MNRAS 000,1–8 (2021) Preprint 19 February 2021 Compiled using MNRAS LATEX style file v3.0 X-raying the galaxy pair Arp 41: no collision in NGC 1232 and three ultraluminous sources in NGC 1232A Roberto Soria1,2 ¢, Manfred W. Pakull3 1College of Astronomy and Space Sciences, University of the Chinese Academy of Sciences, Beijing 100049, China 2Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, Sydney, NSW 2006, Australia 3 Université de Strasbourg, CNRS, Observatoire astronomique, CNRS, UMR 7550,F-67000, Strasbourg, France Accepted XXX. Received YYY; in original form ZZZ ABSTRACT We studied the apparent galaxy pair NGC 1232/NGC 1232A with Chandra, looking for evi- dence of interactions and collisions. We report that there is no cloud of diffuse emission in NGC 1232, contrary to previous claims in the literature. Instead, we find that the small “com- panion” galaxy NGC 1232A contains three ultraluminous X-ray sources with peak 0.3–10 keV luminosities above 1040 erg s−1 (assuming a cosmological distance of ≈93 Mpc for this galaxy). For its mass, morphology, metal abundance and bright ULX population, NGC 1232A is analogous to the more nearby late-type spiral NGC 1313. Key words: galaxies: individual: NGC 1232, NGC 1232A – accretion, accretion disks – stars: black holes – X-rays: binaries 1 INTRODUCTION cized claim2, revisiting the original data and including subsequent Chandra observations from 2012–2015. The population properties of X-ray binaries and ultraluminous X- Our second objective was to study the apparent companion, ray sources (ULXs) are a tracer of stellar mass and star formation NGC 1232A. Although at first sight this late-type barred spiral looks rate in their host galaxies (Lehmer et al. 2019). In turn, ULXs affect like a Magellanic-dwarf satellite of the main galaxy, it has a helio- the surrounding interstellar medium with their radiative power and centric recession speed of ≈(6600±45) km s−1 (Jones et al. 2009), mechanical luminosity, creating X-ray photo-ionized nebulae and/or which corresponds to a distance modulus of ¹34.84 ± 0.17º mag, shock-ionized bubbles (Pakull & Mirioni 2002). or a luminosity distance of ¹93 ± 7º Mpc (HyperLEDA database3). As part of this investigation, we are doing a long-term search This is the reason why NGC 1232 + NGC 1232A were cited by Arp for ULXs in star-forming galaxies of different morphological types. (1982) as one of the most striking examples of “discrepant redshift” The galaxy pair NGC 1232/NGC 1232A (Arp 41) is a particu- in galaxy pairs. Unfortunately, no reliable redshift-independent dis- larly interesting target. The larger galaxy in the pair is the face-on tance measurement exists for this galaxy. The peak rotational veloc- spiral NGC 1232, of Hubble type SABc, with a Hubble distance ity of the disk in NGC 1232A sometimes listed in galaxy databases = ¹21.3 ± 1.5º Mpc (based on its recession speed of ≈1450 km s−1 (e.g., HyperLEDA) is in fact meaningless (Fouque et al. 1990), be- with respect to the cosmic background), and a redshift-independent cause of contamination from the outer spiral arm of NGC 1232. distance ≈ 14.5 Mpc. See the NASA/IPAC Extragalactic Database Therefore, this value cannot be used for Tully-Fisher estimates of (NED1) for the full reference list of those two distance values; for distance and baryonic mass. the redshift-independent distance, we took the median value of 19 different measurements reported in NED, mostly based on the Tully- Fisher relation (Tully & Fisher 1977). In this paper, we will use the arXiv:2102.08961v1 [astro-ph.GA] 17 Feb 2021 value of 14.5 Mpc. A study based on three Chandra observations 2 OBSERVATIONS AND DATA ANALYSIS from 2008–2010 claimed (Garmire 2013) that there was X-ray ev- The galaxy pair was observed by Chandra/ACIS on seven occa- idence for a minor collision, perhaps a dwarf satellite that passed sions between 2008 and 2015: six times with ACIS-I and once with through the disk of NGC 1232 and shock-ionized the gas around ACIS-S, for a total of 295 ks (Table 1). We downloaded the data the impact region. Our first objective was to test this widely publi- from the public archives, then reprocessed and analysed them with the Chandra Interactive Analysis of Observations (ciao) software ¢ Email: [email protected] (RS) 2 See, e.g., https://chandra.harvard.edu/photo/2013/ngc1232/ 1 https://ned.ipac.caltech.edu. 3 http://leda.univ-lyon1.fr © 2021 The Authors 2 R. Soria & M. W. Pakull NGC 1232 Table 1. Summary of the Chandra observations for NGC 1232/NGC 1232A. ObsID Instrument Obs. Date Exp. Time (ks) NGC 1232B 10720 ACIS-I 2008-11-03 47.0 10798 ACIS-I 2008-11-05 52.9 12153 ACIS-I 2010-09-29 50.0 121980 ACIS-S 2010-11-30 46.5 142361 ACIS-I 2012-06-13 48.9 Nucleus 15391 ACIS-I 2013-10-13 21.3 16486 ACIS-I 2013-10-14 28.3 17463 ACIS-I 2015-11-10 44.5 0 X-1 : Its field of view covers NGC 1232A but only half of NGC 1232. X-3 1: Its field of view covers only the north-east corner of NGC 1232, and X-2 does not include NGC 1232A. NGC 1232A Table 2. Diffuse emission in NGC 1232 2’ ~ 8.3 kpc :) :) 퐹 0 !1 NGC 1232 1 (kev) 2 (keV) Sector 0.5−2 0.5−2 . ¸0.23 . ¸0.13 2 . ¸1.13 . ¸0.31 0 30−0.09 0 82−0.11 West 4 24−0.97 1 15−0.26 . ¸0.94 . ¸0.25 NGC 1232B North 3 27−0.85 0 88−0.23 . ¸0.88 . ¸0.24 East 2 59−0.81 0 70−0.22 . ¸2.23 . ¸0.65 South 5 01−1.49 1 38−0.42 Nucleus 0: observed (absorbed) 0.5–2 keV surface brightness, in units of 10−15 erg cm−2 s−1 arcmin−2; 1: unabsorbed 0.5–2 keV surface luminosity, in units of 1038 erg s−1 arcmin−2, assuming a distance of 14.5 Mpc; X-1 X-3 2: the alleged hot cloud centre (red circle in Garmire 2013). X-2 NGC 1232A the spectra to 1 count per bin. Finally, we modelled the spectra with xspec (Arnaud 1996) version 12.11.0, fitting them with the Cash 2’ ~ 8.3 kpc statistics (Cash 1979). As a further check of our main results, we rebinned the same spectra to ¡15 counts per bin and re-fitted them 2 Figure 1. Top panel: stacked Chandra/ACIS colour image of NGC 1232 and with the j statistics. NGC 1232A. Red = 0.3–1.2 keV, green = 1.2–2.6 keV, blue = 2.6–7 keV. North is up and east to the left. The outline of the optical galaxies (roughly corresponding to the D25 isophotes) is overplotted in magenta. The X-ray 3 MAIN RESULTS nucleus of NGC 1232, the ULXs in NGC 1232A, and the background star- burst galaxy NGC 1232B are labelled. Bottom panel: Digitized Sky Survey 3.1 No evidence of galaxy collisions in NGC 1232 image of the system, with the Chandra sources overplotted. We analyzed the stacked ACIS-I images, looking for an excess of diffuse hot plasma emission to the west and to the north of the version 4.12 (Fruscione et al. 2006), Calibration Database 4.9.1. nucleus, as claimed by Garmire(2013). Neither the distribution of In particular, we used the ciao tasks chandra_repro to create new count rates or the fluxes measured with srcflux suggest any such level-2 event files, reproject_obs to create stacked images, srcflux excess. However, we do notice that our stacked images contain to determine model-independent fluxes, and specextract to create many more point sources (obviously, because they are deeper than spectra and associated response and ancillary response files (both the images available to Garmire 2013). Several well-resolved point- for individual observations and combined for all datasets). like sources were just below the signal-to-noise threshold of the More specifically, when we extracted the spectra of extended adaptive-smoothing task csmooth used by Garmire(2013), based on regions (for the study of the diffuse emission in NGC 1232), we built the first three Chandra observations alone. As a result, we suspect spatially weighted ancillary response functions (specextract param- that csmooth treated the photons associated to those faint sources as eter “weight = yes”) without a point-source aperture correction diffuse emission, and smeared them into a large cloud, whose size (specextract parameter “correctpsf = no”). Conversely, when we ex- is a function of the smoothing-scale parameter. tracted the spectra of point-like sources (the ULXs in NGC 1232A), To investigate the matter in more details, we reproduced the we set ”weight = no” and “correctpsf = yes”. Our definition of the circular extraction region used by Garmire(2013) (red circle in our extended source and background regions in NGC 1232 is described Figure 2, corresponding to the red circle in their Fig. 2, with a radius in details in Section 3.1. For the point-like sources in NGC 1232A, of 4000). However, in our case, we removed several point sources we used source extraction circles of 300 radius, and local background inside the large circle, because we are looking only for the diffuse annuli at least 4 times the size of the source regions. hot gas. The detection threshold for our identification of point-like We then used the ftools (Blackburn 1995) task grppha to rebin sources was 12 net counts (0.3–7 keV band) in the stacked ACIS-I MNRAS 000,1–8 (2021) ULXs in NGC 1232A 3 NGC 1232 N −1 10−3 keV −1 10−4 −5 E normalized counts s 10 500 C−Statistic ∆ 0 1’ ~ 4.2 kpc 0.5 1 2 Energy (keV) Figure 3.