Volume 627, July 2019
|Number of page(s)||19|
|Published online||09 July 2019|
CO observations of major merger pairs at z = 0: molecular gas mass and star formation⋆
Departamento de Física Teórica y del Cosmos, Universidad de Granada, Spain
2 Instituto Carlos I de Física Téorica y Computacional, Facultad de Ciencias, 18071 Granada, Spain
3 National Astronomical Observatories, Chinese Academy of Sciences, 1000012 Beijing, PR China
4 South American Center for Astronomy, CAS, Camino El Observatorio 1515, Las Condes, Santiago, Chile
5 Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, PR China
6 Georgia College & State University, CBX 82, Milledgeville, GA 31061, USA
7 School of Space Science and Physics, Shandong University, 264209 Weihai, Shandong, PR China
8 Shandong Provincial Key Laboratory of Optical Astronomy & Solar-Terrestrial Environment, 264209 Weihai, Shandong, PR China
9 Department of Astronomy, University of Massachusetts, Amherst, MA 01002, USA
10 Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, 100871 Beijing, PR China
Accepted: 28 May 2019
We present CO observations of 78 spiral galaxies in local merger pairs. These galaxies represent a subsample of a Ks-band-selected sample consisting of 88 close major-merger pairs (HKPAIRs), 44 spiral–spiral (S+S) pairs, and 44 spiral–elliptical (S+E) pairs, with separation <20 h−1 kpc and mass ratio <2.5. For all objects, the star formation rate (SFR) and dust mass were derived from Herschel PACS and SPIRE data, and the atomic gas mass, MHI, from the Green Bank Telescope HI observations. The complete data set allows us to study the relation between gas (atomic and molecular) mass, dust mass, and SFR in merger galaxies. We derive the molecular gas fraction (MH2/M*), molecular-to-atomic gas mass ratio (MH2/MHI), gas-to-dust mass ratio and SFE (= SFR/MH2) and study their dependences on pair type (S+S compared to S+E), stellar mass, and the presence of morphological interaction signs. We find an overall moderate enhancement (∼2×) in both molecular gas fraction (MH2/M*) and molecular-to-atomic gas ratio (MH2/MHI) for star-forming galaxies in major-merger pairs compared to non-interacting comparison samples, whereas no enhancement was found for the SFE nor for the total gas mass fraction ((MHI + MH2)/M*). When divided into S+S and S+E, low mass and high mass, and with and without interaction signs, there is a small difference in SFE, a moderate difference in MH2/M*, and a strong difference in MH2/MHI between subsamples. For the molecular-to-atomic gas ratio MH2/MHI, the difference between S+S and S+E subsamples is 0.55 ± 0.18 dex and between pairs with and without interaction sign 0.65 ± 0.16 dex. Together, our results suggest that (1) star formation enhancement in close major-merger pairs occurs mainly in S+S pairs after the first close encounter (indicated by interaction signs) because the HI gas is compressed into star-forming molecular gas by the tidal torque; and (2) this effect is much weakened in the S+E pairs.
Key words: galaxies: evolution / galaxies: general / galaxies: interactions / galaxies: starburst / ISM: molecules
A copy of the reduced spectra and full Tables 1 and 2 are available at the CDS via anonymous ftp to cdsarc.ustrasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/627/A107
© ESO 2019
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