Strong Far-ultraviolet Fields Drive the [C ii]/Far-infrared Deficit in z ∼ 3 Dusty, Star-forming Galaxies

Abstract

We present 0.15-arcsec (1 kpc) resolution ALMA observations of the [C II] 157.74 µm line and rest-frame 160-µm continuum emission in two z ∼ 3 dusty, star-forming galaxies -ALESS 49.1 and ALESS 57.1, combined with resolved CO(3-2) observations. In both sources, the [C II] surface brightness distribution is dominated by a compact core ≤1 kpc in radius, a factor of 2-3 smaller than the extent of the CO (3-2) emission. In ALESS 49.1, we find an additional extended (8-kpc radius), low surface-brightness [C II] component. Based on an analysis of mock ALMA observations, the [C II] and 160-µm continuum surface brightness distributions are inconsistent with a single-Gaussian surface brightness distribution with the same size as the CO(3-2) emission. The [C II] rotation curves flatten at 2 kpc radius, suggesting the kinematics of the central regions are dominated by a baryonic disc. Both galaxies exhibit a strong [C II]/FIR deficit on 1-kpc scales, with FIR-surface-brightness to [C II]/FIR slope steeper than in local star-forming galaxies. A comparison of the [C II]/CO(3-2) observations with PDR models suggests a strong FUV radiation field (G 0 ∼ 10 4 ) and high gas density (n(H) ∼ 10 4 − 10 5 cm −3 ) in the central regions of ALESS 49.1 and 57.1. The most direct interpretation of the pronounced [C II]/FIR deficit is a thermal saturation of the C + fine-structure levels at temperatures ≥ 500 K, driven by the strong FUV field.

The Redshift and Star Formation Mode of AzTEC2: A Pair of Massive Galaxies at z = 4.63

“…Likewise, for AzTEC2-B we find log(µ B L [C II] /L ) = 10.07 ± 0.10 and log(µ B L IR /L ) = 13.07 ± 0.17. Although here we neglect possible differences in the L [C II] /L IR ratio of AzTEC2-A and AzTEC2-B arising from distinct physical conditions of the ISM (e.g., ultraviolet radiation field and/or metal enrichment; Katz et al 2017;Rybak et al 2019), these new and independent L IR estimates corroborate those derived via SED fitting.…”

Full of Orions: a 200-pc mapping of the interstellar medium in the redshift-3 lensed dusty star-forming galaxy SDP.81

“…11), as well as the Herschel observations of ULIRGs from the GOALS sample (Díaz-Santos et al 2017). However, as the [C ii] emission is significantly more extended than the FIR continuum (in line with Gullberg et al 2018;Rybak et al 2019), the source-averaged [C ii/FIR measurement provides only an upper limit on the [C ii]/FIR ratio in the central, FIR-bright region. The large extent of the [C ii] emission compared to the FIR continuum implies that the spatiallyaveraged [C ii]/FIR measurements in high-redshift sourcesas inferred from unresolved observations -mask the potentially very strong [C ii]/FIR deficit in the actual star-forming regions.…”

“…The PDRToolbox models are calculated for a semi-infinite slab geometry, assuming a solar metallicity and stopping the calculation at the depth corresponding to a visual extinction AV = 10. The PDRToolbox models have been widely used in interpreting atomic and molecular line observations from both z ∼ 0 ULIRGs (e.g., Díaz-Santos et al 2017) and high-redshift DSFGs (e.g., Valtchanov et al 2011;Gullberg et al 2015;Wardlow et al 2017;Rybak et al 2019); we now apply the same approach to SDP.81. As the FIR studies of metallicity tracers in z ≥ 1 DSFGs indicate metallicities ≥ 1 Z (Wardlow et al 2017), we consider the Z = 1 Z default PDRToolbox model as appropriate for dust-rich ISM of SDP.81.…”

“…Compared to both samples, SDP.81 shows a similar FUV field strength, with density ≥ 1 dex above the Wardlow et al (2017) range, and comparable to the densest Gullberg et al (2015) sources. Using resolved [C ii] and CO(3-2) and FIR ALMA imaging, Rybak et al (2019) inferred G ∼ 10 4 G0, n ∼ 10 4 − 10 5 cm −3 in the central regions of two z = 2.94 DSFGs from the ALESS sample; SDP.81 shows slightly higher G and lower n(H). On sub-pc scales, we compare the inferred PDR properties with the resolved (∼ 0.05 pc) study of Orion by Goicoechea et al (2015), who used [C ii], FIR continuum and CO(2-1) observations to infer the PDR conditions in the OMC 1.…”

“…Consequently, resolved multi-tracer observations are necessary to robustly infer the PDR conditions in DSFGs. For example, Rybak et al (2019) have used resolved [C ii], CO(3-2) and FIR continuum ALMA observations to study the properties of the star-forming ISM in the central regions (R ≤ 2 kpc) of two z ∼ 3 DSFGs from the ALESS sample (Hodge et al 2013;Karim et al 2013), finding FUV fields in excess of 10 4 G0 and moderately high densities 1 G 0 denotes the Habing field, 1 G 0 = 1.6 × 10 −3 erg s −1 cm −2 . n(H)=10 3.5 −10 4.5 cm −3 .…”