The scenario of galaxy formation is believed to follow a structure that
builds up from the bottom, with large galaxies being formed by several merging
episodes of smaller ones. In this scenario a number of galaxies can be expected
to be seen in the merging phase, with their external regions already mixed,
while their nuclei, with stronger self-gravitation, are still recognizable as
such. During a photometric monitoring of AGNs in the field of a long-exposure
INTEGRAL pointing, we serendipitously found an elliptical galaxy in the center
of the X-ray cluster (EXO 0422-086) with two nuclei. We performed surface
photometry on our images and those of the SDSS archive and obtained slit
spectra of both nuclei. Aperture photometry of the two stellar-like nuclei
showed very similar colors in the SDSS image and in our Johnson BVRI images,
which is typical of an elliptical galaxy nucleus. The spectra of the nuclei
showed the typical absorption lines of an elliptical galaxy without appreciable
emission lines. The redshifts derived from each nucleus were equal and fully
consistent with the literature value (0.0397). We can therefore exclude the
possibility that one of the nuclei is a foreground star or a background AGN and
consider this elliptical galaxy as a bona fide example of a galaxy merger.Comment: 4 pages, 3 figures. Accepted on Astronomy and Astrophysic

Bassani, Loredana

Fiocchi, Mariateresa

Nesci, Roberto

Parisi, Pietro

Astronomy and Astrophysics

English

arXiv

The scenario of galaxy formation is believed to follow a structure that builds up from the bottom, with large galaxies being formed by several merging episodes of smaller ones. In this scenario a number of galaxies can be expected to be seen in the merging phase, with their external regions already mixed, while their nuclei, with stronger self-gravitation, are still recognizable as such. During a photometric monitoring of active galactic nuclei (AGNs) in the field of a long-exposure INTEGRAL pointing, we serendipitously found an elliptical galaxy in the center of the X-ray cluster (EXO 0422-086) with two nuclei. We performed surface photometry on our images and those of the SDSS archive and obtained slit spectra of both nuclei. Aperture photometry of the two stellar-like nuclei showed very similar colors in the SDSS image and in our Johnson BVRI images, which is typical of an elliptical galaxy nucleus. The spectra of the nuclei showed the typical absorption lines of an elliptical galaxy without appreciable emission lines. The redshifts derived from each nucleus were equal and fully consistent with the literature value (0.0397). We can therefore exclude the possibility that one of the nuclei is a foreground star or a background AGN and consider this elliptical galaxy as a bona fide example of a galaxy merger

FIOCCHI, MARIATERESA

BASSANI, LOREDANA

OA@INAF - Istituto Nazionale di Astrofisica

2015Publication Year2020-07-13T17:00:34ZAcceptance in OA@INAFDouble-nucleus elliptical MCG-01-12-005 in an X-ray emitting cluster of galaxiesTitleNesci, Roberto; FIOCCHI, MARIATERESA; BASSANI, LOREDANA; Parisi, PietroAuthors10.1051/0004-6361/201525713DOIhttp://hdl.handle.net/20.500.12386/26433HandleASTRONOMY & ASTROPHYSICSJournal576NumberAstronomy & Astrophysics manuscript no. 25713_ap_revised_2col c©ESO 2018November 10, 2018Double-nucleus elliptical MCG-01-12-005 in an X-ray emittingcluster of galaxies(Research Note)Roberto Nesci1, Mariateresa Fiocchi1, L. Bassani2, Pietro Parisi11 INAF-IAPS, via Fosso del Cavaliere 100, I-00033 Roma, Italiae-mail: roberto.nesci@iaps.inaf.it2 INAF-IASFBO via P. Gobetti 101, 40129 Bologna, Italia13 mar 2015ABSTRACTThe scenario of galaxy formation is believed to follow a structure that builds up from the bottom, with large galaxies being formedby several merging episodes of smaller ones. In this scenario a number of galaxies can be expected to be seen in the merging phase,with their external regions already mixed, while their nuclei, with stronger self-gravitation, are still recognizable as such. During aphotometric monitoring of AGNs in the field of a long-exposure INTEGRAL pointing, we serendipitously found an elliptical galaxyin the center of the X-ray cluster (EXO 0422-086) with two nuclei. We performed surface photometry on our images and those ofthe SDSS archive and obtained slit spectra of both nuclei. Aperture photometry of the two stellar-like nuclei showed very similarcolors in the SDSS image and in our Johnson BVRI images, which is typical of an elliptical galaxy nucleus. The spectra of the nucleishowed the typical absorption lines of an elliptical galaxy without appreciable emission lines. The redshifts derived from each nucleuswere equal and fully consistent with the literature value (0.0397). We can therefore exclude the possibility that one of the nuclei is aforeground star or a background AGN and consider this elliptical galaxy as a bona fide example of a galaxy merger.Key words. galaxies: clusters: individual –galaxies: evolution – galaxies: interactions – galaxies: elliptical and lenticular, cD –galaxies: nuclei1. IntroductionThe formation of elliptical galaxies is generally assumed to bethe result of several merging events, as demonstrated by severalsimulations (see, e.g., van Wasselhove et al. 2012; Ascaso et al.2014). As the timescale of this process is estimated to be short,catching a galaxy during the capture of a satellite is very rare:the definition of a sample of merging galaxies is therefore usefulto better constrain the details of the process from an observa-tional point of view (e.g., Darg et al. 2010a, 2010b; Hwang &Chang 2009). We observed the galaxy MCG-01-12-005 in thecluster EXO 0422-086 during an optical monitoring of a sampleof blazars included in the field of view of a deep exposure (3 Ms)of a field in the Orion region with the INTEGRAL satellite. Theaim of our data-rights proposal on this field was to take advan-tage of the long integration time to better measure the hard X-rayflux of the blazars serendipitously included in the field and thusto improve the measure of their spectral energy distribution: tothis purpose, we collected nearly simultaneous optical data withthe Loiano 1.5m and Asiago 1.8m telescopes. To select the tar-gets, we used the RomaBZCat catalog (Massaro et al. 2009),where this source is indicated as a possible blazar of uncertainclassification (BZU J0425-0833).We found two point-like sources in the center of the galaxy,aligned in the N-S direction, separated by just 3 arcsec, withsimilar magnitudes: in the Sky Surveys plates (both from the USand the UK Schmidt telescopes) the nucleus of the galaxy seemselongated N-S, but the two sources are not resolved because thecentral part of this galaxy is saturated.The presence of a double nucleus was not reported in anyprevious optical study; the galaxy is listed in the catalog of inter-acting galaxies (as VV 760) by Vorontsov-Velyaminov (1977)mainly because of the nearby spiral galaxy MCG-01-12-006(Bassani et al. 1999). Our galaxy is the brightest member of acluster, recognized as such only in 1993 as a result of its X-ray emission, which was detected with the EINSTEIN satellite(David et al 1993). Several subsequent papers were then devotedto studying the X-ray cluster properties, but few papers werededicated to an optical study of its central galaxy. De Grandiet al. (1999) only reported a redshift, taken from an ESO KeyProject. Bauer et al. (2000) performed a cross-correlation ofROSAT and NVSS catalogs, associating a radio flux of 114 mJywith the cluster. Belsole et al. (2005) performed a detailed spatialanalysis of the cluster and of its central galaxy at arcsec resolu-tion in X-rays with the Chandra satellite and in the radio (at 1.4,4.9 and 8.4 GHz) with the VLA, finding a complex radio and X-ray morphology in the very center of MCG-01-12-005, but usedPalomar Survey plates for a comparison with the optical mor-phology. Guzzo et al. (2009) reported the redshift of this galaxyin a comprehensive spectroscopic study of a sample of brightROSAT X-ray clusters (REFLEX survey, Boeringer et al. (2004)), but again with no mention of its double optical nucleus.We examined the images of this galaxy in the Sloan DigitalSky Survey (SDSS DR8 and later), which clearly showed the twonuclei, thanks to a spatial resolution and dynamic range muchbetter than the DSS plates; but they were not identified as sepa-rate sources, and the catalog lists only a single entry. We there-fore decided to perform a new photometric and spectroscopicArticle number, page 1 of 4arXiv:1503.04442v1  [astro-ph.GA]  15 Mar 2015A&A proofs: manuscript no. 25713_ap_revised_2colTable 1. Observations logdate Telescope filters/grism2013-01-31 Asiago BVRI2013-09-24 Loiano BVRI2013-12-09 Loiano grism-4, 2"2014-10-30 Loiano grism-4, 2"2014-10-30 Asiago grism-4, 1.69"2014-11-02 Asiago grism-4, 1.69"analysis of the galaxy, obtaining slit spectra of both optical nu-clei, to derive their redshift and understand their nature.We note that MCG-01-12-005 is not included in the GalaxyZoo merging galaxies catalog by Darg et al. (2010), which isbased on the SDSS DR6 release, nor is it listed in the merginggalaxies catalog by Hwang and Chan (2009).Darg et al. (2010) reported a fraction of about 3% for merg-ing galaxies, with a higher fraction of spiral or elliptical galaxiesthan in the normal field. In their scheme, our galaxy should bebest represented by the "approaching post-merger" class, wherethe progenitor cores are typically within 5 arcsec of each otherin the images, quite comparable to the case of MGC-01-12-005.This class contains only 10% of their merging galaxies sample,which are therefore the least common case.Hwang and Chan (2009) used an automatic algorithm tomeasure the isophotal shapes instead of employing human in-spection: they found that about 0.38% of galaxies are merg-ers. Their classificaton scheme include mergers, close pairs, andclose multiples, but they do not have a double nucleus subset,and their technique is not tailored to find them. No detailed X-ray or radio study of the merging galaxies is presented in any ofthese papers.As a final remark, we note that MCG-01-12-005 resemblesa cD galaxy, that is, a giant elliptical galaxy located near thecenter of a cluster of galaxies: cD galaxies are believed to havegrown so large through galaxy mergers that they spiral in to thecenter of the cluster. The merger hypothesis is supported by fre-quent observations of complex nuclei structures in cD galaxies:for example, Hoessel (1980) analyzed the central galaxies of 108nearby (z<0.1) Abell galaxies and found that 33 contain doubleor multiple nuclei. It is therefore not surprising that MCG-01-12-005 also shows a double nucleus. This source is interestingfor the wealth of optical radio and X-ray data that are availabledespite the confusion in the literature about the entire system(cluster misclassified as a blazar, MCG -01-12-005 misclassifiedas an AGN).2. ObservationsWe observed the galaxy with the Loiano 1.5m telescopeequipped with the Bologna Faint Object and Spectrograph Cam-era (BFOSC), using grism n.4 (dispersion 4.0 A/pixel, 2" slit), and the Asiago 1.8m equipped with the Asiago Faint Objectand Spectrograph Camera (AFOSC) grism n.4 (dispersion 5.0A/pixel, 1.69" slit). The slit was aligned with the two nuclei tocover both spectra in a single shot. Direct images were also ob-tained at Asiago with AFOSC in the BVRI Johnsons-Cousinsfilters, with a scale of 0.57"/pixel, and at Loiano with BFOSCwith a quite similar scale of 0.58"/pixel: all observations wereperformed in service mode.The observations log is collected in Table 1.2.1. AstrometryThe angular resolution of the SDSS images is appreciably betterthan that of our Asiago or Loiano images: we therefore usedthe best-exposed SDSS image (r filter) to derive the astrometryof the nuclei. The southern spot is at 04h 25m 51.33s -08 33’39.9", the northern spot at 51.31s 37.1". The astrometric solutionfor our Asiago and Loiano images using the GSC2.3.2 catalogprovided coordinates equal to those of the SDSS.The angular separation of 3" at the galaxy distance of 160Mpc (from standard cosmology) gives a distance between thesestructures of 2.4 kpc, if located on the plane of the sky.We compared these positions with those of the two radiospots detected at 4.9 GHz with the VLA reported in Belsole etal. (2005); the last digits for the coordinates are SW 51.238s37.51", NE 51.325s 35.96". The radio spots are therefore alignedat about 45 degrees from the N-S direction, separated by only 2":the optical northern nucleus is nearly between the radio spots,while the optical southern nucleus is definitely located away(south) from them.We remark that a systematic shift of just 1" of the VLA radioimage with respect to the optical image would be enough to putthe optical source between the two radio spots. Unfortunately,there are no other strong X-ray and radio sources in the field tocheck for possible systematic offsets between the coordinates ofthe optical, X-ray, and radio sources at arcsec accuracy. If thedouble radio source is related to the central black hole in one ofthe cores, then most likely it must be in the northern optical spot.We show in Fig. 1 the radio and X-ray map of the galaxy centertaken from Fig. 2 by Belsole et al. (2005) with the positions ofthe optical nuclei overplotted.We then tried to estimate the position of the galaxy cen-ter from the isophotal lines of the galaxy: an isophotal map,built with SAOImage from the r image of the SDSS, is given inFig. 2. To determine the light barycenter, we excluded the innerisophote, which is dominated by the two bright point sources,and used the subsequent five isophotes: The centers of the sec-ond, third, and fourth isophote are definitely on the northernspot, while the centers of the sixth and seventh (more external)isophotes are located between the two spots.Overall, we conclude that the light barycenter of the galaxy ismore consistent with the northern spot, which also has a slightlymore diffuse appearence (FWHM of 6 instead of 4 pixels), asexpected for the core of an elliptical galaxy.If the galaxy is not at rest with respect to the intraclustermedium (ICM), as found for the central galaxy in a number ofclusters by Coziol et al. (2009), and if the radio source is an-chored to the ICM, then the shift between the optical nucleusand radio source might have been produced in just a few millionyears.2.2. SpectroscopyWe extracted the spectra of the two nuclear sources withIRAF/apall, using a slit length of 3.4" (±2 pixels), which is com-parable to the FWHM. Wavelength calibration was made withan Hg-Cd lamp (Asiago) or He-Ar (Loiano), as is routinely usedin these observatories. The Asiago and Loiano spectra gave verysimilar results, making us confident of their physical interpreta-tion.The two nuclei only showed absorption lines, as is typicalof the stellar population of an elliptical galaxy, as expected. Noemission lines were detected in either spectrum. A redshift of0.040 was derived using the CaII H-K , G-band, Mg-b triplet, andArticle number, page 2 of 4R. Nesci et al.: A double nucleus elliptical (RN)Fig. 1. Radio, X-ray, and optical map of the galaxy center. Black circlesare the optical nuclei, gray shades the Chandra X-ray emission, whitelines the radio isophotes.Fig. 2. Isophotes of MCG-01-12-005 from the r image of the SDSS.Levels are at 0.030 0.052, 0.090, 0.155, 0.269, 0.467, 0.808, and 1.400flux units.the NaI D doublet (blended), in full agreement with the resultsby Guzzo et al. (2009) and Huchra et al. (2012). The portionof the Asiago spectra including these lines is reported in Fig. 2,with the main absorption lines flagged.It is therefore evident that neither of the two nuclei is a back-ground QSO/BLLac or a foreground star: MCG-01-12-005 lookslike a bona fide giant elliptical galaxy with two nuclei.2.3. PhotometryThe public ugriz images of the SDSS DR8 were obtained on2007-02-07 with a scale of 0.40"/pixel. In this database, as wellas in the following DR9 and DR10, the photometry of the galaxyis flagged as unreliable, and the two nuclear sources are not re-ported as separate sources. Therefore we measured the SDSSmagnitudes of the two sources with iraf/apphot, using as photo-metric radius for all filters the average FWHM of the stellar im-ages; the sky value was taken from a corona with an inner radius45" and width 10", chosen to be substantially outside the galaxyboundary as evaluated from the visible image of the galaxy inthe r band. The mode of the histogram of the sky values was as-Fig. 3. Spectra of the southern (below) and northern (above) spots withthe main absorption lines, both giving a redshift of 0.04.Table 2. SDSS and Loiano aperture photometryFilter South North diff erru 19.97 20.42 0.45 0.10g 18.08 18.18 0.10 0.06r 17.03 17.11 0.08 0.05i 16.57 16.64 0.07 0.05z 16.16 16.21 0.05 0.07B 18.31 18.36 0.05 0.07R 16.83 16.89 0.06 0.04I 15.97 16.03 0.06 0.05sumed as the sky level in all cases. The same criteria were usedto perform the photometry in the B, R, and I Loiano images; inthe Asiago images the seeing was definitley poorer and the twosources were more blended, so we did not reduce their photom-etry. The results are collected in Table 2: Col. 1 is the filter, Col.2 the southern nucleus, Col. 3 the northern nucleus, Col. 4 thedifference, and Col. 5 the error, given by the rms deviation of thecomparison stars from the calibration linear fit.Detecting possible magnitude variations of each nucleus be-tween the two epochs (2007 and 2013) is challenging, both be-cause of the conversion between Sloan and Johnson-Cousins fil-ters and because of the different seeing conditions, which includea variable portion of the surrounding galaxy in the point source.However, given the lack of emission lines in their spectra, we donot expect that these nuclei are variable.The most remarkable photometric result from the SDSS im-ages is that the southern spot is much brighter in the u band thanthe northern spot, while at longer wavelengths the two sourceshave rather similar colors. This is confirmed by the BRI magni-tudes of the Loiano images. As we noted above, the radio sourceseems associated with the northern spot, suggesting that it mightbe an active galactic nucleus and therefore likely to have an UVexcess, while the observation gives an opposite result, which israther puzzling. A possible explanation is that the radio emissionis just a remnant of a past nuclear activity. However, the u-r colorArticle number, page 3 of 4A&A proofs: manuscript no. 25713_ap_revised_2colof both nuclei is ∼ 3, while it is generally lower than 1.4 in bonafide Blazars (see Massaro et al. 2012).The galaxy was barely detected by GALEX in the NUV band(2270 Å) at mag 19.49±0.14, but its angular resolution (5 arcsec)is insufficient to resolve the two sources. In the GALEX image,the nearby MCG-01-12-006 Seyfert galaxy is much brighter inthe UV (NUV=17.6, FUV=17.9).3. ConclusionsThe galaxy MCG-01-12-005 is the giant elliptical in the centerof the X-ray cluster EXO 0422-086. A detailed X-ray study ofthe cluster with Chandra (Belsole et al. 2005) ruled out the pres-ence of an X-ray emitting AGN, showing that the emission is dueto the intracluster medium and the interstellar medium of thecentral elliptical galaxy. Our optical observations showed twoclearly separated nuclei 3" that are located apart and are nearlyaligned in the N-S direction. The optical spectra of these nucleiare very similar, with no emission lines, showing the absorp-tion features typical of an elliptical galaxy: both sources havethe same redshift, z=0.04, consistent with previous data in theliterature, which did not mention these two nuclei. Images fromthe SDSS confirm the presence of the two nuclei. A morpho-logical study of the isophotes shows that the northern nucleus islocated in the light barycenter of the galaxy, suggesting that itis also in the mass center. A radio high-resolution image takenwith the VLA (Belsole et al. 2005) shows two small sources sep-arated by few arcsecs, nearly symmetric to the northern opticalnucleus.The southern nucleus is most likely the remnant of a satellitegalaxy with a velocity vector nearly in the plane of the sky. Inprinciple, it could also be interpreted as an independent galaxyof the cluster that by chance is located in front of the giant el-liptical: the isophotes do not show any trace of the external partof this putative galaxy, however, so we prefer the interpretationof a merger in the final phase, when the outskirts of the smallergalaxy have already been absorbed by the giant, while the corestill retains its shape because of its stronger self-gravity.Finally, our spectroscopic data confirm the lack of emissionlines in the galaxy, as already found by Guzzo et al. (2009).The (u-r) color for both nuclei is definitely higher than the typ-ical value for an AGN. We therefore conclude that there is noAGN currently active in MCG-01-12-005. The faint double ra-dio source might be the remnant of a past activity, now left offcenter by the peculiar motion of the galaxy in the intraclustermedium.Acknowledgements. We thank the Asiago and Loiano Observatories Time Allo-cation Committes for the allocated telescope times; Roberto Gualandi (Loiano)and Paolo Ochner (Asiago) for making the observations in service mode. Wethank the anonymous referee for useful suggestions.ReferencesAscaso, B., Lemaux, B.C., Lubin, L.M., et al. 2014, MNRAS 442, 589Bassani, L., Dadina, M, Maiolino, R., et al. 1999, ApJS 121, 473Bauer, F.E., Condon, J.J., Thuan, T.X., Broderick, J.J., 2000, ApJS 129, 547Belsole, E., Birkinshaw, M., Worrall, D.M., 2005, MNRAS 358, 120Boehringer, H., Schueker, P., Guzzo, L., et al. 2004, A&A 425, 367Coziol, R., Andernach, H., Caretta, C. A., Alamo-Martinez, K. A., Tago, E.,2009, AJ 137, 4795Darg, D.W., Kaviraj, S., Lintott, C.J., et al. 2010, MNRAS 401, 1043Darg D.W., Kaviraj, S., Lintott, C.J., et al. 2010, MNRAS 401, 1552David, L.P., Slyz, A., Jones, C. et al. 1993, ApJ 412, 479De Grandi, S., Boehringer H., Guzzo, L., et al. 1999, ApJ 514, 148Guzzo, L., Shuecker, P., Boeringer, H., et al. 2009, A&A 499, 357Hoessel J.G. 1980, ApJ 241 493Huchra, J.P., Macri, L.M., Masters, K.L. et al. 2012, ApJS 199, 26Hwang, C.Y., & Chang, M.Y., 2009, ApJS 181, 233Massaro, E., Giommi, P., Leto, C. et al. 2009, A&A 495, 691Massaro, E., Nesci, R., Piranomonte, S. 2012, MNRAS 422, 2322Van Wasselhove, S., Volonteri, M., Mayer, L., et al. 2012, ApJ 748, L7Vorontsov-Velyaminov, B.A., 1977, A&AS 28, 1Article number, page 4 of 4

Double-nucleus elliptical MCG-01-12-005 in an X-ray emitting cluster of galaxies

Roberto Nesci

Mariateresa Fiocchi

Loredana Bassani

Pietro Parisi

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https://openaccess.inaf.it/bitstream/20.500.12386/26433/1/1503.04442.pdf

Double-nucleus elliptical MCG-01-12-005 in an X-ray emitting cluster of
  galaxies

Double-nucleus elliptical MCG-01-12-005 in an X-ray emitting cluster of galaxies

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