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Register a new userThe Effect of Galaxy Interactions on Molecular Gas Properties
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Galaxy interactions are often accompanied by an enhanced star formation rate (SFR). Since molecular gas is essential for star formation, it is vital to establish whether, and by how much, galaxy interactions affect the molecular gas properties. We investigate the effect of interactions on global molecular gas properties by studying a sample of 58 galaxies in pairs and 154 control galaxies. Molecular gas properties are determined from observations with the JCMT, PMO, CSO telescopes, and supplemented with data from the xCOLD GASS and JINGLE surveys at $^{12}$CO(1-0) and $^{12}$CO(2-1). The SFR, gas mass ($M_mathrm{H_{2}}$), and gas fraction ($f_{gas}$) are all enhanced in galaxies in pairs by $sim$ 2.5 times compared to the controls matched in redshift, mass, and effective radius, while the enhancement of star formation efficiency (SFE $equiv$ SFR/$M_{H_{2}}$) is less than a factor of 2. We also find that the enhancements in SFR, $M_{H_{2}}$ and $f_{gas}$ increase with decreasing pair separation and are larger in systems with smaller stellar mass ratio. Conversely, the SFE is only enhanced in close pairs (separation $<$ 20 kpc) and equal-mass systems; therefore most galaxies in pairs lie in the same parameter space on the SFR-$M_{H_{2}}$ plane as controls. This is the first time that the dependence of molecular gas properties on merger configurations is probed statistically with a relatively large sample and with a carefully-selected control sample for individual galaxies. We conclude that galaxy interactions do modify the molecular gas properties, although the strength of the effect is merger configuration dependent.
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We use a combination of new NOrthern Extended Millimeter Array (NOEMA) observations of the pair of [CI] transitions, the CO(7-6) line, and the dust continuum, in addition to ancillary CO(1-0) and CO(3-2) data, to study the molecular gas properties of Q1700-MD94, a massive, main-sequence galaxy at $zapprox2$. We find that for a reasonable set of assumptions for a typical massive star-forming galaxy, the CO(1-0), the [CI](1-0) and the dust continuum yield molecular gas masses that are consistent within a factor of $sim2$. The global excitation properties of the molecular gas as traced by the [CI] and CO transitions are similar to those observed in other massive, star-forming galaxies at $zsim2$. Our large velocity gradient (LVG) modeling using RADEX of the CO and [CI] spectral line energy distributions (SLEDs) suggests the presence of relatively warm ($T_{rm kin}=41$K), dense ($n_{rm H_2}=8times10^{3}~{rm cm}^{-3}$) molecular gas, comparable to the high-excitation molecular gas component observed in main-sequence, star-forming galaxies at $zsim1$. The galaxy size in the CO(1-0) and CO(7-6) line emission are comparable, which suggests that the highly-excited molecular gas is distributed throughout the disk powered by intense star formation activity. To confirm this scenario will require spatially resolved observations of the CO and [CI] lines which can now be obtained with NOEMA upgraded capabilities.
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