [C II] emission reported for z > 6-7 galaxies might suggest either a high star formation efficiency or a small fraction of UV light from star formation reprocessed by dust.

Measuring the Heating and Cooling of the Interstellar Medium at High Redshift: PAH and [C ii] Observations of the Same Star-forming Galaxies at z ∼ 2

“…The expected radii of sub-mm and HST H 160 emission in z ∼ 2 star-forming galaxies is < 8 kpc (e.g., Fig. 3, Zanella et al 2018;Calistro Rivera et al 2018;Lang et al 2019), so it is unlikely that our observations are missing flux on large scales due to interferometric spatial filtering. Six galaxies in our proposal were observed for ∼ 18 minutes onsource, achieving the target sensitivity of 5 mJy over 300 km s −1 bandwidth at a native resolution of 31.250 MHz (13.6 km s −1 ) which was later re-binned to lower spectral resolutions.…”

“…To search for marginally detected emission lines, we used a circular aperture with radius 0.5 to extract a 50 km s −1 spectrum centered on the source's dust continuum position. Next, we extracted additional spectra through the same circular apertures offset by 0.5 from the source's center at various angles, as optical light, dust continuum and [C II] emission can be spatially offset from one another in high redshift ULIRGs (e.g., Zanella et al 2018;Calistro Rivera et al 2018). From the set of extracted spectra, we searched each spectral window for the presence of three channels greater than 2× the local rms.…”

“…The horizontal dotted lines correspond to ±1σ noise, and the shaded blue region indicates where line emission was integrated: the peak is detected at an SNR of 19.1σ and the integrated emission at an SNR of 34.3σ. Gaps in the spectra are due to the observation's spectral window configuration.landscape of [C II] observations at z ∼ 2, we also compare our [C II] measurements to z ∼ 2 − 3 galaxies with data from ALMA, APEX, or Herschel FTS(Ivison et al 2010;Valtchanov et al 2011;Schaerer et al 2015;Gullberg et al 2015;Zanella et al 2018;Hashimoto et al 2018;Rybak et al 2019). Prior observations of both PAH and [C II] in the same galaxy at z ∼ 2 are limited to a handful of systems observed with Spitzer and the Redshift (z) and Early Universe Spectrometer (ZEUS) on the Caltech Submillimeter Observatory (CSO)…”

The ALPINE-ALMA [CII] survey: Data processing, catalogs, and statistical source properties

“…In addition, at z > 4, it is conveniently redshifted to the >850 µm atmospheric windows. This line has a variety of different scientific applications since it can be used to probe the interstellar medium (e.g., Zanella et al 2018), the SFR (e.g., De Looze et al 2014;Carniani et al 2018a), the gas dynamics (e.g., De Breuck et al 2014;Jones et al 2020), or outflows (e.g., Maiolino et al 2012;Gallerani et al 2018;Ginolfi et al 2020a). It has now been detected in ∼35 galaxies at z > 4, but most of them are magnified by lensing or starbursts and only one third of them are normal star-forming systems (see compilation in Lagache et al 2018).…”

The ‘Red Radio Ring’: ionized and molecular gas in a starburst/active galactic nucleus at z ∼ 2.55

“…Far-IR fine-structure lines (FSLs) offer an additional probe of Hii regions in obscured sites of SF, as they are less susceptible to dust attenuation when compared to optical or mid-IR lines (see Fernández-Ontiveros et al 2016;Díaz-Santos et al 2017). This motivates the use of these far-IR FSLs as powerful line diagnostics of the evolving ISM at high-z (Maiolino et al 2005(Maiolino et al , 2009Ferkinhoff et al 2010Ferkinhoff et al , 2011Riechers et al 2014;Zavala et al 2018;Zhang et al 2018;Lamarche et al 2018;Marrone et al 2018;Vishwas et al 2018;Zanella et al 2018). Unfortunately, the atmospheric coverage of many important mid-/far-IR FSLs makes observations difficult to execute, if not impossible to observe from the ground.…”