The Formation of Low Surface Brightness Galaxies

The formation of low surface brightness galaxies is an unavoidable prediction of any hierarchical clustering scenario. In these models, low surface brightness galaxies form at late times from small initial overdensities, and make up most of the faint end o

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The formation of low surface brightness galaxies is an unavoidable prediction of any hierarchical clustering scenario. In these models, low surface brightness galaxies form at late times from small initial overdensities, and make up most of the faint end o

with progressively larger objects forming at progressively later times through merging of the earlier generations of smaller objects. Galaxies form fairly late in these models, with clusters of galaxies assembling only in the most recent times. Hundreds of analytical and numerical papers have explored the predictions of hierarchical structure formation and have been able to reproduce many of the properties of normal galaxies and clusters (see for example White& Frenk 1991, Efstathiou& Silk 1983, and the recent review by White 1994). However, as discussed in these papers, hierarchical structure formation consistently predicts many more faint galaxies than are actually observed. The faint-end of the predicted luminosity function is much steeper than luminosity functions generated from catalogs of nearby galaxies. Simultaneously, there has been increasing attention focused on the existence of an often overlooked population of low surface brightness galaxies (LSB's). Because of the brightness of the night sky, obser

vers are naturally biased towards detecting high-surface brightness galaxies, whose high contrast against the background makes them easily detectable. This strong selection e ect, noted by Zwicky (1957) and further explored by Disney (1976), can lead to strong correlations in the properties of observed galaxies that are not intrinsic to the objects (Disney 1976, Allen& Shu 1979), most notably the universal surface brightness of spiral disks discovered by Freeman (1970). Whenever the bias against nding low surface brightness galaxies has been reduced, however, LSB's have appeared which violate the Freeman relation (see surveys by Impey, Bothun,& Malin 1988, Bothun, Impey,& Malin 1991, Schombert et al. 1992, Schombert& Bothun 1988,Irwin, Davies, Disney,& Phillipps 1990, Turner, Phillipps, Davies,& Disney 1993, Sprayberry et al 1995, and references therein). These surveys show that LSB's do exist and have been previously overlooked as a potentially signi cant species in the galaxy menagerie. The existence of elusive but omnipresent LSB's implies that we have been attempting to solve the puzzle of galaxy formation with many pieces missing. Thankfully, our ignorance is succumbing to a growing body of observations of low surface brightness galaxies. First, LSB's exist at every surface brightness to which surveys have been sensitive. They have been detected down to central surface brightnesses of 26.5 mag/arcsec2 in V (Dalcanton 1995); fainter than this, it is di cult to separate LSB's from true uctuations in the optical extragalactic background due to distant clusters of galaxies (Shectman 1973, Dalcanton 1994). Second, LSB's exist at every size, from minute dwarf galaxies in the Local Group, through galaxies with scale lengths typical of\normal" high surface brightness (HSB) galaxies (McGaugh& Bothun 1994), up to a handful of truly giant galaxies with scale lengths of 50 kpc, typi ed by Malin I1 (Bothun et al. 1987). Third, LSB's are roughly a factor1

The exceptional giant\Malin-type" galaxies, while being fascinatingly odd, are unlikely to be the dominant type of LSB galaxy. Their large sizes imply that 2-dimensional surveys have extremely large accessible

The formation of low surface brightness galaxies is an unavoidable prediction of any hierarchical clustering scenario. In these models, low surface brightness galaxies form at late times from small initial overdensities, and make up most of the faint end o

of two less correlated than HSB galaxies in the CfA and IRAS catalogs on scales> 1 Mpc (Mo, McGaugh,& Bothun 1994), and even less correlated on smaller scales (Bothun et al. 1993). Finally, extensive studies of LSB colors (McGaugh& Bothun 1994, Knezek 1993) and spectroscopy of HII regions (McGaugh 1994, McGaugh& Bothun 1994) show that LSB's have rather blue colors (although with large scatter) that cannot be explained by either low metallicity or high current star formation rates; the blue colors are more readily explained by LSB's having a relatively young mean age with a relatively long time-scale for star formation. The properties of known LSB's t naturally with their likely formation scenario. Their clustering properties and long formation timescales immediately suggest that

LSB's form from the collapse of smaller amplitude overdensities than normal HSB galaxies (Mo, McGaugh,& Bothun 1994), with the exception of Malin-type LSB's which likely form from rare, isolated 3 peaks (Ho man, Silk,& Wyse 1992). For a simple top-hat collapse of an overdense region, small amplitude peaks in the background density take longer to reach their maximum size and longer to recollapse than higher amplitude peaks, implying that galaxies that collapse from the smaller peaks will have later formation times and longer collapse times. In any theory with Gaussian uctuations, small amplitude galaxy-sized peaks are more likely to be found in underdense regions, where their mean level is pulled down by the large scale underdensity, suggesting that objects which collapse from small amplitude peaks will be less correlated than those that collapse from larger ones (Kaiser 1984, White et al. 1987). These are exactly the properties being uncovered for LSB galaxies. In this paper, we …… 此处隐藏：42594字，全部文档内容请下载后查看。喜欢就下载吧 ……