Abstract
ABSTRACT We present a sub-kpc resolved study of the interstellar medium properties in SDP.81, a $z$ = 3.042 strongly gravitationally lensed, dusty star-forming galaxy, based on high-resolution, multiband ALMA observations of the far-infrared (FIR) continuum, CO ladder, and the [C ii] line. Using a visibility-plane lens modelling code, we achieve a median source-plane resolution of ∼200 pc. We use photon-dominated region (PDR) models to infer the physical conditions – far-ultraviolet (FUV) field strength, density, and PDR surface temperature – of the star-forming gas on 200-pc scales, finding a FUV field strength of ∼103−104G0, gas density of ∼105 cm−3, and cloud surface temperatures up to 1500 K, similar to those in the Orion Trapezium region. The [C ii] emission is significantly more extended than that FIR continuum: ∼50 per cent of [C ii] emission arises outside the FIR-bright region. The resolved [C ii]/FIR ratio varies by almost 2 dex across the source, down to ∼2 × 10−4 in the star-forming clumps. The observed [C ii]/FIR deficit trend is consistent with thermal saturation of the C+ fine-structure-level occupancy at high gas temperatures. We make the source-plane reconstructions of all emission lines and continuum data publicly available.
Highlights
Dusty star-forming galaxies (DSFGs) with star formation rates (SFRs) of 100–1000 M yr−1 play a key role in the epoch of peak star-forming activity of the Universe, in the z = 2−4 redshift range (e.g. Casey, Narayanan & Cooray 2014; Swinbank et al 2014)
The inferred Tdust, β are in line with those derived for the general highredshift DSFG population
3 × 104 G0, n ≥ 105 cm−3 for the region closest (R ≤0.1 kpc) to the Trapezium cluster, decreasing down to G 103 G0, n 104 cm−3. These are, respectively, on the upper and lower end of the conditions seen in the FIR-bright region of SDP.81; in SDP.81, these describe physical conditions averaged over 200pc scales, underlining the extreme nature of the star-forming regions in DSFGs
Summary
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 104 G0 and moderately high densities n(H) = 103.5−104.5 cm−3 In this regime, the surface temperature of the PDR regions is of the order of few hundred K and the [C II] emission becomes thermally saturated, resulting in a pronounced [C II]/FIR deficit (Munoz & Oh 2016).
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