H i Observations of Major-merger Pairs at z = 0: Atomic Gas and Star Formation

Abstract

We present a study of the Hi gas content of a large K-band selected sample of 88 close major-merger pairs of galaxies (H-KPAIR) which were observed by Herschel . We obtained the 21 cm Hi finestructure emission line data for a total of 70 pairs from this sample, by observing 58 pairs using the Green Bank Telescope (GBT) and retrieving the Hi data for an addition 12 pairs from the literature. In this Hi sample, 34 pairs are spiral-spiral (S+S) pairs, and 36 are spiral-elliptical (S+E). Based on these data, we studied the Hi-to-stellar mass ratio, the Hi gas fraction and the Hi star formation efficiency (SFE Hi = star formation rate/M Hi ) and searched for differences between S+S and S+E pairs, as well as between pairs with and without signs for merger/interaction. Our results showed that the mean Hi-to-stellar mass ratio of spirals in these pairs is = 7.6 ± 1.0%, consistent with the average Hi gas fraction of spiral galaxies in general. The differences in the Hi gas fraction between spirals in S+S and in S+E pairs, and between spirals in pairs with and without signs of merger/interaction are insignificant (< 1σ). On the other hand, the mean SFE Hi of S+S pairs is ∼ 4.6× higher than that of S+E pairs. This difference is very significant (∼ 4σ) and is the main result of our study. There is no significant difference in the mean SFE Hi between galaxies with and without signs of merger/interaction. The mean SFE Hi of the whole pair sample is 10 −9.55±0.09 yr −1 , corresponding to a Hi consumption time of 3.5 ± 0.7 Gyrs.

NOEMA Observations of CO Emission in Arp 142 and Arp 238

“…Previous observations of the SFR (Cao et al 2016) and the molecular (Lisenfeld et al 2019) and atomic (Zuo et al 2018) gas content of the H-KPAIR sample have shown pronounced differences between S+E and S+S pairs: The sSFR…”

“…There is a large uncertainty for the conversion factor α CO , in particular for (U)LIRGs (Bolatto et al 2013;Downes & Solomon 1998). We compare the molecular gas mass estimated using the adopted α CO with the HI mass (Zuo et al 2018) and dust mass (Cao et al 2016) for Arp 142 and Arp 238, in order to constrain the effect of this uncertainty to our results. As shown Table 2, the dust-to-gas ratios of the two pairs are 0.0095 ± 0.0024 and 0.0076 ± 0.0017, both consistent with the value (∼ 0.007) of Draine et al (2007) obtained for local spiral galaxies.…”

“…It was suggested (Park & Choi 2009;Hwang et al 2011) that the lack of star-formation enhancement in S+E pairs could be due to stripping of cold gas of the spiral component by ram-pressure of the hot-gas halo surrounding the elliptical component. But this hypothesis is rejected by the results of Zuo et al (2018) and Lisenfeld et al (2019). Lisenfeld et al (2019) carried out IRAM CO observations for 78 spiral galaxies selected from the H-KPAIR sample of 88 close major-merger pairs that have Herschel FIR observations (Cao et al 2016).…”

“…Lisenfeld et al (2019) carried out IRAM CO observations for 78 spiral galaxies selected from the H-KPAIR sample of 88 close major-merger pairs that have Herschel FIR observations (Cao et al 2016). Combining with the GBT HI observations of Zuo et al (2018) for pairs selected from the same H-KPAIR sample, Lisenfeld et al (2019) found no significant difference between the total gas abundances of star-forming galaxies in S+E and in S+S pairs. Indeed, their results show that the reason for spiral galaxies in S+E pairs to have a significantly lower sSFR than their counterparts in S+S pairs (Xu et al 2010;Cao et al 2016) is because they have a significantly lower molecular-to-total-gas ratios (M H2 /(M H2 + M HI )) and a lower star-formation efficiency (SFE = SFR/M H2 ).…”

Central star formation in double-peak gas rich radio galaxies

“…It is still an open question whether the neutral hydrogen gas fraction changes in merging systems: some studies find little differences in close galaxy pairs (e.g. Zuo et al 2018;Braine & Combes 1993) or post-merger galaxies (e.g. Ellison et al 2015), others find an enhancement of the atomic gas fraction in recently merged galaxies (Huchtmeier et al 2008;Ellison et al 2018) or even a deficit in the final stages of merging (Hibbard & van Gorkom 1996).…”