ON THE [C ii]–SFR RELATION IN HIGH REDSHIFT GALAXIES

Abstract

After two ALMA observing cycles, only a handful of [C II] 158 µm emission line searches in z > 6 galaxies have reported a positive detection, questioning the applicability of the local [C II]-SFR relation to high-z systems. To investigate this issue we use the Vallini et al. (2013, V13) model, based on high-resolution, radiative transfer cosmological simulations to predict the [C II] emission from the interstellar medium of a z ≈ 7 (halo mass M h = 1.17 × 10 11 M ) galaxy. We improve the V13 model by including (a) a physically-motivated metallicity (Z) distribution of the gas, (b) the contribution of Photo-Dissociation Regions (PDRs), (c) the effects of Cosmic Microwave Background on the [C II] line luminosity. We study the relative contribution of diffuse neutral gas to the total [C II] emission (F diff /F tot ) for different SFR and Z values. We find that the [C II] emission arises predominantly from PDRs: regardless of the galaxy properties, F diff /F tot ≤ 10% since, at these early epochs, the CMB temperature approaches the spin temperature of the [C II] transition in the cold neutral medium (T CMB ∼ T CNM s ∼ 20 K). Our model predicts a high-z [C II]-SFR relation consistent with observations of local dwarf galaxies (0.02 < Z/Z < 0.5). The [C II] deficit suggested by actual data (L CII < 2.0 × 10 7 L in BDF3299 at z ≈ 7.1) if confirmed by deeper ALMA observations, can be ascribed to negative stellar feedback disrupting molecular clouds around star formation sites. The deviation from the local [C II]-SFR would then imply a modified Kennicutt-Schmidt relation in z > 6 galaxies. Alternatively/in addition, the deficit might be explained by low gas metallicities (Z < 0.1 Z ).

Extended ionised and clumpy gas in a normal galaxy at z = 7.1 revealed by ALMA

“…We conclude that about 70% (±20%) of the flux in the [Cii] system is extended and diffuse on scales larger than about 1 kpc, which is consistent with the idea that the bulk of the [Cii] emission is associated with relatively diffuse gas on scale larger than the star formign regions, as expected by recent model of primeval galaxies (e.g. Vallini et al 2013Vallini et al , 2015Pallottini et al 2017;Katz et al 2016).…”

“…Vallini et al (2017) suggests that an offset between [Cii] emission and UV-stellar emission could result from a strong feedback effect, which could clean the most vigorous star forming regions, or even the entire galaxy, from the bulk of the molecular gas. Low metallicity and chemical inhomogeneity are also invoked as possible causes of the faintness of the [Cii] emission; indeed, metallicity effects were already proposed in Vallini et al (2015) to be one of the major cause of the deficit of the [Cii]-SFR relation observed at high-redshift relative to the local one (e.g. De Looze et al 2014).…”

“…However, [Oiii] should be colocated with the UV counterpart. Vallini et al (2015) and Pallottini et al (2017) expand further the model by including (i) the [Cii] emission from dense PDR surrounding molecular clouds, (ii) a proper modelling of the metallicity profile in the simulated galaxy and, (iii) by taking into account the excitation by the CMB. In these simulations, although circumgalactic [Cii] is present, the peak of the [Cii] emission is coincident with the location of molecular clouds, where stars form.…”

SIMULATOR OF GALAXY MILLIMETER/SUBMILLIMETER EMISSION (SÍGAME): THE [C ii]–SFR RELATIONSHIP OF MASSIVEz= 2 MAIN SEQUENCE GALAXIES

“…Various sub-grid "gas physics" approaches have been applied to both semi-analytical (Muñoz & Furlanetto 2014;Popping et al 2014a) and hydrodynamical (Nagamine et al 2006;Vallini et al 2013Vallini et al , 2015 simulations of galaxies. The simulations by Nagamine et al (2006) focus on the C II [ ] detectability of lyman break galaxies (LBGs) at z=3, while Vallini et al (2013) apply their simulations to observed upper limits on the C II [ ] emission from the z=6.6 Lyα emitter (LAE) Himiko (Ouchi et al 2013) finding that its metallicity must be subsolar.…”

“…Both set of simulations consider a two-phase ISM consisting of a cold and a warm neutral in pressure equilibrium, and both find that the C II [ ] emission is dominated by the cold neutral medium (CNM). In an update of their 2013 simulation, that includes the C II [ ] contribution from PDRs and a non-uniform metallicity distribution in the gas phase, Vallini et al (2015) finds that most of the C II [ ] emission from their z=6.6 models originates in the PDRs with only~10% coming from the CNM.…”