5.1. Primary Objects
NGC 221 (M32). - A satellite to M31, M32 is a compact dwarf elliptical galaxy noted for its high optical surface brightness. The UV images show a compact, centrally concentrated source projected onto the disk of M31. Enlarged UV images of M32 are shown in Figure 4a, while the full-field versions from which these were extracted are shown in the NGC 224 / M31 mosaics in Figures 5a - 5b. M32 is the bright object near the left hand (eastern) edge of the M31 UIT 25 mosaic, just below the overlap region.
OB stars in the M31 disk adjacent to M32 produce a complex background which has been modeled and removed from our photometry, but which produces greater uncertainty in the integrated UV magnitudes. M32 exhibits a good fit to a de Vaucouleurs r0.25 profile for r 40" in both the FUV and MUV bands. Axial ratios of the UV isophotes match those of the optical bands; this is true in general for the normal ellipticals in our sample. M32 has the reddest central (FUV-MUV) color (after extinction corrections) of any of the five normal early-type systems (M32, NGC 1399, NGC 1404, and the bulges of M31 and M81) included in our sample. This is consistent with results from IUE (Burstein et al. 1988; O'Connell et al. 1992). The R-band photometry plotted in Figure 4b was interpolated from the major axis profile data taken from Kent (1987). The sharp central rise in the R-band flux with respect to the UV flux is partially an artifact of mismatched spatial resolution.
There are strong radial gradients in both the (FUV-MUV) and (FUV-B) colors. The colors become bluer with increasing radius, contrary to the UV color gradients observed in other dynamically hot systems (O'Connell et al. 1992; Ohl et al. 1998). As in the case of M31 (below), individual massive stars hotter than B1 V would be detectable in our UV images if they were present. No isolated point sources are visible within M31 in the UV imagery. Instead, the UV light distribution shows an azimuthally smooth, radial decline. This UV morphology is typical of other normal E galaxies and spiral bulges and is evidence that the far-UV light originates in low-mass stars (reviewed in O'Connell 1999). HST UV imagery of the central region of M32 has been presented by Bertola et al. (1995); Cole et al. (1998); and Brown et al. (1998) with the FOC and WFPC2 cameras, and more recently by Brown et al. (2000) with STIS. These images confirm the absence ofluminous, massive OB stars. The STIS images of Brown et al. (2000) are sufficiently deep to resolve the horizontal branch (HB) and to confirm that the UV light in M32 is produced by a mixture of evolved hot HB stars and more luminous, but shorter-lived, post-HB stars. The absence of a UV-bright nucleus is notable since M32 is the densest resolvable extragalactic system (Lauer et al. 1998). Evidently, high stellar density alone is not sufficient to produce an active nucleus or a nuclear population of UV-bright stars.
NGC 224 (M31). - The Andromeda galaxy, a member of the Local Group, is a disk system, type SA(s)b, ~ 690 kpc in distance, and inclined 78° to our line of sight. Several low-resolution, wide field UV images of M31 were obtained with both balloon (Deharveng et al. 1980) and rocket experiments (Deharveng et al. 1976; Carruthers, Heckathorn, & Opal 1978; Bohlin et al. 1985; Bohlin et al. 1988), the latter two in both FUV and MUV bands. The best of these images achieved a spatial resolution of ~ 15" and showed that the UV emission from M31 is dominated by the bulge and a broken ring of OB associations.
UIT imagery was obtained in two overlapping fields during Astro-1; M31 completely fills the field of view in both. One field is centered on the bulge; the other is centered ~ 40' to the southwest along the major axis. Images in each filter have been combined and are displayed as mosaics in Figures 5a - 5b. The radial UV surface brightness and color profiles plotted in Figures 5a - 5b, respectively, are based on mean values within those segments of the elliptical apertures which lie within our mosaic. The profiles therefore reflect behavior only near the southwestern major axis for r < 50'. The R-band profile shown in Figures 5a - 5b was interpolated from the major axis profile data taken from Kent (1987).
The bulge is extended in both FUV and MUV and displays a relatively smooth appearance; neither bright substructure nor dust lanes are evident. As in the case of M81, the bulge of M31 is considerably more prominent at longer wavelengths, though the contrast is not as extreme as in M81. In both FUV and MUV light, the inner bulge follows a reasonably good de Vaucouleurs r0.25 profile to r ~ 90". Except at very small radii where the spatial resolution of the data are mismatched, the bulge shows only a small color gradient in (25-R). However, the FUV light is more centrally concentrated, producing a strong radial color gradient in (FUV-MUV) for 0 r 150", in which the light gets redder with increasing radius. This behavior is typical of large spiral bulges and elliptical galaxies but is much larger in amplitude and is reversed in sense from gradients in the optical bands such as (B-V) or (V-K) (O'Connell et al. 1992; Ohl et al. 1998).
There are no detected UV point sources in the bulge, even though individual stars as faint as B1 V (m(MUV) ~ 18.4) would be above the MUV threshold (O'Connell et al. 1992). If the FUV light from the bulge originated in recently formed massive stars with a normal initial mass function, there would be ~ 200 OB stars visible within r 2'. Their absence supports the interpretation of the FUV light as originating from low-mass, post-giant branch stars (e.g., Greggio & Renzini 1990; O'Connell 1999). HST UV observations of the core of M31 have been presented by King et al. (1992); Bertola et al. (1995); King, Stanford, & Crane (1995); and Brown et al. (1998). At HST resolution, the nucleus of M31 is double, and the component coincident with the dynamical center has UV colors which are bluer than those of the background bulge and therefore may contain massive stars formed through stellar collisions (King et al. 1995; Lauer et al. 1998). The second, off-nuclear component has UV colors similar to the bulge. These sources are not resolved in the UIT photometry. Brown et al. (1998) have partially resolved the hot stellar population responsible for the FUV light in the central 14" × 14" using HST/FOC images. Color-magnitude diagrams of the resolved stars are consistent with objects that have evolved from the extreme horizontal branch.
Outside the bulge, the MUV light profile roughly parallels the R-band profile. However, the FUV profile flattens and even rises slightly with increasing radius, meaning that the (FUV-MUV) color becomes systematically bluer with radius. This result is preliminary because of the difficulty in determining the sky background (see Section 4). The existence of a smooth radial distribution of FUV emission at small radii suggests the presence of an FUV-bright inner disk which spatially overlaps with the outer bulge. Such an inner disk is not seen in the case of M81 (NGC 3031, see Fig. 17a), where the combined FUV light of the bulge and disk is undetectable at inner radii (r < 3'.5 = ~ 3.5 kpc). The surface brightness profiles are quite different between these two objects, despite the fact that M81 is only slightly earlier in Hubble type (Sab rather than Sb).
The outer disk of M31 is rich in UV-bright massive stars, the brightest concentration of which is NGC 206, located ~ 5' from the southwestern edge of the mosaics shown in Figures 5a - 5b. Hill et al. (1992a) provide UV photometry of 30 resolved stars in NGC 206; most are evolved OB stars with 30 M / M 60. Hill et al. (1993c, 1995b) presented enlargements of the UIT images for some 60 OB associations in M31 and photometered individual stars and integrated light. They infer a maximum stellar mass of 85 M. Only about 40% of the integrated MUV light and 20% of the FUV light of a typical association comes directly from the resolved stars; the remainder may be dust-scattered starlight with a smaller contribution from fainter stars with T 10,000 K. Bohlin et al. (1993) obtained UV photometry for 43 compact clusters; most of these are old globulars, but seven appear to be compact young clusters. The disk shows a strong (FUV-MUV) color gradient, becoming systematically bluer from r ~ 3' to r ~ 50' (~ 10 kpc) by about 2 mag. This reflects the increasing contribution of OB associations to the total integrated light at larger radii.
NGC 598 (M33). - This object, of type SA(s)cd at a distance of D ~ 700 kpc, 52' × 56' in size, and inclined ~ 55°, is the least luminous of the three spiral galaxies in the Local Group. The UIT images were centered on OB association NGC 595 to permit the Hopkins Ultraviolet Telescope to obtain spectra during the UIT exposures. M33 extends slightly beyond the UIT field, but almost all of the UV-luminous regions are contained within the 40' field. The UIT, H, and R-band images are presented in Figure 6a.
The UIT images are almost completely filled with UV-bright knots and clusters, which roughly trace out spiral arms. High surface brightness UV features are found at all radii, but there is considerable diffuse UV light as well. The galaxy is considerably more irregular and much less centrally concentrated in the UV than in the optical bands. Qualitatively, the spiral structure is traced better by the UV continuum than by the (younger) H-emitting regions. Several giant extragalactic H II regions (GEHRs), including NGC 604, are visible in these frames. NGC 604 is located ~ 12' NE of the nucleus and is prominent in both the UV and H images. Massey et al. (1996) have cataloged the 356 brightest UV objects on the UIT frames and obtained follow-up spectroscopy for about half of these. Most are classified as O and early-B supergiants.
The UV morphology of M33 has been discussed by Landsman et al. (1992) using UIT data and by Buat et al. (1994) using lower resolution data obtained with the FOCA balloon-borne telescope. The FOCA image contains the entire galaxy at 20" resolution in a bandpass centered at 2000 Å. Buat et al. (1994) find that about 80% of the FUV light is contributed by discrete sources rather than a diffuse background. This is a larger discrete contribution than found in more luminous spirals like M74 (Cornett et al. 1994). Keel (1998) has used Voyager 2 data to obtain one-dimensional spatial profiles for M33 in Ly and the 800-1200 Å continuum and compared this to the UIT photometry.
The UV brightness profiles shown in Figure 6b display a steep decline to a radius of 3', followed by a long plateau, then a gradual decline. This pattern has been noted in other disk galaxies imaged by UIT (Fanelli et al. 1997b) and is distinct from the more smoothly declining optical broadband radial profiles. For r 3', the R-band profile is well fitted by an exponential, whereas the UV bands exhibit considerably more structure. Inflections in the UV profiles are caused by luminous OB associations, in particular the "bump" at r ~ 14' due to NGC 604. Landsman et al. (1992) find scale lengths for the best-fit exponentials of 5'.6 in the MUV and 7'.0 in the FUV. These values imply a gradient toward bluer (UV-visible) and (FUV-MUV) colors with increasing radius, as seen in the color profiles in Figure 6b, and in other disk galaxies imaged by UIT (M74 and M81). Over the entire disk (FUV-MUV) ~ -0.6 mag, about 1/3 of the gradient found in M31. Over the same radius, (NUV-R) ~ -1.0 mag. Both internal reddening and changes in the history of star formation can produce the observed color changes (Landsman et al. 1992; Cornett et al. 1994), although the larger amplitudes seen in objects like M74 and M81 argue for the latter.
NGC 628 (M74). - NGC 628, a face-on spiral of type SA(s)c at a distance of ~ 7.6 Mpc, shows considerable grand design spiral structure and has been used for density wave studies (Cepa & Beckman 1990). In the UV (see Fig. 7a), the spiral arms are highlighted as "beads on a string," where bright knots embedded in diffuse emission trace the spiral pattern (Chen et al. 1992; Cornett et al. 1994). The pattern is much more organized than in the case of the slightly later type system M33. Many of the UV-bright knots are also bright in H, though, as in the other spiral disks with UV data, there are large variations in the ratio of H to far-UV continuum among the knots. As seen best in the MUV image, the brightest knots tend to lie on the inside edges of the spiral arms formed by the diffuse UV continuum. This is in the correct sense for an age gradient if gas in the disk rotates through the spiral pattern from inside to outside, such that the youngest and most massive stars are located at the inner edge of the pattern. Over 70% of the UV light in both bands originates from the more diffuse and presumably older component, rather than from the younger, compact regions.
As in M33, the UV surface brightness profiles (Fig. 7b) show smaller radial gradients than those observed in the optical bandpasses. The stronger central concentration of the optical light is evident in the images presented in Figure 7a and is consistent with Cornett et al. (1994), who find that azimuthally averaged scale lengths for the continuum emission decrease with increasing wavelength. However, the UV profiles are clearly nonexponential despite the approximately exponential behavior of the R-band profile. UV/optical colors become systematically bluer by over 1.5 mag with increasing radius in the disk. The H/UV flux ratio also systematically increases with radius. Using color-color diagrams, Cornett et al. (1994) have analyzed these color gradients and conclude that they reflect the star formation history rather than metallicity or internal extinction. The entire disk has undergone active star formation within the past ~ 500 Myr, but the inner regions have experienced more rapidly declining star formation than the outer regions. What is surprising is not that the star formation history changes with radius but that it exhibits such a remarkably smooth and organized pattern of change. Other spirals, e.g., M33 and M81, likewise have organized patterns but with different color-color relations. Such patterns are important clues to the global mechanisms by which disk galaxies regulate star formation.
NGC 1068 (M77). - An SA(r)b disk system, NGC 1068 is the prototype Type 2 Seyfert galaxy and one of the nearest galaxies harboring an AGN (D ~ 15.1 Mpc). In addition to the active nucleus, the optical disk exhibits a number of unusual features, most prominently a high surface brightness inner disk and a population of luminous knots.
Neff et al. (1994) provide a detailed description of the UV morphology of NGC 1068; the UV photometry is discussed by Fanelli et al. (1997a). The UV images, presented in Figure 8a, show the bright AGN, a population of very luminous starburst knots, a bright oval inner disk, and a fainter, more circular halo. The UV knots were first studied with IUE spectra and were found to be sites of intense recent massive star formation (Snijders, Briggs, & Boksenberg 1982; Weedman & Huenemoerder 1985; Bruhweiler, Truong, & Altner 1991; Hutchings et al. 1991). The brightest knot complex, located ~ 750 pc from the nucleus in PA ~ 315°, has ~ 80 times the luminosity of 30 Dor and gives NGC 1068 a "double nucleus" appearance at UV wavelengths, in marked contrast to its optical morphology. The radial decline of the azimuthally averaged UV surface brightness (Fig. 8b) can be followed for over 8 mag. The plateau in the light profiles associated with the inner disk (r 60") is more pronounced in the UV than the R band. Both the MUV and R profiles flatten at r ~ 100". Neff et al. (1994) suggest that the UV halo is either indirect light from the UV-bright inner disk that is scattered by dust grains, or direct UV radiation from the stellar population in the outer disk. The absence of a significant change in the MUV-R color suggests the latter model, since one expects the dust to preferentially scatter the UV light. Despite the bright AGN, the stellar disk produces 81% of the integrated FUV and 83% of the MUV flux. The bright disk emission in NGC 1068 implies that even at UV wavelengths, the contribution of an active nucleus to the total light can be small and that composite AGN + starburst objects will be difficult to interpret at large redshift.
NGC 1275, Perseus A. - The central dominant galaxy in the Perseus Cluster, NGC 1275 is a peculiar E galaxy whose unusual features and evolutionary state have been extensively studied (e.g., Burbidge & Burbidge 1963; Rubin et al. 1977; McNamara, O'Connell, & Sarazin 1996). It contains a Seyfert-type AGN, evidence for recent star formation throughout its main body, an enormous system of line-emitting filaments near the systemic velocity, and a distinct system of emission line knots with a redshift ~ 3000 km s-1 greater than that of the galaxy. It is one of the best examples of mass accretion from an X-ray cooling flow.
NGC 1275 is detected in both the FUV and MUV (Fig. 9a). The nonthermal nucleus dominates the UV light for r 5", but it contributes only ~ 20% to the integrated FUV flux and 8% to the MUV flux. The off-nuclear FUV and MUV light distributions are clearly asymmetric. Smith et al. (1992) describe the UIT data in more detail. They subtracted symmetric elliptical fits to the MUV and found the residual UV continuum light to be spatially coincident with the extended lower (systemic) velocity H emission. There was evidence for UV emission from the high-velocity system as well, consistent with the photoionization requirements of the high-velocity H emitting gas. The UV colors of the main body of the galaxy (after exclusion of the AGN and correction for significant internal extinction of AFUV ~ 3.5) indicate the presence of stars with masses up to ~ 5 M but not above, suggesting either a cessation of star formation in the last 50-100 Myr or a truncated initial mass function. The "super star clusters" discovered by HST (Holtzman et al. 1992; Carlson et al. 1998) are below our detection threshold. The unusual properties of this galaxy have been interpreted as products of either the cooling flow or a recent interaction/merger.
NGC 1316 (Fornax A, Arp 154). - A member of the Fornax cluster, this peculiar elliptical or S0 galaxy is at a distance of 19 Mpc. A system of dust filaments cross the minor axis, and there are numerous indications for the infall of material (Schweizer 1980). At MUV wavelengths, a central source is observed embedded in diffuse light from the prominent galactic bulge (Fig. 10a). The galaxy is only barely detected in the FUV. The diffuse extension to the southwest on the MUV image is spurious, produced by residual phosphorescence from a preceding exposure of the planet Jupiter. This artifact has been masked out of our photometry.
NGC 1317. - This early-type barred spiral (SAB(r)a) is located 6' north of NGC 1316. The UV images (Fig. 11a) show a bright, broken, elliptical ring of UV knots ~ 17" in diameter (~ 2.1 kpc at the adopted distance), roughly coincident with the H ring. The UV morphology is very different from the optical continuum morphology, which shows only traces of the knotty ring. These strong differences in morphology are found to be most pronounced for early-type disk systems. NGC 4736, observed during Astro-2, is another example of an early-type spiral, (SA(r)ab), with a strong, UV-bright central ring. Lacking multiwavelength imagery, NGC 1317 might be erroneously classified as a "ring" galaxy based solely on the UV morphology. Ring galaxies which have undergone a central penetrating collision (Marcum, Appleton, & Higdon 1992) are extremely rare systems; however, circumnuclear rings of massive star formation are much more common (Crocker, Baugus, & Buta 1996).
NGC 1399. - NGC 1399 is the central and brightest elliptical galaxy in the nearby (D ~ 19 Mpc) Fornax cluster of galaxies and is well detected in both UV bands (see Fig. 12a). The central FUV surface brightness is exceptionally high among the normal E galaxies and spiral bulges we have observed. There is a superposed foreground star visible 13" northeast of the nucleus on the MUV frame. As discussed by O'Connell et al. (1992) and Ohl et al. (1998), the radial light profiles in both the FUV and MUV (Fig. 12b) are consistent with a de Vaucouleurs (µ ~ r0.25) profile over a 5 mag range, suggesting that the FUV light is produced by low-mass stars from the dominant old stellar population. At larger radii, the axial ratio of the UV isophotes matches that of the optical, but there is a flattening of the UV isophotes for r 5" in both UV bands which appears to be real but has not been reported at longer wavelengths (Mackie, Visvanathan, & Carter 1990; Bicknell et al. 1989). There are strong (FUV-MUV) and (UV-optical) color gradients, in which the colors become redder with increasing radius. The central (FUV-MUV) color (~ -0.9, see Fig. 12b) is among the bluest known among normal elliptical galaxies. UIT images of the Fornax cluster field, including NGC 1399 and NGC 1404> are discussed by O'Neil et al. (1996).
NGC 1404.-This elliptical galaxy is also a member of the Fornax Cluster, located ~ 10' southeast of NGC 1399. The UV images (Fig. 13a) show a strong centrally peaked source. The bright object southeast of the galaxy core visible in the MUV image is a foreground star, which was masked out of the photometry. As in the case of most of the other old populations with good detections in the UV, NGC 1404 exhibits a significant UV color gradient (Fig. 13b) in which the colors become redder with increasing radius.
NGC 2146. - This is an infrared-luminous system, probably a postmerger object undergoing a burst of star formation (Hutchings et al. 1990). NGC 2146 appears to be a nearly edge-on spiral with two or more prominent dust lanes and a disturbed morphology. The MUV images show a compact central source, faint emission from the spiral arms (up to 2' from the nucleus), several discrete knots, and the strong dust lane. No emission is evident in the FUV image, presumably because the extensive dust layer lies in the foreground of the young population, similar to the morphology of M82. The UV/optical photometry is discussed further by Hennessy (1995).
NGC 2992 (Arp 245). - This highly inclined (i 70°) Sa galaxy hosts a Seyfert 1.9 nucleus and is interacting with the peculiar galaxy NGC 2993 at a projected separation of ~ 25 kpc. A strong interaction is inferred from the tidal tails and connecting bridge observed optically (Schombert, Wallin, & Struck-Marcell 1990). Optically, NGC 2992 shows a galactic bulge bisected by a dust lane (Ward et al. 1980). NGC 2992 is quite faint in the MUV and is not detected in the FUV (Fig. 15a). The MUV image reveals patchy, diffuse light at the position of the optically defined bulge (Fanelli et al. 1997a). Despite the AGN emission line spectrum seen in the optical (e.g., the bright H emission shown in Fig. 15a), there is no evidence of a compact UV nuclear source, most likely due to the dust lane and high inclination of the galaxy.
NGC 2993 (Arp 245). - This Sa spiral is a companion to NGC 2992. It is oriented more face-on than NGC 2992. By contrast with NGC 2992, the UV images (Fig. 16a) show a bright, centrally peaked source, identified as a probable starburst nucleus. The tracking was poor for the FUV exposure, leading to a slight tailing of the images.
NGC 3031 (M81). - At a distance of ~ 3.6 Mpc and having large angular size (14' × 26'), the grand design SA(s)ab spiral galaxy M81 is morphologically similar to M31 and has often been used as a test of density wave theories (e.g., Adler & Westphal 1996). FOCA images of M81 at 2000 Å with resolution ~ 20" have been analyzed by Blecha et al. (1990) and Reichen et al. (1994), who compared the UV continuum in detail to other tracers of spiral structure, including H, H I, and free-free radio emission.
A preliminary discussion of the UIT images (Fig. 17a) was given by Hill et al. (1992b). In the FUV, the bulge is very faint, and its surface brightness drops precipitously with radius, by 5.5 mag within 50", exhibiting a 2.5 mag increase in the (FUV-MUV) color (Fig. 17b). In both the MUV and FUV bands, the bulge follows a de Vaucouleurs r0.25 profile within r 50". M81 contains a low-luminosity LINER/Seyfert 1 active nucleus. This AGN is not detectable in the UV at our 3" resolution, and our FUV profile shows no evidence of a nuclear point source. However, the active nucleus was recently detected in the UV with HST (Ho, Filippenko, & Sargent 1996; Devereux, Ford, & Jacoby 1997) as a faint point source (d 0".04) which photoionizes a small gas disk extending to r ~ 7".
In both the FUV and MUV frames of Figure 17a, the galaxy has a "ringlike" morphology, produced by the pattern of UV knots strung out along the dominant spiral arms and the faintness of the central regions of the galaxy out to r ~ 200". Not only do the outer parts of the bulge become very faint in the FUV, but there is evidently little underlying disk light inside 200". The inner edge of the disk near r ~ 250" also exhibits a large (FUV-MUV) color gradient, evidence of a rapid change in the mean age of the disk population. There are remarkable changes of morphology with wavelength. Comparing the MUV to the R band, we see that the outer disk becomes more prominent and structured at shorter wavelengths while the central bulge is much less prominent (see Fig. 17a). The contrast between the large UV color gradients and the tiny (B-R) gradient is striking in Figure 17b. The (FUV-MUV) color is roughly uniform at radii larger than 300" in the disk. The (UV-V) colors in the disk are redder than those observed in M33 and M74, indicating an older characteristic age for the disk (Cornett et al. 1994). In the rest-frame FUV, a high-redshift galaxy like M81 would not appear to be a normal spiral galaxy.
UV, H, and extinction properties of the 52 brightest resolved sources are discussed by (Hill et al. 1992b, 1995a). They estimate the total mass in recently formed massive stars (5-120 M) to be 1.4 × 105 M. The inferred star formation rate is 0.13 M yr-1. Allen et al. (1997) have compared the UIT images to H and H I maps of M81 at 9" resolution. They find that while bright H peaks are always associated with bright UV continuum peaks, the converse is not always true, which is expected because the UV continuum persists for many times the duration of strong photoionization. H I is closely associated with the UV continuum, suggesting that it is a produced by stellar UV photodissociation from a massive underlying H2 layer and is not a precursor to star formation. Obscuration by dust has little effect on the morphology of M81 (or most of the other normal galaxies in this atlas), and Allen et al. (1997) argue that local chimneys blown in the dust layer by concentrated star formation permit the UV light to escape with little extinction.
NGC 3034 (M82). - M82 is the peculiar irregular companion to M81. Its central 500 pc contain a concentrated starburst (star formation rate ~ 10 M yr -1) with a bolometric luminosity 100 times larger than a corresponding volume in our own Galaxy (Telesco 1988; Reike et al. 1993; Waller, Gurwill, & Tamura 1992; O'Connell et al. 1995). The starburst has probably been triggered by a tidal interaction with M81 (now ~ 36' distant). M82 is observed nearly edge-on, and a complex network of dust lanes crisscross the galaxy.
M82 was first observed in the UV at 2000 Å with the FOCA experiment at ~ 15" resolution (Courvoisier et al. 1990). Our MUV image (Fig. 18a), with ~ 3" resolution, shows similar global features but with more detail. M82 was not detected in the 270 s FUV exposure, giving a limiting magnitude m(FUV) 14.5 within an aperture of 4'.5 radius.
In the MUV, the galaxy clearly has a peculiar morphology. There are two extended bright structures along the major axis and a fan-shaped plume along the minor axis to the southeast. Fainter regions are detectable at larger radii (to r ~ 5'). Two dust lanes (also visible on the B-band image) are prominent in the MUV image: the strong central lane on the east side of the minor axis plume and another lane about 1' west of this. The brightest central knot on the B- or V-band images (labeled "A" in the notation of O'Connell & Mangano 1978) is considerably less conspicuous in the MUV but is still detectable despite extinction along the line of sight of AV ~ 2 mag. The brightest region in the galaxy in the MUV is region "B," which lies about 1' east of the center. In optical bands this region has a high surface brightness, comparable to region A's, but a significantly lower extinction of AV ~ 0.5 (Marcum & O'Connell 1996). The large UV/optical color excursions (see Fig. 18b) found within the main body appear to be produced mainly by the complex geometry of the dust layer within M82 (Hennessy 1995).
The fanlike MUV extension below the plane of M82 coincides with the southeastern base of the bright H plume discovered by Lynds & Sandage (1963), which is now thought to be associated with a superwind driven by supernova explosions in the starburst (Lehnert, Heckman, & Weaver 1999). However, the MUV light is much fainter on the northwest minor axis, and the general light distribution is much more asymmetrical than in H or optical continuum bands. The MUV/optical colors become bluer at larger radii in the southeastern plume. The MUV band does not contain any prominent emission lines from hot gas, so the light is virtually entirely continuum. Given evidence for extensive dust scattering in the halo of M82 (e.g., Schmidt, Angel, & Cromwell 1976; Scarrott, Eaton, & Axon 1991), the MUV light is best interpreted as scattering by dust grains of photons from the central starburst regions. This would require that lines of sight from the plume to the starburst regions have relatively low UV extinction. The asymmetry can be produced by preferential forward-scattering by grains in the southeastern plume, which kinematic evidence suggests is nearer us (e.g., Bland & Tully 1988). A more detailed discussion of the UV images of M82 is given by Hennessy (1995).
UGC 5336 (Holmberg IX, DDO 66). - UGC 5336 is a low surface brightness, dwarf irregular companion to M81. The MUV image (Fig. 19a), while shallow, does show diffuse irregular emission, but no prominent knots attributable to OB associations or individual main-sequence stars hotter than B0 are detected. Individual cooler stars with masses up to ~ 12 M are detectable distributed across the face of the galaxy in the optical bands, apparently from a recent (~ 20-200 Myr old) population superposed on an older population which dominates the V-band light (Davidge & Jones 1989). The color profiles displayed in Figure 19b indicate that recently formed stars are spatially well mixed with the older population. The star formation history of Holmberg IX as revealed by the UV/optical imagery is discussed by Hill et al. (1993b). UIT data on other star-forming dwarf galaxies from the Astro-2 sample are discussed by Stewart et al. (2000).
UGC 6697. - UGC 6697 is a peculiar, late-type disk system, observed to be the brightest UV source in the field of the cluster Abell 1367. With a observed UV diameter of ~ 1', this system is apparently viewed close to edge-on, yet is remarkably bright in the UV (see Fig. 20a), in striking contrast to the inclined systems M82 and NGC 891. The latter apparently suffer much greater extinction by dust located external to most of the stellar component. UGC 6697 shows blue (FUV-MUV) colors throughout, with the UV/optical colors becoming significantly bluer at larger radii (Fig. 20b). Abell 1367 was also observed at 2000 Å with the FOCA experiment by Donas et al. (1990), who estimated star formation rates for UGC 6697 and other cluster members.
NGC 4151. - NGC 4151 is an early-type spiral, oriented approximately face-on, which hosts the prototypical Seyfert 1.5 AGN. The active nucleus is centered within a weak barlike or oval distortion with a PA ~ 130° (Simkin 1975). Near the ends of the bar are H-emitting knots symmetrically bracketing the nucleus (see Fig. 21a). Low surface brightness spiral arms are visible extending from the end of the barlike structure. The outer disk of NGC 4151 is optically faint and contains no prominent H II regions.
Fanelli et al. (1997a) discuss the UV morphology of NGC 4151. The unresolved, bright AGN dominates the light in both UV bands, producing ~ 81(82)% of the total FUV (MUV) light, unlike most of the other AGN systems in this sample. This result is produced by the absence of substantial massive star formation in the disk, and the face-on orientation of the galaxy, which minimizes absorption of the AGN continuum by dust. In addition to the AGN, two emission regions are seen in both UV bandpasses, on opposite sides of the nucleus. Each region is resolved into a few discrete knots, which correspond to the structure observed in the H image. The UV light in the knots is produced by recent massive star formation which appears to be localized near the end of the central bar. For the assumed distance, these star-forming regions extend to a radius of ~ 6 kpc. In the FUV, an additional diffuse component is detected, oval-shaped and brighter toward the nucleus. The lower surface brightness outer disk present in deep optical (cf. Arp 1977) and H I images (Pedlar et al. 1992) is not seen in the UV. In Figure 21a, note that the center of the H image is saturated.
NGC 4321 (M100). - NGC 4321 is a well-studied, grand design spiral galaxy, type SAB(s)bc, located within the Virgo Cluster. A prominent circumnuclear ring of recent star formation is inferred from H imagery, with a radius of ~ 15". The Astro-1 images are underexposed (see Fig. 22a), revealing the overall spiral arm pattern only faintly. The principal feature observed in the UV is a horseshoe-shaped broken circumnuclear ring, resolved into ~ 15-20 knots, bracketing the optically defined nucleus, which itself is fainter than the ring. This structure is ~ 16" × 24" in diameter, corresponding to a physical size of 1.2 × 1.7 kpc. The star-forming ring appears to be associated with a weak stellar bar observed at I (Pierce 1986) and in the NIR (Knapen et al. 1995). Despite the shallow exposure, it is apparent that the circumnuclear ring dominates the global UV light in this system.
5.2. Secondary Objects
Objects in the sample were classed as "secondary," due either to intrinsic faintness, truncated exposures, or those showing a point-source morphology. We present the UV imagery along with matching optical imagery where available in Figures 23a - 23b.
Mrk 335. - Classified as a Seyfert 1, the active nucleus is embedded in an early type galaxy (E or S0) at a redshift of z = 0.025 (Heckman & Balick 1981; MacKenty 1990). The UIT frames (Fig. 23a) show a bright, unresolved point source in both bandpasses. Fanelli et al. (1997a) find no excess UV flux which could be attributed to the host galaxy after modeling and subtraction of the central point source. The active nucleus dominates the light at UV and optical bandpasses, producing 100% of the UV light, 92% of the R-band flux (MacKenty 1990) and ~ 50% of the light at K (McLeod & Reike 1995).
NGC 891. - As one of the nearest edge-on systems, NGC 891 has been used to explore the vertical structure of galaxy disks (e.g., Morrison et al. 1997). It is unusual in showing bright diffuse H emission out to more than 4 kpc from the plane of the galaxy (Rand, Kulkarni, & Hester 1990), and deep continuum images show dust filaments extending out half that distance (Howk & Savage 1997).
The MUV image is underexposed (Fig. 23a). It shows a faint, spatially resolved linear structure coincident with the midplane of the galaxy. The UV light is most likely produced by stellar associations in the outer disk of the galaxy and by scattering of light from the inner disk by the extended dust. In the FUV (not shown), only very faint extended emission is observed.
NGC 1267, NGC 1268, NGC 1270, NGC 1272, NGC 1273, NGC 1277, NGC 1278, and NGC 1282. - Six of these galaxies detected on the MUV frame centered on NGC 1275 are members of the Perseus cluster. Most are early types, the latest being NGC 1268 (Sb). The brightest of these in the MUV is NGC 1267 (type E+), which shows a very compact source (see Fig. 23a). This could be a mild starburst or an active nucleus, although no radio emission has been reported. Although these objects are faint, we believe their (MUV-R) colors are accurate to 0.1-0.2 mag. After correction for foreground extinction, these have the reddest (MUV-R) colors (see Table 6) of any objects in our sample. A sufficiently large error ( 0.5 mag) in the estimate of foreground AV seems unlikely. Instead, these may be metal-rich systems that lack the large extreme horizontal branch component which boosts the MUV flux in ellipticals like NGC 1399.
UGC 2665. - This late-type, highly inclined disk galaxy is barely detected in the MUV (see Fig. 23a). It is a background galaxy located in the field of view of the Perseus cluster. Like similar late-type disk systems in the primary object sample, the central bulge in this galaxy disappears at UV wavelengths.
NGC 1396. - This early-type galaxy, type SAB0-, is a member of the Fornax cluster. It is detected as a faint but clearly extended source in the MUV (Fig. 23b), but it is not seen in the FUV.
NGC 2551. - This disk system, type SA0/a, was seredipidously observed in a UIT pointing centered on Z Cam. In the MUV (Fig. 23b), the galaxy has a bright nucleus and a moderately bright disk showing clumpy structure.
NGC 3837. - This E galaxy is located in the Abell 1367 cluster. It shows as a slightly elongated structure on the MUV frame (Fig. 23b).
MCG 3-30-71. - This galaxy, of uncertain classification, is located in the Abell 1367 frame. It appears elongated in the MUV image (Fig. 23b), with the same orientation seen in POSS images. There is also a definite detection in the FUV frame.
NGC 3842. - This is another E galaxy located in the Abell 1367 cluster. It has a concentrated core detected in the MUV (Fig. 23b).
CGCG 97-114. - This compact galaxy is located in the Abell 1367 cluster. It appears as point source in both the FUV and MUV.
NGC 4156. - NGC 4156 is an Sb spiral located in the field of view of NGC 4151, but with a substantially larger redshift. Both the FUV and MUV images (Fig. 23b) show a centrally concentrated source and extended emission corresponding to the optical spiral arms.
UGC 7188. - This faint, late-type spiral is also located in the field of view of NGC 4151. The redshift quoted in Schneider et al. (1992) suggests that UGC 7188 is associated with NGC 4151. UGC 7188 appears as a resolved, diffuse source in both UV bandpasses (Fig. 23b). There are evidently large excursions in the UV/optical flux ratio with position.
NGC 4470. - This type Sa disk system is a member of the Virgo cluster, and located in the UIT frame centered on NGC 4472. Faint diffuse emission is seen in both the FUV and MUV frames (Fig. 23b).
NGC 4472. - The UIT frame for this well-known bright elliptical member of the Virgo cluster was badly underexposed. It was only faintly detected in the MUV (Fig. 23b).
NGC 4476. - This early-type galaxy (type SA0-), a member of the Virgo cluster, is located in the UIT frame centered on NGC 4472. It is detected only in the MUV image (Fig. 23b).
NGC 4478. - This elliptical, a member of the Virgo cluster and located in the UIT frame centered on NGC 4472, is faintly detected in the MUV (Fig. 23b) as a point source. Due to a large number of point source-like artifacts in this image (for example, note the bright source to the east), caution should be exercised in using this detection or associated photometry.
NGC 4486 (M87). - This is a well-known giant E galaxy in the Virgo cluster with an active nucleus, bright nonthermal jet, and X-ray cooling flow. The Astro-1 UIT images are underexposed. Only the central parts of the galaxy, including the nucleus and the nonthermal jet (the jet being brighter), are detected (Fig. 23b). Ohl et al. (1998) analyzed deeper exposures of M87 from Astro-2. The brightness of the nucleus and jet notwithstanding, they find that only 10% of the FUV light within a radius of 10" is contributed by these features and conclude that most of the UV light of M87 is stellar.