Copyright © 1997 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 1997. 35:
267-307 Copyright © 1997 by Annual Reviews. All rights reserved |
Over the past 15 years, a number of surveys have succeeded in locating low
surface brightness galaxies. The distribution in surface brightness is
continuous, but operationally we choose to define galaxies with
µ0 
23
mag arcsec-2 as low surface brightness. In terms of the
narrow surface brightness distribution of
Freeman (1970),
a disk galaxy this diffuse should be extremely
rare. In practice, low surface brightness galaxies are a mixed bag,
including objects as diverse as giant gas-rich disks and dwarf
spheroidals. This of course
is the reason for their importance: such galaxies offer a new window
onto the diversity of galaxy morphology and evolution.
Unsurprisingly for someone who left his idiosyncratic mark on much of extragalactic astronomy, Zwicky (1957) was one of the first to speculate on the existence of low surface brightness galaxies. His claim of a steeply inceasing tail of faint galaxies was at odds with Hubble's (1936) earlier Gaussian form for the luminosity function. In retrospect, both were correct; Hubble had identified mostly galaxies of high surface brightness, Zwicky had discovered an exponential tail of mostly low surface brightness dwarfs. The David Dunlap Observatory (DDO) catalog (van den Bergh 1959) was the first catalog to contain significant numbers of diffuse galaxies, although most of them had low mass and so were not representative of the full range of LSB types. Meanwhile, Reaves (1956) and Arp (1965) had identified the selection effect which might lead low surface brightness galaxies to be missed. This selection bias was first clearly formulated by Disney (1976).
The discovery of low surface brightness galaxies advanced considerably in the 1980s. In a prescient piece of work, Longmore et al (1982) obtained optical and 21 cm data on a sample of 151 LSB galaxies selected by visual inspection of UK Schmidt plates. Many early studies were based on the diameter limited Uppsala General Catalog of Galaxies (UGC; Nilson 1973). With no explcit surface brightness selection, the UGC contains for example an order of magnitude more LSB galaxies than the catalog of Fisher & Tully (1981). LSB galaxies from the UGC catalog were subsequently studied by Romanishin et al (1982), who noted that they had relatively large amounts of gas for their luminosity.
Large numbers of LSB dwarfs were detected in the monumental photographic
survey of the Virgo cluster by
Binggeli, Sandage, &
Tammann (1985)
using plates taken with
the Du Pont 100-inch telescope. Another large survey was carried out in
the nearby Fornax cluster using both du Pont plates
(Ferguson & Sandage
1988,
Ferguson 1989)
and sky survey plates from the UK Schmidt Telescope
(Phillipps et al 1987).
The surface brightness limits of both these surveys were
µlim
25 B mag
arcsec-2, however the spatial resolution and morphological
classification is superior on the du Pont plates, which gives an
advantage in defining cluster membership in the absence of redshifts.
The next improvement was offered by visual searches of the POSS-II
plates, which reach to a deeper limiting isophote,
µlim
26 B mag
arcsec-2
(Schombert & Bothun
1988,
Schombert et al 1992).
Binggeli, Tarenghi &
Sandage (1990)
used deep Palomar plates to identify several hundred predominantly LSB
dIm and dE galaxies.
Impey, Bothun & Malin
(1988)
used photographically amplified images of
Virgo to push the limiting isophote down to
µlim
27.5 B mag arcsec-2 ,
and a similar surface brightness can be reached by automated scans of UK
Schmidt plates
(Irwin et al 1990a).
These studies yielded new samples of extremely LSB
galaxies in Virgo and Fornax
(Davies et al 1988;
Bothun, Impey & Malin
1991).
A further gain in sensitivity can be achieved by digitally stacking scans of
existing sky survey plates, or by using Tech Pan emulsions; large-scale
surveys are currently underway with limits of
µlim
27 R mag
arcsec-2
(Schwartzenberg, Phillipps
& Parker 1995b).
Digital detectors can survey for LSB galaxies in the field down to a
much lower limiting isophote, but over much smaller areas
(Schwartzenberg et al
1995a).
CCD surveys of nearby
(Turner et al 1993,
Bernstein et al 1995)
and more distant clusters have been undertaken
(Driver et al 1994).
The Texas Survey for field LSB galaxies adds a new dimension with red
selection down to µlim
27.5 R mag
arcsec-2
(O'Neil et al 1997).
Dalcanton et al (1997b)
have used strip scans made with the Palomar 200-inch telescope operating
in transit mode to find galaxies with
23 < µ0 < 27 V mag
arcsec-2 . This approaches the
limit below which individual galaxies cannot be distinguished from distant
clusters of galaxies
(Schectman 1973).
At the limit of deep surveys, LSB
galaxies are being mined from the WFPC2 images of the Hubble Deep Field by
several groups. The next major step forward in large area surveys will come
with the Sloan Digital Sky Survey
(Gunn & Knapp 1993).
The diversity of galaxies uncovered by these surveys is striking. An early surprise was the accidental discovery of the giant LSB disk galaxy Malin 1 in a survey of the Virgo cluster (Bothun et al 1987). This remarkable galaxy is the prototype of systems which have low surface mass density stellar disks, large physical sizes, and enormous amounts of neutral hydrogen (Impey & Bothun 1989, Knezek 1993). These galaxies are extreme cousins of the gas-rich LSB galaxies discussed by Longmore et al (1982), characterized by large exponential scale lengths (s > 10 kpc) and low central surface brightnesses (µ0 > 25 B mag arcsec-2 ). Further examples have recently been found (Bothun et al. 1990, Sprayberry et al. 1993, 1995a). Dwarf spirals have also been detected (Schombert et al 1995); both the smallest and the largest spiral galaxies known have low surface brightness. At the other extreme are LSB dwarfs, with similar surface brightness but much smaller scale lengths (s ~ 1-2 kpc). Surface brightness selection accounts for the fact that dwarf members of the Local Group continue to be discovered (Irwin et al 1990b). Even at the modest distance of the Coma cluster, many Local Group dwarf spheroidals would be too low in surface brightness to be detected in a shallow survey, and too compact to be distinguished from stars in a deep survey.