THE REST-FRAME SUBMILLIMETER SPECTRUM OF HIGH-REDSHIFT, DUSTY, STAR-FORMING GALAXIES

Abstract

We present the average rest-frame spectrum of high-redshift dusty,\nstar-forming galaxies from 250-770GHz. This spectrum was constructed by\nstacking ALMA 3mm spectra of 22 such sources discovered by the South Pole\nTelescope and spanning z=2.0-5.7. In addition to multiple bright spectral\nfeatures of 12CO, [CI], and H2O, we also detect several faint transitions of\n13CO, HCN, HNC, HCO+, and CN, and use the observed line strengths to\ncharacterize the typical properties of the interstellar medium of these\nhigh-redshift starburst galaxies. We find that the 13CO brightness in these\nobjects is comparable to that of the only other z>2 star-forming galaxy in\nwhich 13CO has been observed. We show that the emission from the high-critical\ndensity molecules HCN, HNC, HCO+, and CN is consistent with a warm, dense\nmedium with T_kin ~ 55K and n_H2 >~ 10^5.5 cm^-3. High molecular hydrogen\ndensities are required to reproduce the observed line ratios, and we\ndemonstrate that alternatives to purely collisional excitation are unlikely to\nbe significant for the bulk of these systems. We quantify the average emission\nfrom several species with no individually detected transitions, and find\nemission from the hydride CH and the linear molecule CCH for the first time at\nhigh redshift, indicating that these molecules may be powerful probes of\ninterstellar chemistry in high-redshift systems. These observations represent\nthe first constraints on many molecular species with rest-frame transitions\nfrom 0.4-1.2mm in star-forming systems at high redshift, and will be invaluable\nin making effective use of ALMA in full science operations.\n