An Atlas of Hubble Space Telescope Ultraviolet Images of Nearby Galaxies1 Dan Maoz2, Alexei V. Filippenko3, Luis C. Ho3,4, F. Duccio Macchetto5, Hans-Walter Rix6, and Donald P. Schneider7 1 Based on observations with the Hubble Space Telescope which is operated by AURA, Inc., under NASA contract NAS 5-26555 2 School of Physics & Astronomy and Wise Observatory, Tel-Aviv University, Tel-Aviv 69978, Israel. [email protected] 3 Department of Astronomy, University of California, Berkeley, CA 94720-3411 4 Center for Astrophyics, 60 Garden Street, Cambridge, MA 02138 5 Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218 6 Steward Observatory, University of Arizona, Tucson, AZ 85721 7 Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 ABSTRACT We present an atlas of UV (∼ 2300 A)˚ images, obtained with the Hubble Space Telescope (HST) Faint Object Camera, of the central 22′′ × 22′′ of 110 galaxies. The observed galaxies are an unbiased selection constituting about one half of a complete sample of all large (D > 6′) and nearby (V < 2000 km s−1) galaxies. This is the first extensive UV imaging survey of normal galaxies. The data are useful arXiv:astro-ph/9605170v1 28 May 1996 for studying star formation, low-level nuclear activity, and UV emission by evolved stellar populations in galaxies. At the HST resolution (∼ 0.05′′), the images display an assortment of morphologies and UV brightnesses. These include bright nuclear point sources, compact young star clusters scattered in the field or arranged in circumnuclear rings, centrally-peaked diffuse light distributions, and galaxies with weak or undetected UV emission. We measure the integrated ∼ 2300 A˚ flux in each image, classify the UV morphology, and examine trends between these parameters and the optical properties of the galaxies. Subject headings: atlases – ultraviolet: galaxies – galaxies: structure – galaxies: active – galaxies: star clusters – galaxies: nuclei 1 1. Introduction are probably bound. In a forthcoming work (Ho et al. 1996b) we will use the UV images to analyze the star- Ultraviolet (UV) imaging is a powerful tool for the formation properties of all the galaxies in the sample. study of galaxies. Recent star formation and low- In this paper, we present images and basic data for the level nuclear activity can be traced and measured in 110 galaxies observed. The images provide a reference UV images, unhindered by the bright background of for the UV brightness and morphology of the galaxies the older stellar populations which dominate in visual in the sample, which can be used as a general guide bands. The source of UV emission in some quiescent to the UV properties of the centers of galaxies, and early-type galaxies can also be studied with UV imag- as an aid in extracting the actual data from the HST ing. Prior to the launch of the Hubble Space Telescope archive. We also provide a classification of the UV (HST), few galaxies were imaged in the UV. These few morphology, additional notes on some of the galaxies were mostly Local Group spiral galaxies observed at and what is seen in their images, and a measurement low angular resolution using rocket-borne telescopes of the ∼ 2300 A˚ flux integrated over the image. We on brief flights (see Bohlin et al. 1990, and refer- tabulate these together with the larger-scale proper- ences therein). This situation has improved in recent ties of the galaxies and (for the northern galaxies) years with the flight of UV imaging telescopes on the the spectroscopic nuclear classification from Ho, Fil- Space Shuttle (e.g., Landsman et al. 1992; Hill et ippenko, & Sargent (1995, 1996a). In §2, below, we al. 1992), although the angular resolution is still low describe the sample selection and the observations. In and the number of galaxies observed is small, due to §3 we present tables listing the galaxy parameters, a the limited mission duration. In this respect, HST, pictorial atlas of the data, and notes on some of the with its high sensitivity and angular resolution, has individual galaxies. In §4 we intercompare the UV opened up a new frontier. Summaries of recent HST and optical properties of the galaxies and provide a UV-imaging studies of galaxies appear in Benvenuti, brief summary. Macchetto, & Schreier (1996). We have carried out a UV imaging survey with 2. Observations the pre-COSTAR HST Faint Object Camera (FOC) of the central regions of 110 large nearby galaxies, se- The sample from which the observed galaxies were lected randomly from a complete sample of 240 galax- chosen consists of all galaxies in the UGC and ESO ies. The main purposes of the survey are (a) to de- catalogs (Lauberts & Valentijn 1989) with heliocen- tect low-luminosity active galactic nuclei (AGNs), ex- tric velocities available in the literature and less than ploiting the high contrast of a nuclear UV continuum 2000 km s−1, and photographic diameters (as defined source above the low UV background from stars, and in the catalogs) greater than 6′. A total of 22 galaxies (b) to study star formation in the central regions of were removed from this initial sample of 262 galaxies the galaxies; by observing at ∼ 2300 A,˚ the survey during subsequent stages of the sample definition. In images detect primarily the youngest existing stel- 21 of these, no nuclear position could be defined be- lar populations and thus provide a clean probe of cause they were too diffuse, too low in surface bright- the most recent sites of active star formation, un- ness, or edge-on with strong dust lanes. One galaxy contaminated by light from more evolved stars. The (ESO 1331−4517) had a bright foreground star near power of this technique, especially when augmented the nucleus, which would have endangered the HST by the high HST angular resolution, has been recently instruments. This left a well-defined sample of 240 demonstrated in UV studies of starbursts by Conti & galaxies with observable nuclei. Vacca (1994), Meurer et al. (1995), and Maoz et al. Twenty-seven galaxies from this sample were not (1996). included in our target list because they were proposed Maoz et al. (1995) presented data from the survey targets of other HST UV-imaging programs. How- for nine galaxies displaying bright compact nuclear ever, we have subsequently obtained from the HST UV sources, five of which are potential low-luminosity archive the data for seven of these galaxies which were AGNs. Maoz et al. (1996) analyzed the images of five actually observed with an instrumental configuration additional galaxies having circumnuclear star-forming similar to the one we used (FOC + UV filter at ∼ 2300 rings, and showed that much of the star formation in A).˚ the rings is confined to massive compact clusters that Digitized photographs from the GASP archive at 2 Space Telescope Science Institute (STScI) of all po- have used, the FOC is limited in its dynamic range to tential target galaxies were examined and the coordi- 255 counts (8 bits) per zoomed pixel; additional signal nates of the nucleus determined to ∼ 1′′ −2′′ precision causes the counts to “fold over” and start again from by computing the centroid of the light distribution. In 0. Another problem is that the detected count rate a few cases, as evidenced by the eventual HST images, becomes nonlinear, gradually saturating for bright the galaxy nucleus position was off by up to ∼ 5′′, due sources (see Baxter et al. 1994). The central pix- to isophote distortion by dust features in the GASP els of most of the compact bright sources detected images. in the images may be in the nonlinear regime, and The observations were done in Snapshot mode – the brightest of them are clearly saturated. In sev- i.e., targets were chosen from the target list by the eral galaxies where we report the brightness and/or STScI staff based on the convenience of their location angular size of individual compact sources, our analy- on the sky. The brief exposure was used to fill the sis relies mainly on the wings of the PSF, which have −1 −1 gaps left in the observing schedule after other science low count rates (∼< 0.05 s pixel ), using the algo- programs had been scheduled. The observed galaxies rithms described by Maoz et al. (1995, 1996). We are therefore an unbiased selection from the complete model the PSF using a well-exposed F220W image of sample. a star observed with the FOC f/96 256 × 256 pixel mode, which has a large dynamic and linear range Among the galaxies described in this paper, 103 (but small field of view). Such empirical PSFs are were successfully observed with HST while the pro- required for work in the UV (Baxter et al. 1994). gram was active, in 1993, March through July. We supplement these observations with data for the seven As in Maoz et al. (1995, 1996), we translate the out of the 27 “embargoed” galaxies that were ob- FOC counts to a flux density fλ at 2270 A˚ assuming −1 −17 −1 −2 −1 served with HST between 1991 and 1995. We there- 1 count s =1.66 × 10 erg s cm A˚ , based fore have data for a total of 110 galaxies out of on the on-line calibration data available from STScI the complete sample of 240 galaxies. Observations for the FOC and F220W filter, with a 25% increase in were unsuccessful for four galaxies due to wrong co- sensitivity of the 512 × 1024 zoomed-pixel mode rela- ordinates (NGC 660) or telescope malfunction (NGC tive to the 512 × 512 pixel mode (Baxter et al. 1994).