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ABSTRACT We report the serendipitous discovery of a dust-obscured galaxy observed as part of the Atacama Large Millimeter Array (ALMA) Large Program to INvestigate [C ii] at Early times (ALPINE). While this galaxy is detected both in line and continuum emissions in ALMA Band 7, it is completely dark in the observed optical/near-infrared bands and only shows a significant detection in the UltraVISTA Ks band. We discuss the nature of the observed ALMA line, that is [C ii] at $$z$$ ∼ 4.6 or high-J CO transitions at $$z$$ ∼ 2.2. In the first case, we find a [C ii]/FIR luminosity ratio of $$\mathrm{log}{(L_{[\mathrm{ C}\, \rm {\small {II}}]}/L_{\mathrm{ FIR}})} \sim -2.5$$, consistent with the average value for local star-forming galaxies (SFGs). In the second case instead, the source would lie at larger CO luminosities than those expected for local SFGs and high-z submillimetre galaxies. At both redshifts, we derive the star formation rate (SFR) from the ALMA continuum and the physical parameters of the galaxy, such as the stellar mass (M*), by fitting its spectral energy distribution. Exploiting the results of this work, we believe that our source is a ‘main-sequence’, dusty SFG at $$z$$ = 4.6 (i.e. [C ii] emitter) with $$\mathrm{log(SFR/M_{\odot }\, yr^{-1})}\sim 1.4$$ and log(M*/M⊙) ∼ 9.9. As a support to this scenario our galaxy, if at this redshift, lies in a massive protocluster recently discovered at $$z$$ ∼ 4.57, at only ∼1 proper Mpc from its centre. This work underlines the crucial role of the ALPINE survey in making a census of this class of objects, in order to unveil their contribution to the global SFR density at the end of the Reionization epoch.
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[CII] 158 μ m emission as an indicator of galaxy star formation rate
Abstract Observations of local star-forming galaxies (SFGs) show a tight correlation between their singly ionized carbon line luminosity ($$L_{\rm [C\, \small {II}]}$$) and star formation rate (SFR), suggesting that $$L_{\rm [C\, \small {II}]}$$ may be a useful SFR tracer for galaxies. Some other galaxy populations, however, are found to have lower $$L_{\rm [C\, \small {II}]}{}/{}\rm SFR$$ than local SFGs, including the infrared-luminous, starburst galaxies at low and high redshifts as well as some moderately star-forming galaxies at the epoch of re-ionization (EoR). The origins of this ‘$$\rm [C\, \small {II}]$$ deficit’ is unclear. In this work, we study the $$L_{\rm [C\, \small {II}]}$$-SFR relation of galaxies using a sample of z = 0 − 8 galaxies with M* ≈ 107 − 5 × 1011 M⊙ extracted from cosmological volume and zoom-in simulations from the Feedback in Realistic Environments (fire) project. We find a simple analytic expression for $$L_{\rm [C\, \small {II}]}$$/SFR of galaxies in terms of the following parameters: mass fraction of $$\rm [C\, \small {II}]$$-emitting gas ($$f_{\rm [C\, \small {II}]}$$), gas metallicity (Zgas), gas density (ngas) and gas depletion time ($$t_{\rm dep}{}={}M_{\rm gas}{}/{}\rm SFR$$). We find two distinct physical regimes: $$\rm H_2$$-rich galaxies where tdep is the main driver of the $$\rm [C\, \small {II}]$$ deficit and $$\rm H_2$$-poor galaxies where Zgas is the main driver. The observed $$\rm [C\, \small {II}]$$ deficit of IR-luminous galaxies and early EoR galaxies, corresponding to the two different regimes, is due to short gas depletion time and low gas metallicity, respectively. Our result indicates that the $$\rm [C\, \small {II}]$$ deficit is a common phenomenon of galaxies, and caution needs to be taken when applying a constant $$L_{\rm [C\, \small {II}]}$$-to-SFR conversion factor derived from local SFGs to estimate cosmic SFR density at high redshifts and interpret data from upcoming $$\rm [C\, \small {II}]$$ line intensity mapping experiments.
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- PAR ID:
- 10486538
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- ISSN:
- 0035-8711
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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