Relation of the Jet in M 87 to nearby Galaxies in the Virgo Cluster

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1 J. Astrophys. Astr. (1986) 7, Relation of the Jet in M 87 to nearby Galaxies in the Virgo Cluster Halton Arp* Mount Wilson and Las Campanas Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA , USA Received 1985 November 16; accepted 1986 January 8 Abstract. The massive and active galaxies in the core of the Virgo Cluster were shown in 1968 to be aligned on either side of Μ 87, along the direction of the jet and counter-jet. Recent observations confirm the significance of this alignment by showing that the brightest X-ray sources, including additional large galaxies, define the same line. Detailed X-ray maps of Μ 86 show that this galaxy, which is part of the alignment, is probably blown by a wind from Μ 87 due to its close alignment with the jet. But the large radio Ε galaxy, Μ 84, which is 1.4 degrees away from Μ 87, and exactly along the line of the jet, is shown by the compression of its X-ray isophotes to be actually moving out along the line of the jet. This evidence furnishes rather spectacular support for the earlier conclusion that Μ 84 had originated as a proto-galaxy within Μ 87 and had been ejected out along the line of the jet. Key words: galaxies, jets radio sources X-ray sources Virgo Cluster The typical giant radio galaxy consists of an elliptical (E) galaxy with radio lobes and jets extending far out in space on either side of the central galaxy. As soon as enough of these objects were mapped it became apparent that companion galaxies in the vicinity of the central Ε tended to be aligned along these directions of radio ejection. In 1968 all the strongest radio sources that were associated with Ε galaxies of bright apparent magnitude were studied (Arp 1968). The investigation revealed that ten of the thirteen radio E's turned out to be members of well marked lines of galaxies. In six cases the line of components of the radio emission was well defined. In five of these latter cases this direction of the radio components was coincident with the line of galaxies. Since lines of galaxies cannot arise from an originally disordered cluster the galaxies must be formed along the lines. It was concluded in 1968 that the only possible explanation of these observations was that the aligned galaxies had originated as protogalaxies within the central, large Ε and been ejected out along the line of the radio ejection. This strongly established observational phenomenon has subsequently gone quite unremarked however, undoubtedly because the conclusion to which it so inexorably leads is so foreign to current assumptions about galaxy formation. It is important to realize, however, that a decade earlier Ambartsumian (1958,1961, 1965) had independently advanced the conclusion that galaxies ejected material and * Alexander von Humboldt Senior Scientist awardee at the Max-Planck-Institut für Astrophysik, Garching, FRG.

2 72 H. Arp that some of this material formed new galaxies. The observational demonstrations of this process which have accumulated in the ensuing years (see Arp 1978 for review) attest to the fundamental importance of Ambartsumian's original insight. One of the most crucial galaxy alignments is shown below in Fig. 1. All the Ε galaxies in this densest region of the Virgo cluster were classified by de Vaucouleurs in They are shown by filled circles and the position angle of the jet by a dashed line. Since the mass-to-light ratio (M/L) is the greatest for Ε galaxies, and since these are generally the brightest galaxies in the vicinity, this result tells us that the most massive galaxies in the core of the Virgo Cluster are well aligned along the jet and (approximate) counter-jet direction. The arrows point to the two brightest radio sources in the region, Μ 87 and Μ 84, and so we see that the most active galaxies, in a radio sense, also fall along this line. The reality of this line of galaxies is so important that its confirmation by recently published X-ray observations is one of the major points of this note. As Fig. 2 shows, the brightest X-ray sources in this region, which are generally different galaxies than pictured in Fig. 1, independently and very accurately confirm this same alignment that was so striking in The identification of these X-ray sources has been drawn from the contribution of Forman, Jones & de Faccio (1984) to the ESO Workshop on the Virgo Cluster. In their partially complete mosaic X-ray map of the Virgo Cluster the large, diffuse X-ray sources which comprise Μ 87 and Μ 86 are very conspicuous. These two galaxies almost fill in the region between them with X-rays. But the next three brightest X-ray sources in the area are moderately compact and fall almost exactly along the line previously defined by the jet in Μ 87. These additional X-ray sources are plotted as small crosses in Fig. 2 and identified in Table 1. Table 1 lists all the Ε galaxies in the region as defined by Sandage & Tamman (1984) in the Revised Shapley-Ames Catalog. Two fainter Ε galaxies have been omitted from de Vaucouleurs' original listing and some classifications have been changed from Ε to S0 or vice versa. The 11 Ε galaxies listed in Table 1 give an even tighter definition of the Figure 1. Filled circles represent all E galaxies in pictured region as classified by de Vaucouleurs (1961). Strongest radio sources are marked by arrows. Diagram adopted from Arp (1968).

3 M 87 jet and nearby galaxies 73 Figure 2. Symbols represent strongest X-ray source found in partially complete, mosaic X-ray map of the Virgo Cluster by Forman, Jones & de Faccio (1984). Large symbols represent extended X-ray sources, and smaller symbols the compact ones. Table 1. Galaxies in the Μ 87 line*. * All Ε galaxies in the area defined by Sandage & Tamman (1981) and strong X-ray sources from Forman, Jones & de Faccio (1984).

4 74 H. Arp line through Μ 87 than 1961 de Vaucouleurs, classification shown in Fig. 1. Brighter X ray galaxies are also identified in Table 1. Since this line of galaxies, radio sources and X-ray sources corresponds so closely to the line of the famous jet which originates from the centre of Μ 87, we could not help but examine with great interest the extended X-ray isophotes around some of the galaxies in this alignment which have also recently become available (Forman, Jones & de Faccio 1984). Fig. 3 shows the X-ray isophotes around Μ 86, one of the galaxies falling closely along the line to the northeast. It is evident that the outer X-ray isophotes are blowing very closely in a direction away from the line going back to Μ 87. The inner isophotes suggest ejection of X-ray material initially almost north-south. But as soon as the X-ray material emerges further from Μ 86 it clearly becomes elongated along a line almost exactly away from Μ 87. The position angle of Μ 86 from Μ 87 is p.a. = 296, only about 6 degrees greater than the position angle of the jet in Μ 87, that is more in the direction of the 'fan jet' which comes out more broadly from Μ 87 (Walker 1968; Ford & Butcher 1979). The position angle of the jet has always been taken as p.a. = 290. For the optical jet de Vaucouleurs & Nieto (1979) measured p.a. = ± 0.4. But the optical knot A is slightly north of the general line of the jet. Measures of the centre line of the radio jet, as in Fig. 4, gives p.a. = 290 ± 1. Now the astonishing fact about the bright radio Ε galaxy, Μ 84, is that it is at p.a. = from Μ 87, that is within the accuracy of measurement, exactly along the line of the Μ 87 jet. The fact that Μ 84 at 1.4 degrees away on the sky lies exactly along the line of the Μ 87 jet was first pointed out by Wade (1960). The alignment of Ε galaxies along this line was discussed by Arp (1968) and the exact alignment of Μ 84 with the Μ 87 jet was again emphasized by Burbidge & Burbidge (1969). But now Fig. 4 shows the most astonishing fact of all. The high resolution, HRI X-ray map of Μ 84 demonstrates that it is moving through the inter galactic medium of the Virgo Cluster almost exactly along this same line from Μ 87. The originating X-ray astronomers noted that Μ 84 appeared to be moving westward through the cluster medium. Fig. 4 shows that this motion is not quite west, more exactly WNW, or along the line of the jet. The inner X- ray isophotes indicate a direction about p.a. = 280 and the outer isophotes a direction about p.a. = 300. It seems reasonable to adopt the motion of Μ 84 to be p.a. = 290 ±10. In order to estimate a rough probability of these coincidences happening by chance we take the maximum misalignment as the estimated accuracy of the angle determinations and compute But, of course, just visual judgement of the accuracy with which the jet is defined and is pointing toward Μ 84 and now the direction of motion of Μ 84 would seem to be adequate to establish the physical significance of the configuration. We should also remember that this observed phenomenon was predicted. In 1968 it was already stated that the observations showed: 'The only alternative... to be the ejection of the galaxies, or progenitors of the galaxies, from an initially large central galaxy., It was always clear that the high surface brightness optical knots seen in the jet of Μ 87 represented probable examples of such protogalaxies (see review in Arp 1978).

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7 Μ 87 jet and nearby galaxies 75 Additional remarks should include the fact that the dispersion of measured redshifts of the galaxies which define the lines such as exhibited in Fig. 1, would indicate the lines cannot persist for long times. In a time about two orders of magnitude less than the usually accepted ages of the galaxies, the lines should be disrupted. Since the lines visibly exist, this may be an indication that some components of the measured galaxy redshifts are non-velocity. There is, of course, abundant other evidence now available for non-velocity redshifts in galaxies (Arp 1982, 1985, 1986). Μ 86 is particularly interesting in this regard because it is one of the six galaxies in the sky with appreciably negative redshifts. All six are concentrated in this small, central region of the Virgo, cluster so there can be hardly any question of their membership. (Two are X-ay sources in the line defined in Fig. 2.) The fact that Μ 86 is blown by a wind from Μ 87, or falling toward Μ 87 though the medium, is additional proof of its membership in the core of Virgo Cluster despite its large negative redshift (cz 0 = -367kms -1 ). On the other hand the galaxy Μ 84 appears to be traveling with the jet wind and passing Μ 86 on the way out. Μ 84 may be travelling faster than the general wind in that direction. In a model where the galaxies were moving along the line, and the line is tilted away from us on the NW and towards us on the SE, the eight E's (excluding Μ 84 and Μ 86) would be moving outward from Μ 87 with ~ 660 km s -1 and the four X-ray galaxies inward toward Μ 87 with Δv ~ 860 km s -1. Finally there is a newly discovered quasar which falls very close to Μ 84 as indicated in Fig. 4. The probability of this being a chance occurrence is only p Altogether there is an association of quasars with the bright Ε galaxies in the M87 alignment that has only about 10 4 chance of being accidental (see the immediately following paper). This reinforces our conclusion that redshift values of the objects ejected from Μ 87, or from objects in the Μ 87 line, cannot be necessarily interpreted as velocity redshifts. References Ambartsumian, V.A.1958, OnziemeConseilde PhysiqueSolvay,Ed.R. Stoops,Bruxelles. Ambartsumian, V.A.1961, Astr.J., 66, 536. Ambartsumian, V. Α.1965, in The Structure and Evolution of Galaxies, Interscience,New York, p. 1. Arp,H.1968, Publ. astr. Soc. Pacific, 80,129. Arp, H. 1978, Problems of Physics and Evolution of the Universe, publ. Armenian. Acad. Sci. Arp, Η. 1982, Astrophys,J., 263,54. Arp, Η. 1986, Quasars,Redshift sand Controversies, Arp, Η.,Sulentic,J.W.1985, Astrophys. J.,291, 88. Burbidge, G. R., Burbidge, E. M. 1969, Nature, 224, 21. de Vaucouleurs, G.1961, Astrophys. J. Suppl. Ser.,6,213. de Vaucouleurs, G., Nieto, J.L Astrophys. J., 231, 364. Ford,Η.C,Butcher,H.1979,Astrophys. J. Suppl. Ser.,41,147. Forman, W, Jones, C,de Faccio, M.1984, in The Virgo Cluster of Galaxies, ESO Conference and workshop, No. 20, p Sandage, A. R., Tamman, G. A. 1981, A Revised Shapely-Ames Catalog of Bright Galaxies, Carnegie Inst. Washington. Wade, C M. 1960, Observatory, 80,235. Walker, M. F. 1968, Astrophys. Lett., 2,65.