High Dense Gas Fraction in a Gas-rich Star-forming Galaxy at z = 1.2^∗

Abstract

We report observations of dense molecular gas in the star-forming galaxy EGS 13004291 (z = 1.197) using the Plateau de Bure Interferometer. We tentatively detect HCN and HNC J = 2 → 1 emission when stacked together at 4σ significance, yielding line luminosities of L HCN(J=2→1) = (9 ± 3) × 10 9 K km s −1 pc 2 and L HNC(J=2→1) = (5 ± 2) × 10 9 K km s −1 pc 2 respectively. We also set 3σ upper limits of < 7-8 × 10 9 K km s −1 pc 2 on the HCO + (J = 2 → 1), H 2 O(3 13 → 2 20 ) and HC 3 N(J = 20 → 19) line luminosities. We serendipitously detect CO emission from two sources at z ∼ 1.8 and z ∼ 3.2 in the same field of view. We also detect CO(J = 2 → 1) emission in EGS 13004291, showing that the excitation in the previously detected CO(J = 3 → 2) line is subthermal (r 32 = 0.65 ± 0.15). We find a line luminosity ratio of L HCN /L CO = 0.17 ± 0.07, as an indicator of the dense gas fraction. This is consistent with the median ratio observed in z > 1 galaxies (L HCN /L CO = 0.16 ± 0.07) and nearby ULIRGs (L HCN /L CO = 0.13 ± 0.03), but higher than in local spirals (L HCN /L CO = 0.04 ± 0.02). Although EGS 13004291 lies significantly above the galaxy main sequence at z ∼ 1, we do not find an elevated star formation efficiency (traced by L FIR /L CO ) as in local starbursts, but a value consistent with main-sequence galaxies. The enhanced dense gas fraction, the subthermal gas excitation, and the lower than expected star formation efficiency of the dense molecular gas in EGS 13004291 suggest that different star formation properties may prevail in high-z starbursts. Thus, using L FIR /L CO as a simple recipe to measure the star formation efficiency may be insufficient to describe the underlying mechanisms in dense star-forming environments inside the large gas reservoirs.

Low Star Formation Efficiency in Typical Galaxies at z = 5–6

“…The detection of bright CO emission from HZ10 represents the highest redshift CO detection from a "normal", Main Sequence galaxy to date (the next highest redshift CO line from an unlensed Main Sequence galaxy was serendipitously detected by Gowardhan et al 2017Gowardhan et al , 2019 at z ∼ 3.2). We note that HZ10 appears to have a very high gas fraction based on the measured CO luminosity (M gas > M stars with high confidence and likely M gas ∼ 4 − 5 × M stars , Figure 4).…”

“…The cold molecular gas properties in "normal" star-forming galaxies are poorly constrained beyond z ∼ 3. Even at z ∼ 3 − 4 only few significant detections have been achieved, mostly afforded by strong gravitational lensing (Coppin et al 2007;Riechers et al 2010a;Dessauges-Zavadsky et al 2015, a serendipitous detection at z ∼ 3.22 (Gowardhan et al 2017(Gowardhan et al , 2019, and constraining upper limits for unlensed targets (e.g., Tan et al 2013). The low detection rate could suggest a strong evolution in α CO with redshift, possibly driven by a rapid metallicity evolution (Tan et al 2013(Tan et al , 2014.…”

“…Figure 4. The ratio of molecular gas mass to stellar mass (calculated using an α CO =4.5 M (K km s −1 pc 2 ) −1 for all sources) adapted from Carilli & Walter (2013); Leroy et al (2009); Riechers et al (2010a); Daddi et al (2010b); Geach et al (2011); Magnelli et al (2012); Tan et al (2013); Tacconi et al (2013); Dessauges-Zavadsky et al(2015, 2017);Gowardhan et al (2017Gowardhan et al ( , 2019 The grey line shows Mgas/M * ∝ (1 + z) 2(Geach et al 2011). An alternative choice of α CO =0.8 for HZ10 and LBG-1 is also shown in light grey.…”

On the Appearance of Thresholds in the Dynamical Model of Star Formation

“…Observations suggest that the KS relation for HCN sometimes becomes sublinear with too little emission for the SFR at high surface densities (Chen et al 2015;Bigiel et al 2016;Gowardhan et al 2017), and sometimes becomes sublinear with excess HCN for the SFR at low surface densities (Usero et al 2015;Kauffmann et al 2017a). Sublinear molecular emission like this was discussed in the previous section as a possible result of a shallow density profile inside molecular clouds (α < 2) in a large-scale survey, where the projected PDF for the cloud population is steep (γ 2D > 2).…”

The Kennicutt–Schmidt Law and Gas Scale Height in Luminous and Ultraluminous Infrared Galaxies

“…The KS relation for starbursts and (ultra)-luminous infrared galaxies (U/LIRGS) has about the same ∼ 1.5 slope for CO as the total gas relation in galaxy disks (Kennicutt 1998;Gao & Solomon 2004;Krumholz et al 2012;Gowardhan et al 2017;Shi et al 2018). This is presumably because most of the gas in starbursts is dense enough to emit CO and that molecule is no longer a sparse tracer subject to selection effects.…”