Astronomy Reports, Vol. 45, No. 11, 2001, pp. 841–853. Translated from Astronomicheski˘ı Zhurnal, Vol. 78, No. 11, 2001, pp. 963–976. Original Russian Text Copyright c 2001 by Chernin, Kravtsova, Zasov, Arkhipova. Galaxies with Rows A. D. Chernin, A. S. Kravtsova, A. V. Zasov, and V. P. Arkhipova Sternberg Astronomical Institute, Universitetskii˘ pr. 13, Moscow, 119899 Russia Received March 16, 2001 Abstract—The results of a search for galaxies with straight structural elements, usually spiral-arm rows (“rows” in the terminology of Vorontsov-Vel’yaminov), are reported. The list of galaxies that possess (or probably possess) such rows includes about 200 objects, of which about 70% are brighter than 14m.On the whole, galaxies with rows make up 6–8% of all spiral galaxies with well-developed spiral patterns. Most galaxies with rows are gas-rich Sbc–Scd spirals. The fraction of interacting galaxies among them is appreciably higher than among galaxies without rows. Earlier conclusions that, as a rule, the lengths of rows are similar to their galactocentric distances and that the angles between adjacent rows are concentrated near 120◦ are confirmed. It is concluded that the rows must be transient hydrodynamic structures that develop in normal galaxies. c 2001 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION images were reproduced by Vorontsov-Vel’yaminov and analyze the general properties of these objects. The long, straight features found in some galaxies, which usually appear as straight spiral-arm rows and persist in spite of differential rotation of the galaxy 2. THE SAMPLE OF GALAXIES disks, pose an intriguing problem. These features, WITH STRAIGHT ROWS first described by Vorontsov-Vel’yaminov (to whom we owe the term “rows”) have long escaped the To identify galaxies with rows, we inspected about attention of researchers. They have attracted interest 7000 photographs stored in the collection of the in recent years owing to a series of papers by Chernin Sternberg Astronomical Institute, which consists et al., who described straight structural rows in the of enlarged reproductions of nonoverexposed blue images of (mostly spiral) galaxies that are, with nearby galaxies M101 and M51 (see [1] and refer- m ences therein) and suggested a qualitative interpre- few exceptions, brighter than 15 ,adoptedfrom tation of these features. They linked the formation of the Palomar sky atlas. The collection contains − ◦ such rows to the fact that powerful shock fronts form- about one-fourth of all galaxies north of δ = 45 ing along spiral arms have a tendency to straighten catalogued in the five volumes of the Morphological and become flat, so that sites of shock-triggered star Catalog of Galaxies of Vorontsov-Vel’yaminov [3]. It formation can form linear structures. This raises the lacks, however, nearby galaxies with large angular question as to what factors determine the conditions diameters. Excluding nonspiral galaxies and galaxies for the development of rows. To answer this question, with poorly defined morphological structure leaves us we must analyze data for as many galaxies as possible with 4200 objects, which make up the basic sample whose spiral patterns exhibit these features. used below. Chernin et al. [1] described 15 spiral galaxies with An inspection of the photographs enabled identi- rows, which they found primarily by inspecting the fication of about 200 bona fide galaxies possessing NASA Atlas of Galaxies [2]. It was shown that, as (or likely possessing) straight rows, which make up a rule, the row lengths are similar to their galacto- about 5% of all the galaxies considered. Figure 1 centric distances and that these features are found shows several galaxies with rows by way of example. in both two-armed (M51, NGC 4303) and multi- In most cases, the rows appear as straight stretches of armed (M101, NGC 1232) galaxies, which appear to spiral arms (or, more rarely, of ring-shaped features), be quite normal as far as their integrated parameters where they form regular hexagonal structures. How- are concerned. More extensive samples are required ever, the relationship between these straight rows and to estimate the rate of occurrence of galaxies with a regular pattern is by no means evident. Linear rows. features such as bars are not considered to be “rows.” In the current paper, we study a sample of several Themeanmagnitudeofgalaxieswithrowsisap- thousand spiral galaxies whose Palomar sky atlas proximately the same as that of galaxies without them 1063-7729/01/4511-0841$21.00 c 2001 MAIK “Nauka/Interperiodica” 842 CHERNIN et al. NGC 1288 NGC 3351 NGC 4902 NGC 3124 NGC 5085 NGC 5247 PGC 31551 PGC 4189 NGC 309 Fig. 1. The most typical examples of galaxies with rows. (∼ 14m), although the scatter of the magnitude dis- among faint systems. Some galaxies with rows could tribution is large in both cases. However, identifying be overlooked due to insufficiently good seeing, es- galaxies with rows is undoubtedly more uncertain pecially in the case of faint galaxies. On the other ASTRONOMY REPORTS Vol. 45 No. 11 2001 GALAXIES WITH ROWS 843 hand, if identification is based on a visual inspection Relative number of galaxies, % 50 of photographs with insufficient resolution, this same (‡) factor could yield false detections of rows, which then 45 contaminate the final sample. The subsample of 40 galaxies brighter than 14m contains about one thou- 35 sand objects. Of these, we found 157 galaxies (about 30 15%) exhibiting rows. The sample size decreases 25 substantially as we consider brighter galaxies. For 20 example, the NASA Atlas of Galaxies [2] gives photos 15 of about 200 nearby galaxies with well-defined spiral 10 arms, of which about 15 objects [1] exhibit well- 5 defined rows, corresponding to about 8% of the entire 0 25 sample, although the statistical error of this fraction (b) is rather high. The table provides information on the entire sam- 20 ple of galaxies with rows. Its columns give (1) the name of the galaxy according to the PGC and NGC 15 catalogs; (2) its morphological type; (3) its integrated 10 magnitude B0 corrected for Galactic extinction and the inclination of the galaxy to the line of sight; (4) its 5 heliocentric radial velocity V0; (5) its absolute magni- tude MB (H0 =75km/s Mpc); (6) the HI mass-to- 0 light ratio, M(HI)/LB (in solar units), inferred from a abbbcccdd the HI index of the RC3 catalog; (7) the number of Morphological type rows detected in the galaxy; and (8) its VV number in the catalog of interacting galaxies [5]. We adopted Fig. 2. Histograms of morphological types of galaxies (a) with and (b) without rows. Shaded entries correspond to the integrated parameters of the galaxies from the interacting galaxies. LEDA electronic database and RC3 reference catalog of galaxies [4]. Figures 2a and 2b show distributions of the morphological types of galaxies with and with- dotted lines, two regression fits to the observational out rows. It is evident that the fraction of objects data: d =0.87L +0.11 and d =1.13L − 0.25.The with rows is especially high among late Sbc–Scd standard deviation is 0.11. galaxies. Figures 3a and 3b compare the absolute The angle between adjacent rows is usually close magnitudes of the two groups of galaxies. Galaxies ◦ to 120 . Figure 7 shows the distribution of these with rows have absolute magnitudes ranging from ◦ ◦ angles. The median and mean are 122 and 125 , −17m to −22m. 5 with the mean and median over m m respectively. the entire sample equal to MB = −20 . 4 and −20 . 7, respectively. Galaxies without rows exhibit the same The galactic rotation curves, which are available general type of luminosity distribution. only for a small fraction of the galaxies in our sample, The number of rows in a galaxy ranges from one exhibit no apparent anomalies. For three galaxies to nine (Fig. 4), with the mean over the entire sample with rows (M101, M51, and NGC 3631), the gas being close to three rows per galaxy. We found no cor- velocity fields were studied throughout the disk in relation between the number of rows and the absolute great detail using both optical and radio methods. magnitude of the galaxy. In each of these, the gas motions are dominated by The linear sizes L of rows vary over a wide regular differential rotation, although noncircular ve- range and can exceed 20 kpc in some cases (M101, locities (both local and ordered, associated with the Fig. 5). The mean and median row lengths are 4.9 spiral structure) are also present. This pattern is and 4.0 kpc, respectively. We found the length of fairly typical of spiral galaxies. On the other hand, a row to be well correlated with the galactocentric the galaxy with well-developed spiral arms NGC 157, distance d of the farthest tip of the row—the further in which the rotation is only slightly differential (the from the galactic center, the longer the row. Fig- linear rotation velocity increases monotonically with ure 6 illustrates this correlation. The lengths and galactocentric distance throughout the entire region galactocentric distances d of the rows are expressed of the spiral arms) and the wave nature of the spirals as fractions of the optical radii of their galaxies D25/2. is indicated directly through analysis of the velocity The solid line in Fig. 6 shows the relation L = d;the field [6], fails to show even the slightest sign of rows. ASTRONOMY REPORTS Vol. 45 No. 11 2001 844 CHERNIN et al. Galaxies possessing (or possibly possessing) rows PGC/NGC Type B0 V0,km/s MB MHI/LB Number of VV Comments rows PGC 120 SBc 13.6 4389 −21.1 0.21 5 254