7 HI properties of spirals

The HI survey of the Ursa Major cluster presented here provides not only the kinematical information necessary for the study of the Tully-Fisher relation and of the dark and luminous matter for a well defined sample of galaxies. It also serves to investigate the general HI properties of disks and to make a comparison with galaxies in the field and with galaxies in denser environments. The HI studies of the Virgo cluster galaxies by Warmels (1988a, 1988b) and especially by Cayatte et al. (1990) have shown that the spiral galaxies in the central parts of the cluster have smaller HI disks of lower surface density. In the Hydra Cluster McMahon (1993) did not find any such significant HI deficiency. She did find, however, a surprisingly large number of isolated HI-rich dwarf galaxies near the center of the cluster. Dickey (1997) surveyed the more distant Hercules SuperCluster and found a similar HI deficiency of spirals near the X-ray gas as in the case of Virgo.

The Ursa Major cluster differs from those just mentioned. It has no central concentration, no X-ray emitting gas and contains mainly spirals of late morphological types. In many respects its conditions are very similar to those of a field environment. For this reason it is useful to compare the properties of the Ursa Major spirals not only with those of galaxies in dense cluster environments but also with those of field galaxies as found in various recent studies (see e.g. Broeils 1992; Puche & Carignan 1991; Rhee 1996a; Swaters 1999).

Here we give only a brief description of the global parameters and of the main properties of the HI disks of the Ursa Major galaxies. A more detailed discussion and a comparison with results from previous work is beyond the scope if this data paper.

7.1 Global parameters

Integral properties and global parameters of spiral galaxies have been derived for a large number of objects from single-dish observations (cf. Roberts & Haynes 1994). In recent years also synthesis observations (Broeils 1992; Rhee 1996a) have been used to obtain similar information for smaller samples of galaxies.

The $M_{\rm HI}/L$ ratios obtained for the galaxies of the Ursa Major sample listed in Table 5 are shown here in Fig. 5 as a function of absolute magnitude and of morphological type. It is well known (see refs. above) that the $M_{\rm HI}/L$ ratio of galaxies depends on luminosity and morphological type. The present sample of galaxies shows a clear increase of the HI mass fraction with decreasing luminosity and from early to late morphological types. The correlation is clearly stronger for the $K^\prime$-band magnitudes which is a better tracer of the stellar mass.

7.2 Sizes of HI disks and radial surface density profiles

Detailed information on the sizes and radial distributions of HI disks has been obtained recently from synthesis observations of limited samples of field and cluster galaxies (Broeils & Van Woerden 1994; Cayatte et al. 1994; Rhee 1996a and 1996b). Here we present only some of the main results on the comparison of HI and optical diameters, on the relation between HI mass and diameter and on the radial density profiles for the Ursa Major sample.

Figure 6 shows the ratio of the HI diameter $D_{\rm HI}$(defined at an HI surface density of $1~M_{\odot}~{\rm pc}^{-2}$) to the optical diameter D₂₅^b,i as a function of luminosity, morphological type and disk scale-length. The diagrams do not indicate any clear trend or dependence of the diameter ratio on any of those quantities. The spread is large. There may be a hint of a slight increase of the ratio from early to later types and from more luminous to less luminous systems. For almost all galaxies $D_{\rm HI}$ is larger than D^b,i₂₅.

As shown in previous investigations (see refs. above), there is a tight correlation between HI mass and HI diameter as illustrated in Fig. 7. This implies a nearly constant mean HI surface density regardless of size. The HI mass correlates also with the optical diameter, but, as in previous work, with a much larger scatter.

Figure 7: Correlations between HI mass and the isophotal diameters of the stellar (triangles) and HI disks (circles). Solid lines indicate fits to the plotted data points while the dashed lines represent the fits found by Broeils (1992). Filled symbols indicate HSB galaxies and open symbols denote galaxies of the LSB type. Triangles are offset by 0.3 dex to the left

7.3 Warps, asymmetries and interactions

 

 

Table 6: Warps, asymmetries and interactions

Name	Type	Warped	Lopsided		Inter-
			HI distr.	HI kin.	acting
U6399	Sm		$\times$
U6446	Sd			$\times$
N3718	Sa	$\times$
N3726	SBc	$\times$
N3729	SBab
N3769	SBb	$\times$			$\times$
U6667	Scd			$\times$
N3877	Sc			$\times$
U6773	Sm			$\times$
N3893	Sc				$\times$
N3917	Scd		$\times$
U6818	Sd			$\times$
N3949	Sbc		$\times$	$\times$
N3953	SBbc
U6894	Scd
N3972	Sbc			$\times$
U6917	SBd	$\times$
N3985	Sm	$\times$
U6923	Sdm	$\times$		$\times$
U6930	SBd		$\times$
N3992	SBbc
U6940	Scd
N4013	Sb	$\times$
U6962	SBcd			$\times$	$\times$
N4010	SBd	$\times$		$\times$
U6969	Sm
U6973	Sab	$\times$			$\times$
U6983	SBcd
N4051	SBbc			$\times$
N4085	Sc			$\times$
N4088	Sbc	$\times$	$\times$	$\times$
U7089	Sdm		$\times$
N4100	Sbc	$\times$
U7094	Sdm		$\times$
N4102	SBab
N4117	S0
N4138	Sa	$\times$	$\times$
N4157	Sb	$\times$
N4183	Scd	$\times$
N4218	Sm
N4217	Sb
N4220	Sa
N4389	SBbc

The radial distributions of the HI surface densities are shown in Fig. 8. Only galaxies with fully reduced data, more inclined than 80 degrees and with $R_{\rm HI}<$ 1 arcmin are considered in order to avoid the most severe cases of beam smearing. There is clearly a considerable diversity of shapes and intensities. The upper row shows the profiles grouped for galaxies of similar morphological types. The dotted lines represent low surface brightness galaxies. No obvious trend with morphological type or surface brightness can be discerned. However, in the lower row, the profiles are grouped according to the galaxy properties as listed in Table 6. In this case, a clear trend is visible in the sense that galaxies with high HI surface densities in their inner regions are either involved in interactions, are lopsided or display a warped HI disk. Especially in the case of interacting and strongly lopsided systems, no longlived stable gas orbits can be expected and evidently, the cold gas becomes concentrated toward the inner regions of the disk. Note that some galaxies appear in more than one panel.

Figure 8: Azimuthally averaged deprojected radial HI surface density profiles of galaxies with fully reduced data, less inclined than 80 degrees and with $R_{\rm HI}>1$ arcmin. The profiles were scaled in radius by the radius of the HI disk measured at the $1~M_{\odot}~{\rm pc}^{-2}$ isodensity contour. Upper row: $\Sigma _{\rm HI}$ as a function of morphological type. The dashed lines indicate low surface brightness galaxies. Lower row: $\Sigma _{\rm HI}$ as a function of the kinematic state, according to Table 6, and nuclear activity

Warps are thought to be a quite common feature of the outer HI layers of spiral galaxies. But, in spite of attempts made in recent years (Bosma 1991), a good statistics on their occurrence does not exist yet. The information on HI warps provided by the present survey of the Ursa Major sample is somewhat limited mainly because of the already noted small radial extent of the HI layers with respect to the optical. There are pronounced HI warps like the well-known ones found in NGC 3718 (Schwarz 1985) and NGC 4013 (Bottema 1995) and those seen in other galaxies (N3726, N3985 N4010, N4157, N4183). In most cases the warps are visible in the outer parts beyond the optical bright disk and are present in normal, regular, not interacting disks. But there are also systems, like NGC 4088, which are strongly distorted in their optical appearance and also in their kinematics. Similar distortions are found in clearly interacting systems like NGC 3769. In the sample of 43 galaxies (Table 6) there are at least 13 objects with clear indications of warping.

Also asymmetries are thought to occur frequently in field spirals (Richter & Sancisi 1994). In the present sample we see a large number of objects (at least half, see Table 6) with a lopsided HI distribution and/or kinematics. The majority shows kinematical asymmetries: on one side of the disk the rotation curve rises more slowly and reaches its flat part at a larger radius than on the other side (see for example NGC 3877, 3949, 4051). Note that this occurs in the inner parts of non-interacting, regular, normal systems.

Finally, there are four galaxies in this sample of 43 which have close companions and show clear HI signs of tidal interactions. A few more objects (examples NGC 3718, 4088) show distortions or peculiar structures in their density and velocity maps.