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CHAPTER 2. OBSERVATIONAL MATERIAL

At the start of this investigation (1973) we have made a brief inventory of the observational material on the rotation curves of spiral galaxies. At that time optical rotation curves were available mainly from the work of Burbidge, Burbidge and Prendergast in the 1960's. For a few galaxies rotation curves were available from 21-cm line data, i.e. M31, M81, M101, M33, NGC 2403, NGC 4236, NGC 6946 and IC 342 (see Roberts and Rots, 1973; Rogstad and Shostak, 1972); 21-cm line data were also available for other galaxies, but with low spatial resolution. In order to increase and improve the available sample, 21-cm line observations with the Westerbork Synthesis Radio Telescope (WSRT) were undertaken for several galaxies. The line receiver available at that time, an 80-channel filter spectrometer (Allen, Hamaker, and Wellington, 1974), is, however, relatively slow compared to the one in operation now. Thus, the 21-cm line projects carried out with the 80-channel receiver have an exploratory nature, and sometimes full resolution data have not been obtained (cf. chapter 3).

We have made a list of spiral galaxies which are sufficiently large to obtain a reliable rotation curve, i.e. the angular size of the galaxy has to be at least 5, better 10, times larger than the size of the beam. From the Reference Catalogue of Bright Galaxies (De Vaucouleurs and De Vaucouleurs, 1964) we selected the spiral galaxies with 1) angular size larger than 5', 2) declination larger than +20° (to obtain sufficient angular resolution in the north-south direction). This sample of about fifty galaxies has a very heterogeneous distribution of Hubble types: the S0/a, Sa, Sab, Sdm, Sm and Im are underrepresented; also the barred spirals are under- represented as compared to the ordinary spirals.

We have selected from this list a number of galaxies for observation with the WSRT. We required that the galaxies should have 1) inclination smaller than 75°, to permit two-dimensional mapping, 2) a morphological type which was not well represented in the available sample of galaxies with rotation curves, i.e. Scd's and two-armed early type spirals were excluded. In this selection process we have taken into account that certain galaxies were being observed with the WSRT for other purposes by other investigators. The results of some of these studies have been published, for other galaxies the rotation curve data were kindly made available to us. Because of the exploratory nature of the studies with the 80-channel receiver we have observed only five galaxies: NGC 2841, NGC 3198, NGC 5033, NGC 5055 and NGC 2805. The last galaxy in this list is a member of a small group of galaxies; the observations of this group will be discussed elsewhere (Bosma et al., 1978). In the course of our work it became clear that many galaxies have warped HI disks. At a later stage we have therefore also observed NGC 7331, as a galaxy where a possible warp can be studied in some detail.

In Table 1 we present the list of galaxies for which 21-cm line data are available with a spatial resolution such that the ratio radius/beamsize (R/B) is larger than about 3. In practice this meant that we have included most of the aperture synthesis data and also some of the single dish data on galaxies of large angular size. For R we have taken the Holmberg radius, and for B the beamsize in the major axis direction. Most of the galaxies have a R/B smaller than 10, we can therefore expect beam smoothing effects to be present (see chapter 3). The distribution of galaxies over Hubble types is fair for Sab, Sb, Sbc, Sc and Scd, also the luminosity classes are fairly well covered. The ratio of SA:SAB:SB is about 0.44:0.31:0.25; in a sample of 1000 nearby galaxies this is 0.33:0.29:0.38 (De Vaucouleurs, 1963). The edge-on galaxies have been observed to study the z-distribution of the gas; their classification is uncertain. This sample may not yet be ideally suited for a study of the relation between the radial distributions of properties that can be inferred from 21-cm line data and morphological type, but it represents a great improvement with respect to the information available before this study was started. Also, we hope that our attempt to relate morphological type to certain physical properties will indicate which problems must be considered before a more adequate sample can be constructed.

Table 1.

Galaxy Type + L. class Diameterx R/B Ref.+

N3718 SB(s)o/a   8.1 × 4.7 4.0 1
M81 SA(s)ab I-II 35 × 14.4 42,21,9 2,3
N4151 PSAB(rs)ab   ~ 9 × 8 4.5 4
N4736 RSA(r)ab II 15 × 13.3 7.5 5
M31 SA(s)b I-II 197 × 92 40,9.5 6,7,8  
N891 SA b   15 × 3.8 15 9
N2841 SA(r)b I 11.3 × 5.7 5.6 10
N4565 SA b I 20 × 36 6.0 11
N5383 SB(rs)b II ~ 6 × 3 4.3 12
N4258 SAB(s)bc   24 × 9.6 24 13
N5055 SA(rs)bc II 16 × 10.1 8.0 14
M51 SA(s) bc p I 14.2 × 9.5 10.5,5 15,16
N7331 SA(s) bc I-II 13.5 × 7.0 10.0 17
N253 SAB(s)c   ~ 23 × 4.6 5.0 18$
N3198 SB(rs)c II 11.9 × 4.9 10.0 19
N3359 SB(rs)c III 9 × 5.6 3.7 20
N5033 SA(s) c I-II 12.3 × 5.8 4.3 21
N5907 SA c II 15.7 × 2.0 7.5 22
N7640 SB(s) c II 13.5 × 3.6 3.4 23
N300 SA(s)cd III 38 × 26 2.5 24
M33 SA(s)cd II-III 83 × 53 15,20 25,26
I342 SAB(rs)cd I-II ~ 40 × 40 5,40 27,28
N2403 SAB(s) cd III 29 × 15 7.2 29
N4244 SA(s) cd III-IV 18 × 2.9 9.0 30
M83 SAB(s) cd I-II ~ 14.6 × 14 3.7 31
M101 SAB(rs)cd I 28 × 28 28,14,3.5 32,33
N6946 SAB(rs)cd I 14.4 × 12.6 3.6 27
N2805 SAB(rs)d   8.0 × 6.0 4.0 34
N4631 SB(s) d III 19 × 4.4 10.0 35
N4236 SB(s)dm IV 26 × 8.7 6.5 29
I2574 SAB(s)m IV-V 16 × 8.0 4.0 23
N3109 SB(s) m IV-V ~ 13 × 2.0 1.5 37
HoII I m IV-V 11.0 × 8.9 2.7 36
N4449 IB m III 10.1 × 8.7 10.0,2.5 38,39
Galaxy         40

De Vaucouleurs et al. (1976)
x Holmberg (1958); - represent estimates
+ Reference number
$ Data processing in this paper can be questioned

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