J. Astrophys. Astr. (1990) 11, 411– 443 Galaxy Alignments Halton Arp Max-Planck-Institut für Physik und Astrophysik, Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 8046 Garching b. München, Germany Received 1989 November 15; revised 1990 July 16; accepted 1990 July 30 Abstract. It is well-known that galaxies tend to form elongated associ- ations stretching many degrees across the sky. It is shown here that especially galaxies of about 3000 to 5000 km s–1 redshift define narrow filaments of from 10 to 50° in length. The surprising feature is that galaxies of very bright apparent magnitude tend to occur at the centre or ends of these alignments. The 20 brightest galaxies in apparent magnitude north of δ = 0° are investigated here and of the 14 which are uncrowded by nearby bright galaxies, a total of 13 have well marked lines and concentrations of fainter, higher redshift galaxies. Key words: galaxies, alignments—galaxies, discordant redshifts 1. Introduction The most conspicuous linear concentration of galaxies in the sky is the so-called Perseus-Pisces supercluster (Gregory, Thompson & Tifft 1981; Giovanelli & Haynes 1982, 1985; Chincarini, Giovanelli & Haynes 1983). Starting from the Perseus cluster of galaxies, one can trace galaxies about 15 degrees due west and then, dis- continuously, for another 15 degrees to the southwest where their density diminishes and the feature becomes uncertain. All along this filament of galaxies the redshifts characteristically range between cz ≃ 4000 to 6000 km s–1. Other, similar strings and elongated distributions of galaxies are rapidly being discovered. There are already difficulties, however, with the standard interpretation of these filaments. The Perseus-Pisces galaxies form a filament of about two degrees in width over much of its more than 30 degrees in length. The observed dispersion in redshift shows a minimum range of from 4000 to 6000 km s–1. In a time equal to the supposed –1 age of the galaxies, random peculiar velocities of ± 1000 km s would widen the filament to ~ 40 Mpc, or more than ten times the measured value of ~ 3 Mpc. To hold these galaxies on a line with gravitational forces would require an extremely large amount of mass distributed linearly down the core of the filament. It seems we already encounter an insoluble paradox if we make the standard interpretation of the redshifts of these galaxies as velocities. In order to understand this alignment phenomenon better, galaxies in other regions of the sky are first investigated in the present paper. One listing of galaxies that can be used for this purpose is the Rood Catalogue. (Privately distributed in 1980, it lists all the galaxies with redshifts known to Rood at that time.) The galaxies in this catalogue are typically from such sources as Tully-Fisher and Bottinelli-Gougenheim (Η I 412 H. Arp redshifts) and Humason-Mayall-Sandage (predominantly absorption-line redshifts). Redshifts from many other sources are also listed. There is no claim to completeness of galaxies measured over any region of the sky but naturally bright galaxies are more completely surveyed than faint. For the purposes of the present analysis, however, incompleteness can only render the alignments and filaments less conspicuous than they really are. The reason for this is that if long lines of galaxies exist, it is implausible that observers would have unknowingly measured along them. Therefore any incompleteness in catalogues would only serve to degrade the lines. More complete surveys in the future would be expected to enhance these long linear concentrations. Another catalogue which is very useful is the survey of low surface brightness (LSB) galaxies by Bothun, Beers & Mould (1985). That survey encompasses galaxies of fainter apparent magnitude than does, for example the CFA survey (Huchra et al. 1983). It turns out that galaxies in a narrow redshift range but over a wide range in apparent magnitude operationally define the lines better than galaxies of all redshifts to a bright apparent magnitude limit. The two catalogues designated here as Rood and LSB will form the bulk of the source material for the investigations in the following paper. Investigations within these catalogues consisted of plotting large areas of the sky and examining whether galaxies of similar redshifts formed continuous chains or alignments. 2. The longest line of galaxies ⋝ ⋝ Fig. ⋜ 1 shows all the galaxies tested in the Rood Catalogue at northern declinations 0 h m h m δ α ⋝ ( 50 ) between 4 40 13 20 Only galaxies with redsh⋝ ifts corrected to the –1 –1 centre of the Local Group which are between 3100 km s cz0 5100 km s are ⋝ plotted ⋝ (see Table 1). The filled circles represent the narrower redshift range cz ⋝ 3100 0 3700 and give⋝ the most unique definition of the filament. The higher redshift range 3700 cz0 5100 also defines the major, long filament but in addition 0 h m furnishes a short filament at about δ≃ 6 5 and and α ≃ 7 30 one just north of δ= 50° h h between 8 < α < 9 . These shorter alignments will be discussed in the following section. The main concentration of galaxies in Fig. 1 is seen to run from α ≃ 6h, δ ≃ 73° to h 0 o α ≃ 13 , δ ≃ 54 This alignment of galaxies has about the same length ( ≳ 40 ) as the Perseus-Pisces superfilament and is comprised of galaxies of roughly similar redshift. The reality of the Perseus-Pisces filament is widely accepted and the configuration in Fig. 1 represents another example of this large scale chaining or filamentary structure in the Universe. Such structures have recently been discussed extensively (see for recent review separate papers by G. Chincarini, M. Geller & J. Einasto in IAU Symposium 124, Beijing 1986). In order to explore further the nature of these galaxy alignments we examine other regions of the sky. In Fig. 2 is shown the section of sky below the main filament in Fig. 1 (see Table 2).In this restricted range of redshift we see that the filament at the bottom edge of Fig. 1 is more conspicuous and angles to the southeast below δ = 50°. This is the same linear alignment of galaxies that was identified as the Lynx-Ursa Major supercluster by Giovanelli & Haynes (1982). It was also noted by Focardi, Morano & Vettolani (1986). In addition Fig. 2 shows a sparser filament slightly south of δ ≃ 35o. Galaxy alignments 413 Figure 1. All galaxies listed in the Rood redshift catalogue are plotted in the pictured area. The –1 filament is best defined by galaxies 3100 < v0 < 3700 km s (filled circles). Figure 2. An extension of a region southward from Fig. 1. All Rood listings in the designated redshift range are plotted. Upper filament was designated as the Lynx-Ursa Major supercluster by Giovanelli & Haynes (1982). 414 H. Arp ⋜ ⋝ ⋝ h m h m o –1 Table 1. Rood galaxies: 4 40 α 13 20 , δ 50 ; 3100 < cz0 < 5100 km s . Galaxy alignments 415 Table 1. Continued. * V means line through M81 (including M81-N2403 line) X denotes line through NGC 2841. blank denotes galaxy not aligned. There is an amazing property of these galaxy filaments, however, which bears not only on their reality but also on their origin. We proceed to the discussion of this. 3. Association with bright apparent magnitude galaxies Fig. 3 is exactly the same as Fig. 1 except that we have now plotted the three brightest galaxies in the area as open squares. We see that M81, the brightest galaxy in the sky outside the Local Group, falls at the centre of the largest superfilament in the sky outside the Local Group. The two shorter filaments present in Fig. 1 contain the second and third brightest apparent magnitude galaxies in this sector of the sky, NGC 2403 and NGC 2841. Further south than M81, Fig. 4 shows the same galaxy filaments as in Fig. 2 but the brightest galaxies in this region, NGC 2841 and NGC 2683, are now added as open 416 H. Arp Table 2. Galaxies aligned through NGC 2841 and 2683 Figure 3. The same plot as in Fig. 1 except the three brightest apparent magnitude galaxies in the region have now been added (see Table 6 for listing of brightest apparent magnitude spirals) Galaxy alignments 417 Figure 4. The same plot as in Fig. 2 except the two brightest galaxies in the region have been added. squares. Again these bright apparent magnitude galaxies fall near the midpoints of the fainter galaxies in the filaments. 3.1 Small Field Samples In Figs 1–4, galaxies obviously sampled only in small areas of the sky have been omitted when plotting from the Rood Catalogue. One such limited region is the Kirshner, Oemler & Schecter (1978) field which falls close to NGC 2403. Fig. 5a shows, however, that when the limits of that field are outlined by a dashed box that only in the top of this field are found galaxies in the redshift range which belong to the NGC 2403 filament. These galaxies form a straight connection across the KOS field from the direction of M81 to NGC 2403. Fig. 5a shows, however, that this is an extraordinary line of galaxies to be found in a fully sampled field. As KOS themselves state: “There –1 are prominent groupings at 4000 and 11000 km s . The galaxies in the near grouping are loosely clustered but the 11,000 km s–1 galaxies are spread over the entire field.” It would be an almost unbelievable coincidence if this straight line of ~ 4000 km s–1 galaxies, by accident, just fit exactly into the longer line of similar redshift galaxies that was observed earlier passing from M81 through NGC 2403.