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J/ApJS/107/215 UV Images of Nearby Galaxies (Maoz+, 1996)
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An Atlas of Hubble Space Telescope Ultraviolet Images of Nearby Galaxies
Maoz D., Filippenko A. V., Ho L. C., Macchetto D., Rix H., Schneider D. P.
<Astrophys. J. Suppl. 107, 215 (1996)>
=1996ApJS..107..215M
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ADC_Keywords: Galaxies, nearby ; Ultraviolet ; Atlases
Mission_Name: HST
Keywords: Atlases - Galaxies: active - Galaxies: nuclei -
Galaxies: star clusters - Galaxies: structure -
Ultraviolet: galaxies
Abstract:
The accompanying tables are part of a pictorial atlas of UV (2300A)
images, obtained with the Hubble Space Telescope (HST) Faint Object
Camera, of the central 22"x22" 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) galaxies. This is
the first extensive UV imaging survey of normal galaxies. The data are
useful 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 2300A flux in each image, and classify the UV morphology.
The UV and optical parameters are given in the tables.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
atlas.dat 116 110 The atlas
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See also:
http://archive.stsci.edu/HDA/ : Hubble Data Archive Home Page
B/hst : HST Archived Exposures Catalog
Byte-by-byte Description of file: atlas.dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- NGC ?NGC number
5 A1 --- arch *[a] a indicates an archival image
6- 10 I5 --- UGC ?UGC number (northern galaxies only)
12- 20 A9 --- Bname *Name based on B1950 position
22- 23 I2 h RAh Right ascension hours (J2000)
25- 26 I2 min RAm Right ascension minutes (J2000)
28- 32 F5.2 s RAs *Right ascension seconds (J2000)
34 A1 --- DE- Declination sign (J2000)
35- 36 I2 deg DEd Declination degrees (J2000)
38- 39 I2 arcmin DEm Declination minutes (J2000)
41- 44 F4.1 arcsec DEs *Declination seconds (J2000)
46- 50 I5 km/s Hvel *Heliocentric velocity
52- 55 I4 0.1arcmin Dmaj *Major axis diameter from UGC and ESO
57- 59 I3 0.1arcmin Dmin *Minor axis diameter from UGC and ESO
61- 65 F5.2 mag Bmag ?B magnitude from UGC and ESO
67- 68 I2 --- T *de Vaucouleurs type
70- 80 A11 --- class *Hubble type
81- 82 A2 --- lum *Luminosity class
84- 91 A8 "DD/MM/YY" date HST UT observation date
93-101 A9 --- Dataset Dataset name in HST archive
103-107 F5.1 10-17W/m2/nm flux *Total flux at 2270{AA}
109-112 F4.1 10-17W/m2/nm e_flux One sigma uncertainty on flux
114-116 A3 --- morph *UV morphology from HST image
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Note on arch:
An "a" denotes archival images, generally obtained with a different
FOC format, filter, or exposure time than those of the program
galaxies.
Note on Bname:
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.
Note on RAs, DEs:
Coordinates of the nucleus, as measured in the STScI GASP system.
These coordinates are generally accurate to 1"-2".
Note on Hvel:
Heliocentric velocity, as listed in the UGC or ESO catalogs, in km/s.
The selection criterion for inclusion in the sample was
Hvel < 2000 km/s.
Note on Dmaj, Dmin:
In tenths of arcminutes, from the UGC and ESO catalogs. The selection
criterion for inclusion in the sample was Dmaj > 60.
Note on T:
Hubble type, using de Vaucouleurs' T-type classification from the RC3
catalog (de Vaucouleurs et al. 1991, Cat. <VII/155>). The
correspondence is approximately as follows:
E = -6 to -4
S0 = -3 to 0
Sa = 1
Sb = 3
Sc = 5
Sd = 7
Irr = 10
Note on class:
Hubble type and luminosity class using the classification, when
available, from the Revised Shapley-Ames Catalog of Bright Galaxies
(Sandage & Tammann 1987, see Cat. <VII/51>).
Note on lum:
For northern galaxies only, the bytes 81-82 contain the spectral
classification of the nucleus, from Ho et al. (1995ApJS...98..477H,
1996a). The designation is as follows.
L = LINER (low-ionization nuclear emission-line region)
H = H II nucleus
T = "transition" source, between LINER and HII
S = Seyfert nucleus
A = "absorption-line" nucleus with no detected emission lines.
: = an uncertain classification.
The Ho et al. classification is based on the Filippenko & Sargent
(1985ApJS...57..503F, 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 (1985ApJS...57..503F) sample. The effective aperture of the
optical observations is 2"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.
Note on flux:
Total f({lambda}2270{AA}) in units of 10^-15^ erg/s/cm^-2^/{AA}
integrated above the background over the entire area of the image, and
1sigma uncertainty. The area of each image is 22x22", except for
some of the archival exposures. The background was determined as
follows. The mean counts per pixel were measured in seventeen
200x200-pixel squares in the frame, excluding occulting fingers and
distortions in the FOC field (see Baxter et al. 1994), 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 of the paper. The flux uncertainty due to the background
uncertainty was combined in quadrature with a 5% absolute calibration
uncertainty (Meurer 1995AJ....110.2665M) 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{AA} fluxes
measured for those galaxies that have also been observed with IUE
(Kinney et al. 1993ApJS...86....5K; see also Meurer
1995AJ....110.2665M), 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 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.
Note on morph:
UV morphology 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 from the original authors on each of these types
are described in the "UV Morphology" section below.
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UV Morphology:
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. 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 >60deg). 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. 1995AJ....110.2665M; 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. 1988ApJ...328..440B), rather than visual-band emission leaking
through the F220W filter, based on several tests described in section
2 of the paper. In individual cases, however, confirmation by means of
blue and near-UV imaging photometry of the center of each of these
galaxies is required.
Galaxies with bright nuclear UV point sources ("P-type") have been
discussed in detail by Maoz et al. (1995ApJ...440...91M), especially
in the context of low-luminosity AGNs. They showed that 20% of the
northern LINER galaxies display a nuclear point source in the FOC
images, with a UV flux that, if extrapolated beyond the Lyman limit,
could be sufficient to produce the observed strength of optical
emission lines through photoionization. While this was the first
direct detection of what may be the AGN-like ionizing source in LINER
galaxies, it raised the question of why such a source is not detected
in 80% of LINERs. Several of the P-type sources in in this paper,
especially the weak ones and those in archival images, were not
included in Maoz et al. (1995ApJ...440...91M). However, the fraction
of "UV-bright" LINERs, or LINERs plus Seyferts, remains unchanged. For
example, among the 35 northern galaxies with spectral classification
T, L, or S (transition-type, LINER, or Seyfert), nine have a nuclear
UV point source.
The five circumnuclear rings in the sample (designated "R") have been
discussed in detail by Maoz et al. (1996AJ....111.2248M), 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.
References:
Baxter, D. A., Gilmore, D., Greenfield, P. E., Hack, W., Hodge, P.,
Jedrzejewski, R. I., & Nota, A. 1994, in "HST Data Handbook", ed. S.
Baum (Baltimore: STScI)
Burstein, D., Bertola, F., Buson, L.M., Faber, S.M., & Lauer, T.R. 1988,
ApJ, 328, 440 =1988ApJ...328..440B
de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H. G., Buta, R. J.,
Paturel, G., & Fouque, P. 1991, Third Reference Catalog of Bright
Galaxies (New York: Springer)
Filippenko, A. V., & Sargent, W. L. W 1985, ApJS, 57, 503
=1985ApJS...57..503F
Filippenko, A. V., & Sargent, W. L. W. 1986, in Structure and Evolution
of Active Galactic Nuclei, eds. G. Giuricin et al. (Dordrecht:
Reidel), 21
Ho, L. C., Filippenko, A. V., & Sargent, W. L. W. 1995, ApJS, 98, 477
=1995ApJS...98..477H
Ho, L. C., Filippenko, A. V., & Sargent, W. L. W. 1996a, ApJ, in
preparation
Ho, L. C., et al. 1996b, in preparation
Kinney, A. L., Bohlin, R. C., Calzetti, D., Panagia, N., & Wyse, R. F.
G. 1993, ApJS, 86, 5 =1993ApJS...86....5K
Maoz, D., Filippenko, A. V., Ho, L. C., Rix, H. -W., Bahcall, J. N.,
Schneider, D. P., & Macchetto, F. D. 1995, ApJ, 440, 91
=1995ApJ...440...91M
Maoz, D., Barth, A. J. Sternberg, A., Filippenko, A. V., Ho, L. C.,
Macchetto, F. D., Rix, H. -W., & Schneider, D. P. 1996, AJ, in press
=1996AJ....111.2248M
Maoz, D. 1996, in "The Physics of LINERs in View of Recent
Observations", eds. M. Eracleous et al. (Baltimore: STScI), in press
Meurer, G. R. 1995, Instrument Science Report FOC-083 (Baltimore:
STScI)
Meurer, G.R., Heckman, T. M., Leitherer, C., Kinney, A., Robert, C., &
Garnett, D. R. 1995, AJ, 110, 2665 =1995AJ....110.2665M
Sandage, A., & Tammann, G. A. 1987, A Revised Shapley-Ames Catalog of
Bright Galaxies, 2nd edition (Washington DC: Carnegie Institution of
Washington) (See Cat. <VII/51>)
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(End) N. G. Roman [ADC/SSDOO] 24-Feb-1997