The Rest-Frame Submillimeter Spectrum of High-Redshift, Dusty, Star-Forming Galaxies

Abstract

We present the average rest-frame spectrum of high-redshift dusty, star-forming galaxies from 250 − 770 GHz. This spectrum was constructed by stacking ALMA 3 mm spectra of 22 such sources discovered by the South Pole Telescope and spanning z = 2.0 − 5.7. In addition to multiple bright spectral features of 12 CO, [CI], and H 2 O, we also detect several faint transitions of 13 CO, HCN, HNC, HCO + , and CN, and use the observed line strengths to characterize the typical properties of the interstellar medium of these high-redshift starburst galaxies. We find that the 13 CO brightness in these objects is comparable to that of the only other z > 2 star-forming galaxy in which 13 CO has been observed. We show that the emission from the high-critical density molecules HCN, HNC, HCO + , and CN is consistent with a warm, dense medium with T kin ∼ 55 K and n H2 10 5.5 cm −3 . High molecular hydrogen densities are required to reproduce the observed line ratios, and we demonstrate that alternatives to purely collisional excitation are unlikely to be significant for the bulk of these systems. We quantify the average emission from several species with no individually detected transitions, and find emission from the hydride CH and the linear molecule CCH for the first time at high redshift, indicating that these molecules may be powerful probes of interstellar chemistry in high-redshift systems. These observations represent the first constraints on many molecular species with rest-frame transitions from 0.4 − 1.2 mm in star-forming systems at high redshift, and will be invaluable in making effective use of ALMA in full science operations.

The nature of the [C ii] emission in dusty star-forming galaxies from the SPT survey

“…The resulting FWHMs are in the range ∼ 210 − 820 km/s. This is comparable to the typical CO line width of ∼ 460 km s −1 found for SPT DSFGs by stacking 22 spectra (Spilker et al 2014). Nine out of 20 sources have FWHM > 500 km/s, which is large compared to other high-z [CII] detections (e.g.…”

“…9). However, the average CO excitation temperatures in the SPT sample are 50 K (Spilker et al 2014), well below the values required to fit the observed ratio (Fig. 9).…”

“…Even though the ratio is consistent with Bothwell et al (2013b) within the uncertainties, a lower [CO(2-1)/CO(1-0)] ratio is expected for normal star forming galaxies than for starburst galaxies, because the molecular gas in starburst galaxies is expected to be more highly excited. Spilker et al (2014) find an average CO(2-1)/CO(1-0) brightness temperature ratio of 1.1 ± 0.1 for 22 of the SPT DSFGs by stacking ATCA spectra, after scaling them by their 1.4 mm continuum flux density. Given the range of values of these three methods, and for consistency with previous literature, we adopt the scaling factor of 0.9 from Stacey et al (2010).…”

“…However, we know from 12 CO to 13 CO ratios that CO is optically thick (e.g. Spilker et al 2014), which rules out case ii. Only case iii, implying optically thick CO and [CII], is consistent with all observational data.…”

“…In the opposite case, scenario ii (grey dot-dashed curve), the observed ratio is reached only for very low excitation temperatures. However, this optically thin CO scenario can be ruled out because both the 12 CO to 13 CO ratios and the low-J 12 CO line ratios in the SPT sample imply that CO is moderately optically thick with τ CO(1-0) = 1 − 10 (Spilker et al 2014), similar to what is seen in our own galaxy (Penzias et al 1972;Goldreich & Kwan 1974). Scenario iii, where the [CII] and CO(1-0) optical depths are in the same regime is thus the only one that can fit the observed ratios, but only for excitation temperatures 180 K. While we cannot distinguish mathematically between low and high optical depth, the known τ CO(1-0) = 1 − 10 and the equality of both the excitation temperatures and opacities would imply that also [CII] would need to be (nearly or fully) optically thick.…”