1986Aj 92. .7423 the Astronomical Journal
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.7423 THE ASTRONOMICAL JOURNAL VOLUME 92, NUMBER 4 OCTOBER 1986 92. NEUTRAL HYDROGEN IN SMALL GROUPS OF GALAXIES Stephen E. Schneider Department of Astronomy, University of Virginia, Charlottesville, Virginia 22903 1986AJ George Helou Infrared Processing and Analysis Center, 100-22, California Institute of Technology, Pasadena, California 91125 Edwin E. Salpeter Department of Astronomy and Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853 Yervant Terzian Department of Astronomy and National Astronomy and Ionosphere Center, Cornell University, Ithaca, New York 14853 Received 20 March 1986; revised 27May 1986 ABSTRACT Neutral hydrogen in 36 pairs and small groups of galaxies was studied using the Arecibo radio tele- scope. Potential groups were selected from the UGC by a simple algorithm with the aims of determining or improving redshifts for galaxies in possible groups and of mapping some of the larger galaxies to determine their internal dynamics. The H i spectra are examined in detail and problems of confusion in these and previous observations are addressed. A simple analysis of the groups’ mean dynamical mass suggests that the mass determined from the galaxies’ rotation is sufficient to bind them, but there may be important unmodeled selection or contamination problems. We also derive a simple method for estimating errors of H I velocity measurements. L INTRODUCTION chance alignments, and that a degree of isolation from sur- rounding galaxies be maintained, to avoid larger groupings. Small groups of galaxies provide an important link A simple analysis of these samples gave mass-to-light ratios between individual galaxies and clusters of galaxies for esti- of up to 60. White et al (1983) re-examined the Turner mating the amount of mass associated with galaxies over sample, with higher-precision velocities and a more general intermediate scale lengths. Within spiral galaxies the simple model of the orbits (White 1981 ), and found that the orbits geometry of circular rotation allows a direct estimate of their are not well determined given even the better velocity mea- masses from measurements of the rotation, and to the limits surements; thus questions of total mass in the systems must that rotation curves can be measured, galaxies appear to still rely heavily upon assumptions about the nature of the have mass-to-light ratios of —10-20. Clusters, by virtue of orbits. Worse, there are even uncertainties in the most basic the large number of galaxies, can be treated statistically; the assumptions made in analyzing binary galaxy orbits ( Sharp galaxies act like test particles in the cluster’s gravitational 1984). Tifft (1980, 1982a,b) has pointed out that a simple field, and the total mass and mean galactic properties asso- histogram of the velocity differences in pairs of galaxies ap- ciated with the whole system indicate Af/L — 300. Unfortu- pears to be “quantized” with a 72 km s-1 periodicity. Thus nately, the geometry of the orbits is not known in groups, dynamical analyses of group masses must be regarded with and the numbers of galaxies are too few to permit the simpli- some skepticism. fications in large numbers. On the other hand, in many near- Very few groups actually contain only a pair of galaxies. by groups, the internal dynamics of the galaxies can be exam- With slightly less strict selection criteria, many more possi- ined while modeling the galaxies’ interactions over larger ble group members are usually found when examining even distance scales. Additionally, galaxies in groups are less like- the binary galaxy samples. While more complex to analyze, ly to have suffered the modifications that occur in clusters, the larger groupings of galaxies generally include a broader and the interactions between individual galaxies can often be range of galaxy types and sample the group’s gravitational traced. Galaxies in groups also appear to have fundamental field at more points. The smaller galaxies can also be used as relationships of structure and morphology, and this may ul- tracers of the more massive galaxies’ gravitation. timately relate to the manner of their formation and evolu- In deriving lists of groups and pairs it is difficult, though, tion. to obtain complete, uncontaminated samples. When chosen The small numbers of galaxies in each group necessitate by the appearance of nearness on the sky, they are plagued an ensemble approach to group studies. By collecting many with interlopers and questions of selection effects and biases groups, a statistical sample can be derived in which random (e.g., Turner and Gott 1976). On the other hand, those orientations of the orbital elements can be assumed. In par- based on velocity surveys (e.g., Huchra and Geller 1982; ticular, pairs of galaxies provide an important subset in Geller and Huchra 1983), while less subjective, depend al- which specific assumptions about the orbits remain tracta- ready on the acquisition of a large amount of data irrelevant ble. Turner ( 1976a,b) and Peterson ( 1978,1979a,b) derived to group studies, and often overlook faint companions. The samples of binary galaxies by studying the two-dimensional galaxies collected for this study lie somewhere in between. distribution of galaxies on the sky. Where two galaxies were The sample was not defined with strict selection criteria de- found close enough together, they required that the differ- signed solely for statistical analysis, nor is it limited to ence in their luminosities not be too great, to guard against known physical associations. This work aims primarily at 742 Astron. J. 92 (4), October 1986 0004-6256/86/040742-24$00.90 © 1986 Am. Astron. Soc. 742 © American Astronomical Society • Provided by the NASA Astrophysics Data System .7423 743 SCHNEIDER ETAL. : H i IN GROUPS OF GALAXIES 743 92. adding to the data base of galaxy groups. The groups were III. THE 21 cm DATA selected by a simple algorithm with the goals of determining The observations were carried out at the Arecibo Obser- or improving redshifts for possible group members, mapping vatory* mostly in early 1983. Because of the varied circum- the larger galaxies to study their internal dynamics, and ex- stances pertaining to the different groups and pairs of galax- 1986AJ amining the dynamics of a set of groups having accurate ies, the mode of observing was varied as well. redshifts. Either the dual-circular-polarization (“circular”) feed or the linear-polarization (“flat”) feed was used, depending on II. SELECTION OF THE GROUPS the observational requirements of the particular group. The circular feed has a 3!2 beam with a ( ~ 10% ) sidelobe at 5Í4 The procedure used for identifying possible groups of gal- from the beam center. The flat feed is less sensitive and has a axies was similar to the one used by Helou, Salpeter, and 3Í9 beam, but its sidelobes are less than 2% of the peak sensi- Terzian (1982) with the main variation being a fainter limit tivity. In general, the circular feed was preferred because of on the magnitude of the galaxies. The Uppsala General Cata- its greater sensitivity; however, where its relatively high side- logue of Galaxies (UGC, Nilson 1973) was examined for all lobes appeared likely to cause confusion, the flat feed was galaxies in the Arecibo declination range with a blue magni- used. The choice of feed is explained in the descriptions of the individual groups. A few sources were observed with tude brighter than mz = 14.0, and having another galaxy brighter than 15.0 within three diameters of the primary. both feeds, providing a comparison sample, which will be This constraint was relaxed to 6 diameters if the magnitude discussed later. différence between the galaxies was less than 2.0 on the basis The circular-feed observations were generally carried out that the association was then less likely to be only apparent. with a 10 MHz bandpass and four parallel quadrants (two in A 6° region around the Virgo cluster was eliminated from the each polarization) of 252 channels each. The duplication of search as were Local Group galaxies and all galaxies with the inputs provides a small ( ~5%-10%) improvement in major diameters greater than 15' in order to minimize the the noise levels because of the different signal pathways in the back-end receiver systems. This arrangement gives —16 number of spurious associations. -1 For groupings of galaxies in the Arecibo declination range km s resolution after Hanning smoothing. meeting these basic criteria, the surrounding region out to 18 The flat-feed observations were made using three-level galaxy diameters was examined for other galaxies to deter- sampling to improve the sensitivity, and this entailed reduc- mine the degree of isolation. The listings of possible groups ing the bandwidth to 5 MHz. Two parallel channels were were then examined for morphological type; associations observed, each having 504 channels. The resultant channel separation is — 2 km s~!, but the flat-feed data have general- consisting primarily of elliptical and lenticular galaxies were -1 eliminated since they are unlikely to be detected at 21 cm. ly been smoothed to ~8 km s to improve the signal-to- When velocity determinations were available in the litera- noise ratio. ture, the likelihood that the galaxies formed an actual associ- When a galaxy’s velocity was unknown, a search was ation was assessed, and the possibility of improving upon the made over a wide velocity range with the circular feed by published velocity measurements was considered. applying different local oscillator offsets to each of the quad- rants. Separating them by 7.5 MHz yielded four overlapping In the end, a list of 36 small groups and pairs was derived, -1 containing groups of variable quality. In some cases the di- spectra covering velocities from about 0 to 6500 km s .