Noema Observations of Co Emission in Arp 142 and Arp 238
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Accepted Version;JUNE 30, 2021 Preprint typeset using LATEX style emulateapj v. 12/16/11 NOEMA OBSERVATIONS OF CO EMISSION IN ARP 142 AND ARP 238 C. K. XU1,2 , U. LISENFELD3,4 , Y. GAO5,6 , F. RENAUD7 (Received March 14, 2021; Accepted June 27, 2021) Accepted Version; June 30, 2021 ABSTRACT Previous studies have shown significant differences in the enhancement of the star-formation rate (SFR) and the star-formation efficiency (SFE = SFR=Mmol) between spiral-spiral and spiral-elliptical mergers. In order to shed light on the physical mechanism of these differences, we present NOEMA observations of the molecular gas distribution and kinematics (linear resolutions of ∼ 2 kpc) in two representative close major-merger star- forming pairs: the spiral-elliptical pair Arp 142 and the spiral-spiral pair Arp 238. The CO in Arp 142 is widely distributed over a highly distorted disk without any nuclear concentration, and an off-centric ring-like structure is discovered in channel maps. The SFE varies significantly within Arp 142, with a starburst region (Region 1) near the eastern tip of the distorted disk showing an SFE ∼ 0:3 dex above the mean of the control sample of isolated galaxies, and the SFE of the main disk (Region 4) 0.43 dex lower than the mean of the control sample. In contrast, the CO emission in Arp 238 is detected only in two compact sources at the galactic centers. Compared to the control sample, Arp 238-E shows an SFE enhancement of more than 1 dex whereas Arp 238-W has an enhancement of ∼ 0:7 dex. We suggest that the extended CO distribution and the large SFE variation in Arp 142 are due to an expanding large-scale ring triggered by a recent high-speed head-on collision between the spiral galaxy and the elliptical galaxy, and the compact CO sources with high SFEs in Arp 238 are associated with nuclear starbursts induced by gravitational tidal torques in a low-speed coplanar interaction. Keywords: galaxies: interactions — galaxies: evolution — galaxies: starburst — galaxies: general 1. INTRODUCTION and Herschel show that only star-forming galaxies in spiral- It is well documented that mergers can trigger enhanced spiral (hereafter S+S) pairs have significantly enhanced sSFR, star-formation in galaxies (Kennicutt et al. 1987; Sanders & but not those in spiral-elliptical (hereafter S+E) pairs (Xu et al. Mirabel 1996). The most extreme starbursts, such as the ul- 2010; Cao et al. 2016). The low fraction of paired galaxies 12 with enhanced sSFR is often interpreted as due to the fact traluminous infrared galaxies (ULIRGs: LIR ≥ 10 L ), are usually found in the final stage of mergers (Sanders & Mirabel that strong starbursts triggered by interactions are "on" only 1996). Strong star-formation enhancements are also detected for short periods (∼ 100 Myr), while most time a merging in earlier merger stages, particularly in major-mergers (mass galaxy is in the "off" phase of the starburst (DiMatteo et al. ratio less than 3) during close encounters (Xu & Sulentic 2008). However, this interpretation cannot explain the non- 1991; Nikolic et al. 2004; Ellison et al. 2010; Scudder et al. enhancement of sSFR in star-forming galaxies in S+E pairs, 2012). On the other hand, only a small fraction of inter- which represent 34% of star-forming galaxies in a complete acting galaxies show significant star-formation enhancement sample of K-band selected close major-merger pairs (KPAIR, (Horellou et al. 1999; Bergvall et al. 2003; Knapen & James Domingue et al. 2009). It was suggested (Park & Choi 2009; 2009). Spitzer observations of a sample of K-band selected Hwang et al. 2011) that the lack of star-formation enhance- close major-merger pairs (Xu et al. 2010), which preferen- ment in S+E pairs could be due to stripping of cold gas of tially select early mergers during or near the first and second the spiral component by ram-pressure of the hot-gas halo sur- pericentric passages, found that only ∼ 25% of star-forming rounding the elliptical component. But this hypothesis is re- galaxies in the sample show strong enhancement in specific jected by the results of Zuo et al.(2018) and Lisenfeld et al. star-formation rate (sSFR = SFR=M , where SFR is the star- (2019). Lisenfeld et al.(2019) carried out IRAM CO observa- star tions for 78 spiral galaxies selected from the H-KPAIR sample formation rate in M yr-1 and M the stellar mass in M ). star of 88 close major-merger pairs that have Herschel FIR obser- Furthermore, the far-infrared (FIR) observations by Spitzer vations (Cao et al. 2016). Combining with the GBT HI obser- arXiv:2106.15041v1 [astro-ph.GA] 29 Jun 2021 vations of Zuo et al.(2018) for pairs selected from the same Email: [email protected] H-KPAIR sample, Lisenfeld et al.(2019) found no significant 1 Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, China difference between the total gas abundances of star-forming 2 National Astronomical Observatories, Chinese Academy of Sciences galaxies in S+E and in S+S pairs. Indeed, their results show (NAOC), 20A Datun Road, Chaoyang District, Beijing 100101, China that the reason for spiral galaxies in S+E pairs to have a sig- 3 Dept. Física Teórica y del Cosmos, Campus de Fuentenueva, Edificio nificantly lower sSFR than their counterparts in S+S pairs (Xu Mecenas, Universidad de Granada, E-18071 Granada et al. 2010; Cao et al. 2016) is because they have a signifi- 4 Instituto Carlos I de Física Tórica y Computacional, Facultad de Cien- cias, E-18071 Granada, Spain cantly lower molecular-to-total-gas ratios (MH2 =(MH2 +MHI)) 5 Department of Astronomy, Xiamen University, 422 Siming South and a lower star-formation efficiency (SFE = SFR=MH2 ). Road, Xiamen 361005, China In this article, we present NOEMA CO imaging observa- 6 Purple Mountain Observatory & Key Laboratory for Radio Astron- tions of two representative pairs: Arp 142 (S+E) and Arp 238 omy, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China (S+S). In the sample of Lisenfeld et al.(2019), the spiral com- 7 Department of Astronomy and Theoretical Physics, Lund Observatory, ponent of Arp 142 (NGC 2936) has the highest SFR among Box 43, SE-221 00 Lund, Sweden galaxies in S+E pairs. Arp 238-E (UGC8335-E), a luminous 2 C.K. XU ET AL. 11 infrared galaxy (LIRG: LIR ≥ 10 L ), has the second highest For both Arp 142 and Arp 238 we present integrated CO(1- SFR among galaxies in S+S pairs (see Table2). Interestingly, 0) maps, compared to Spitzer-IRAC (Xu et al. 2010) and HST the SFE in Arp 142 is ∼ 30 times lower than that in Arp 238- images 10, in Figure1. The total integrated CO fluxes, to- E(Lisenfeld et al. 2019). With the NOEMA observations, gether with other physical parameters, are presented in Ta- we aim to probe the cause of the strong difference between ble2. As shown in Figure1, the CO in Arp 142 is widely dis- SFEs of the two pairs, which may also shed light on the phys- tributed within a highly distorted disk of NGC 2936. While ical mechanism for the SFE difference between S+E and S+S the CO traces quite well the star-formation as traced by pairs in general. Spitzer (the red areas in Figure1b, which also coincide with the dust lanes in the Hubble images), there is no CO con- 2. OBSERVATIONS centration in the nucleus (Region 2 in Figure1b). The to- CO(1-0) was observed with the Northern Extended Mil- tal CO flux measured in the NOEMA map of Arp 142 (Ta- limeter Array (NOEMA) of the Institute of Radioastronomy ble2) is a factor of 1.35 higher than that detected by IRAM- -1 in the Millimeter (IRAM)8 in C and D array with 10 antennas 30m (175:3 ± 2:8 Jy km s , Lisenfeld et al. 2019), because the NOEMA measurement covers a significantly larger area (project W19BL). The observations were carried out under 00 good weather conditions. For each object we made a small than the IRAM beam (FWHM=22 ). Assuming the stan- -1 -1 2 mosaic consisting of two overlapping regions. The receiver dard conversion factor αCO = 3:2 M K km s pc (Bolatto covers two side-bands, each with a width of 7.744 GHz. The et al. 2013), the total molecular gas mass of Arp 142 is 10:29±0:02 autocorrelator PolyFix was used which has a channel width Mmol = 10 M . This is about a factor of 2 higher of 2 MHz (corresponding to 5.3 km s-1 at the frequency of than that of Bothwell et al.(2014) based on a CO(2-1) obser- our observations). The line frequency was tuned in the Upper vation of APEX, which has a beam (2700) significantly smaller Side Band (USB). Some basic parameters of the observations than the size of the CO emission (Figure1a). On the other are listed in Table1. hand, the Mmol is 0.39 dex lower than that estimated by Lisen- We reduced the data following standard procedures using feld et al.(2019), suggesting that the large aperture correction the GILDAS9 software. The data were calibrated using the (faper = 3:16) adopted by Lisenfeld et al.(2019) might have IRAM package Continuum and Line Interferometer Calibra- been over-estimated. tion (CLIC). The standard pipeline reduction and calibration For Arp 238, CO is detected only in two compact sources was followed to a large extent, only some poor data scans at the centers of the two galaxies.