Dwarf galaxies have received a great deal of observational attention in the last decade for two reasons. The first is that their intrinsic properties provide insight into galaxy formation because dwarf galaxies have a simpler past history of star formation as compared to giant galaxies. Studying the range of galaxy types from dwarfs to giants is also a key test of galaxy formation and evolution models. The second reason deals is the capability of dwarf galaxies to serve as tracers of dark matter. Investigations of dark matter in recent years have questioned the basic premise that light traces the mass on large scales (see Oemler 1988) and there have been suggestions that dwarf, or low mass galaxies, may better sample the distribution of mass in the Universe (Dekel and Rees 1987). The most extreme views question whether the galaxy formation process itself may be biased such that galaxies which formed from low amplitude fluctuations in the early Universe (i.e. dwarf galaxies) are the true indicators of the large scale distribution of mass (Giovanelli, Haynes and Chincarini 1986, Davis and Djorgovski 1985).
The hypothesis that bright, or high mass, galaxies do not trace the true mass distribution of the Universe has come about due to the failure of various cosmological models to correctly predict the amount of large scale structure (voids and walls), or the observed peculiar velocities of the bright galaxies. Parallel to these efforts was the suggestion from grand unified theory (GUT) of new types of stable particles with non-zero mass that interact only weakly with baryons (Turner 1987). The introduction of cold dark matter (CDM) resolves the following cosmological problems: 1) it allows Omega_o=1 without violating the baryon density (Omega_b=0.2) set by primordial nucleosynthesis, 2) CDM allows growth of present day large scale structure from fluctuations currently measured by COBE in the cosmic microwave background (CMB), since the CMB only responds to baryons, 3) CDM can be more smoothly distributed than baryons such that dynamical estimates for Omega_o are too low, 4) it resolves conflicts between cluster models and the galaxy distribution since galaxies do not trace the mass and 5) weak interactions form the basis to segregate baryonic and non-baryonic matter to form dark halos (see Oemler 1988 for a review). The only missing piece is a mechanism to segregate CDM from baryonic matter. On small scales, such as galaxy halos, dissipation is sufficient, but on large scales the mechanism remains undetermined. This has led to the speculation that an inefficiency in galaxy formation produces a bias in the distribution of bright galaxies such that they only trace the peaks of the mass distribution. The theoretical justification for biasing arises from the assumptions that if galaxies form from high sigma fluctuations and if those fluctuations are superimposed on uncorrelated, random, larger-scale fluctuations of small amplitude, then galaxies will be more clustered than the underlying matter distribution (Kaiser 1984).
The theoretical community has also found support for a biased galaxy formation scheme in the many selection effects inherent in galaxy catalogs. Selection effects towards high mass galaxies that would maximize a biased interpretation from observational results such as the autocorrelation function for galaxies. For example, there is a clear surface brightness bias in galaxy catalogs to select against objects with central surface brightnesses near or below the natural sky brightness. Theory predicts that low surface brightness (LSB) galaxies are the result of lower amplitude fluctuations and that redshift surveys, without a complete sample of LSB objects, are biased (Mo, McGaugh and Bothun 1994). Although not all LSB galaxies are dwarfs (Bothun etal 1987) nor are all dwarfs of a LSB nature (Loose and Thuan 1986), a majority of HI-rich dwarfs have central surface brightnesses below 23 B mag arcsec-2 (Schneider etal 1990). Magnitude limited catalogs are known to be sparse in LSB galaxies (Sandage and Tammann 1981) and angular-limited catalogs, such as the UGC (Nilson 1973), that have deeper surface brightness levels will fail to find small dwarf galaxies at useful distances to study large scale structure since they quickly fall below the angular size limits outside of 1000 km sec-1. Searches for dwarf galaxies have primarily focused on nearby rich clusters, such as Virgo or Fornax (Sandage and Binggeli 1984, Caldwell and Bothun 1987), due to the vast number of objects located in a small region of the sky and with distances being determined by association with the cluster. But cluster catalogs are not useful in studying the redshift distribution of dwarfs. All sky catalogs sample the less dense field population (e.g. the DDO catalog, van den Bergh 1960) and are rich in the lowest and faintest type of dwarfs, but are restricted to very local objects.
The purpose of this project is to overcome these complimentary deficiencies in galaxy catalogs by attempting to recover field LSB dwarf galaxies through the use of Second Palomar Sky Survey plates. In addition to the interest in the properties and content of dwarf galaxies with respect to global issues of galaxy formation and evolution, dwarf galaxies are also the leading candidates for the well known faint blue galaxies excess (FBE, see Kron 1980, Tyson 1988, Lilly, Cowie and Gardner 1991). Current models indicated that a majority of the FBE can be explained by a simple, evolving dwarf population (Driver etal 1996). Estimates are that up to 30% of the galaxy luminosity density is due to dwarfs and the number density is 20 times greater than that of normal or giant galaxies. This population is typically low in mean surface brightness and fails to be included in catalogs based on isophotal magnitude or diameter. Thus, a secondary goal of this project is to investigate the optical and HI properties of a unbiased sample of dwarfs, selected by morphology and unrestricted by surface brightness. Papers II and III in this series will deal with these topics.
All distance related values in this paper use values of H_o = 85 km sec-1 Mpc-1, Omega_o = 0.2, a Virgo velocity of 977 km sec-1 and a Virgo infall of 300 km sec-1.
Catalog Paper (Paper I):
HI Paper (Paper III):
Analysis Paper (Paper IV):
To inspect the data on individual dwarfs, click the table below:
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