HERSCHEL EXTREME LENSING LINE OBSERVATIONS: [C ii] VARIATIONS IN GALAXIES AT REDSHIFTS z = 1–3*

Abstract

We observed the [C II] line in 15 lensed galaxies at redshifts 1<z<3 using HIFI on the Herschel Space Observatory and detected 14/15 galaxies at 3σ or better. High magnifications enable even modestly luminous galaxies to be detected in [C II] with Herschel. The [C II] luminosity in this sample ranges from 8 × 107 L e to 3.7 × 10 9 L e (after correcting for magnification), confirming that [C II] is a strong tracer of the ISM at high redshifts. The ratio of the [C II] line to the total far-infrared (FIR) luminosity serves as a measure of the ratio of gas to dust cooling and thus the efficiency of the grain photoelectric heating process. It varies between 3.3% and 0.09%. We compare the [C II]/FIR ratio to that of galaxies at z=0 and at high redshifts and find that they follow similar trends. The [C II]/FIR ratio is lower for galaxies with higher dust temperatures. This is best explained if increased UV intensity leads to higher FIR luminosity and dust temperatures, but gas heating does not rise due to lower photoelectric heating efficiency. The [C II]/FIR ratio shows weaker correlation with FIR luminosity. At low redshifts highly luminous galaxies tend to have warm dust, so the effects of dust temperature and luminosity are degenerate. Luminous galaxies at high redshifts show a range of dust temperatures, showing that [C II]/FIR correlates most strongly with dust temperature. The [C II] to mid-IR ratio for the HELLO sample is similar to the values seen for low-redshift galaxies, indicating that small grains and PAHs dominate the heating in the neutral ISM, although some of the high [CII]/FIR ratios may be due to turbulent heating.

The case for thermalization as a contributor to the [C ii] deficit

“…This is especially true when considering the high-z galaxies with especially pronounced deficit behaviour. For example, the z ∼ 2-6 galaxies observed in Rawle et al (2014), Capak et al (2015), De Breuck et al (2014), Gallerani et al (2012), andMalhotra et al (2017) show [C II]/TIR ratios 10 to 100 times lower than those observed in the local star-forming galaxies included in this work. As the majority of [C II] emission has been found to arise from the neutral phases of the ISM (Pineda et al 2013;Abdullah et al 2017;Croxall et al 2017;Cormier et al 2019;Sutter et al 2019;Bigiel et al 2020), the effects of thermalization solely within the ionized phases of the ISM can only contribute to moderate deficit behaviour.…”

A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies

“…A decade later, the Infrared Space Observatory (ISO; Kessler et al 1996) increased the number of galaxies with FIR fine-structure line detections to the dozens (Malhotra et al 1997Luhman et al 1998Luhman et al , 2003. But twenty more years needed to pass until the Herschel Space Observatory (Herschel hereafter; Pilbratt et al 2010) reopened a new window into the FIR universe, providing spectroscopic data for significantly larger samples of nearby, intermediate-redshift, and high-redshift galaxies (e.g., Graciá-Carpio et al 2011;Díaz-Santos et al 2013Magdis et al 2014;Rigopoulou et al 2014;Brisbin et al 2015;Cormier et al 2015;Rosenberg et al 2015;Malhotra et al 2017).…”

ALMA band 8 observations of DLA 2233+131 at z = 3.150

“…In the case that the [C ii]/FIR ratio is significantly low, the expected [C ii] flux is fainter than the detection limit of our ALMA observations. Although such a lower [C ii]/FIR value suggests higher dust temperature (Malhotra et al 2017), optical observations imply that DLA2233+131 may have only little dust content. This is consistent with the non-detection of dust continuum from DLA2233+131 in this work.…”