Description of of Tables and References

The parameters of the 110 observed galaxies are listed in Table 1 (southern galaxies) and Table 2 (northern galaxies). Following is a column-by-column description of the table entries, and how they are derived.

(1) NGC designation of the galaxy. An "a" next to the number denotes archival images, generally obtained with a different FOC format, filter, or exposure time than those of the program galaxies. See Section 3.3 for details.

(2) UGC designation of the galaxy (northern galaxies only).

(3) 1950 coordinates, to one-minute accuracy, as listed in the UGC and ESO catalogs. This datum can be useful for unambiguously identifying the galaxies in these and other catalogs, since listed coordinates for such large galaxies may vary by arcminutes from catalog to catalog. Also, some galaxies in the ESO catalog are designated solely by means of these coordinates.

(4)(5) J2000 coordinates of the nucleus, as measured in the STScI GASP system (see Section 2). These coordinates are generally accurate to ~ 1''-2''.

(6) V_h -- Heliocentric velocity, as listed in the UGC or ESO catalogs, in km/s. The selection criterion for inclusion in the sample was V_h < 2000 km/s.

(7)(8) D_a, D_b -- Major and minor axis diameters, in tenths of arcminutes, from the UGC and ESO catalogs. The selection criterion for inclusion in the sample was D_a > 60.

(9) B_mag -- Integrated B magnitude, from the UGC and ESO catalogs.

(10) T -- Hubble type, using de Vaucouleurs' T-type classification from the RC3 catalog (de Vaucouleurs et al. 1991; see full citation below). The correspondence is approximately as follows; E: -6 to -4; S0: -3 to 0 ; Sa: 1; Sb: 3; Sc: 5; Sd: 7; Irr: 10.

(11) Classif. -- Hubble type and luminosity class using the classification, when available, from the Revised Shapley-Ames Catalog of Bright Galaxies (Sandage & Tammann 1987).

(12) Sp. -- (northern galaxies only) -- Spectral classification of the nucleus, from Ho et al. (1995, 1996a). The designation is as follows.
L -- LINER (low-ionization nuclear emission-line region);
H -- H II nucleus; T -- "transition" source, between LINER and H II;
S -- Seyfert nucleus; A -- "absorption-line" nucleus with no detected emission lines. A colon
denotes an uncertain classification. The Ho et al. classification is based on the Filippenko & Sargent (1985, 1986) optical spectroscopic survey of the nuclei of a flux-limited (B < 12.5 mag) sample of 486 northern galaxies. All but three of the northern galaxies in the HST survey (which is diameter- and redshift-limited) are included in the Filippenko & Sargent (1985) sample. The effective aperture of the optical observations is 2''x 4''. The Ho et al. (1996a) classification is assigned after careful subtraction of absorption-line template spectra, leaving behind only the emission-line residual; see Ho et al. (1996a) for further details.

(13) HST UT observation date.

(14) Image rootname in the HST archive, useful for retrieving the actual data.

(15)(16) f_UV, sigma -- Total f_lambda(2270 Ang) in units of 10^{-15} erg s^{-1} cm^{-2} Ang^{-1}, integrated above the background over the entire area of the image, and 1 sigma uncertainty. The area of each image is 22'' x 22'', except for some of the archival exposures (marked with footnote "a" in column 1), which were taken with a different FOC format. See Section 3.3 for details. The background was determined as follows. The mean counts per pixel were measured in seventeen 200 x 200-pixel squares in the frame, excluding occulting fingers and distortions in the FOC field. and the median counts per pixel were measured over the entire exposed part of the frame. The mean of the two lowest among these 18 measurements was used as the background value, and the standard deviation of the five lowest among the 18 measurements was used as the uncertainty in the background. The uncertainty in the background was propagated to an uncertainty on the total net counts in the image. The count rate above the background was converted to a UV flux density as described in Section 2. The flux uncertainty due to the background uncertainty was combined in quadrature with a 5% absolute calibration uncertainty (Meurer 1995) to produce the quoted flux uncertainty. Except for bright and concentrated sources, the flux uncertainty is dominated by the uncertainty in the background determination. The cause of artificial background variations across the image is imperfect flat-fielding. Furthermore, a systematic error in background determination is unavoidable due to the small field of view, which covers only a fraction of the optical extent of these galaxies. Some of this systematic error is accounted for by the above procedure for estimating the background uncertainty. Nevertheless, the total UV fluxes quoted here agree well with the 2300 Ang fluxes measured for those galaxies that have also been observed with IUE (Kinney et al. 1993; see also Meurer 1995), which has a comparable entrance aperture. This suggests that the regions of the images with the lowest counts are, in fact, devoid of significant UV emission. The UV fluxes given here should be used with care, and in conjunction with the UV-morphology classification (column 17) and the visual appearance of the image. For example, there is low significance to the flux that is listed for a galaxy whose image appears blank. The UV fluxes are uncorrected for Milky Way or external extinction.

(17) Morph. -- UV morphologies roughly describing the HST image, with the following symbols. B -- blank image; W -- weak or nearly absent UV emission; S -- star-forming morphology, with knots and compact sources of UV emission; F -- diffuse, centrally concentrated emission; P -- unresolved nuclear point-source; R -- circumnuclear star-forming ring. Some comments on each of these types follow.

We have rechecked the coordinates and pointing of the blank ("B" morphology) images, and verified that they are not cases of telescope mispointing. As a check on the pointing accuracy, there are about 40 images which display a feature that can be securely associated with the nucleus of the galaxy. In almost all cases it is within 3'' of the center of the image, as expected from the combined uncertainty in the GASP coordinates of the nucleus and the HST pointing accuracy. In the few cases where the nucleus is further from the image center, this has been traced to inaccurate input coordinates (see Section 3.3). The blank images are also not the result of foreground Milky-Way extinction, except for two galaxies, NGC 1560 and NGC 6946, which lie near the Galactic plane. Dust in the disks of the galaxies themselves is probably a factor, since 10 out of 13 galaxies with B morphologies have minor-to-major axis ratios less than 0.5 (i.e., an inclination >60 degrees). The fact that the centers of many galaxies are weak UV emitters is confirmed by the detection of very weak ("W-type") but significant and centrally concentrated emission in many of the galaxies, which establishes that the galactic nucleus is, indeed, in the field of view.

The compact sources seen in the "S-type" morphologies are probably compact young star clusters, or in some cases individual O and B stars. Similar objects have been detected with HST in a variety of starburst environments (e.g., Meurer et al. 1995; Maoz et al. 1996). They will be studied in further detail by Ho et al. (1996b). The diffuse "F-type" emission occurs in some of the early-type spirals and the ellipticals in the sample. We believe that, in general, this observed feature is dominated by actual UV emission from an evolved spheroidal stellar population (the "UV-upturn"; see, e.g., Burstein et al. 1988), rather than visual-band emission leaking through the F220W filter, based on several tests described in Section 2. In individual cases, however, confirmation by means of blue and near-UV imaging photometry of the center of each of these galaxies is required. The five circumnuclear rings in the sample (designated "R") have been discussed in detail by Maoz et al. (1996), who showed that a large, possibly dominant fraction of the UV light in these objects is emitted by the numerous compact sources distributed along the rings. These sources are probably young and massive star clusters that will remain bound, similar to those seen with HST in other starburst environments.


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