Missing [C ii] emission from early galaxies

Abstract

ALMA observations have revealed that [C$\scriptstyle \rm II$]158μm line emission in high-z galaxies is ≈2 − 3 × more extended than the UV continuum emission. Here we explore whether surface brightness dimming (SBD) of the [C$\scriptstyle \rm II$] line is responsible for the reported [C$\scriptstyle \rm II$] deficit, and the large L[OIII]/L[CII] luminosity ratio measured in early galaxies. We first analyse archival ALMA images of nine z > 6 galaxies observed in both [C$\scriptstyle \rm II$] and [O$\scriptstyle \rm III$]. After performing several uv-tapering experiments to optimize the identification of extended line emission, we detect [C$\scriptstyle \rm II$] emission in the whole sample, with an extent systematically larger than the [O$\scriptstyle \rm III$] emission. Next, we use interferometric simulations to study the effect of SBD on the line luminosity estimate. About 40% of the extended [C$\scriptstyle \rm II$] component might be missed at an angular resolution of 0.8″, implying that L[CII] is underestimated by a factor ≈2 in data at low (<7) signal-to-noise ratio. By combining these results, we conclude that L[CII] of z > 6 galaxies lies, on average, slightly below the local L[CII] − SFR relation (Δz = 6 − 9 = −0.07 ± 0.3), but within the intrinsic dispersion of the relation. SBD correction also yields L[OIII]/L[CII] < 10, i.e. more in line with current hydrodynamical simulations.

ALMA Lensing Cluster Survey: Bright [C ii] 158 μm Lines from a Multiply Imaged Sub-L ⋆ Galaxy at z = 6.0719

“…Following the correction of the lensing magnification, we find that both z6.3 and z6.1/6.2 fall slightly above, but still follow the SFR-L [CII] relation of the local galaxies within the dispersion. This is consistent with recent ALMA results, in that the average SFR-L [CII] relation among high-redshift star-forming galaxies at z ∼ 4-9 is well within the intrinsic dispersion of the local relation (Carniani et al 2018(Carniani et al , 2020Schaerer et al 2020). Given that RXCJ0600-z6 is consistent with being an abundant, sub-L * galaxy at z = 6 (see Section 3.5), these results may suggest that the SFR-L [CII] relation, defined by local galaxies, holds from the spatially resolved sub-kpc ISM to the whole scales in abundant galaxies, even up to the epoch of reionization.…”

“…Since the [C II] line emissivity depends on ISM properties, such as the ionization state, metallicity, and gas density (e.g., Vallini et al 2015), the different L [CII] -SFR relations could be alternatively explained by a larger dispersion of the ISM properties in high-z galaxies than in local galaxies. The uncertainties of the SFR estimates might contribute to the large dispersion in high-z galaxies, due to assumptions regarding star formation history, the dust-attenuation curve, and the stellar population age, as discussed in Carniani et al (2020), andSchaerer et al (2020). Another recent report regarding the extended [C II] line morphology up to a radius of ∼10 kpc (e.g., Fujimoto et al 2019Fujimoto et al , 2020bGinolfi et al 2020;Novak et al 2020) might also be related to some of those nondetections, because the surface brightness of the extended emission is significantly decreased in relatively high-resolution maps (Carniani et al 2020).…”

“…The uncertainties of the SFR estimates might contribute to the large dispersion in high-z galaxies, due to assumptions regarding star formation history, the dust-attenuation curve, and the stellar population age, as discussed in Carniani et al (2020), andSchaerer et al (2020). Another recent report regarding the extended [C II] line morphology up to a radius of ∼10 kpc (e.g., Fujimoto et al 2019Fujimoto et al , 2020bGinolfi et al 2020;Novak et al 2020) might also be related to some of those nondetections, because the surface brightness of the extended emission is significantly decreased in relatively high-resolution maps (Carniani et al 2020). Based on visibility-based stacking, the secondary extended component up to the 10 kpc scale is estimated to have an average contribution to the total line luminosity of ∼50% around star-forming galaxies (Fujimoto et al 2019), and ∼20% around quasars (Novak et al 2020) at z ∼ 6.…”

ALMA Lensing Cluster Survey: A spectral stacking analysis of [C II] in lensed z ∼ 6 galaxies

“…As suggested by Carniani et al (2020) [C ii] extended emission could also lead to reduced emission and an underestimate of the upper limit. Such an effect could be made more probable from stacking objects elongated by gravitational lensing effects.…”

The ALMA Spectroscopic Survey in the HUDF: A Search for [C ii] Emitters at 6 ≤ z ≤ 8

“…Recent observations and reanalyses of earlier ALMA data have now led to a different possible scenario: a significantly larger scatter in L [C II] at high redshift compared to the local SFR-L [C II] relation, instead of a [C II] deficit (see, e.g., Carniani et al 2018;Matthee et al 2019). Additionally, surface brightness dimming could affect the detectability of [C II] emission (Carniani et al 2020). However, the current data sets are still limited in size, and furthermore most of the early galaxies that have been observed with ALMA were selected as Lyα emitters (LAEs) (such that they had a previously known redshift), which can lead to a bias toward young, metal-poor, dust-free systems (see, e.g., Smit et al 2018).…”