eaa.iop.orgDOI: 10.1888/0333750888/2620 Low Surface Brightness GalaxiesErwin de Blok FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:17:53 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsLow Surface Brightness GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSLow Surface Brightness GalaxiesLow surface brightness (LSB) galaxies are galaxies thatemit much less light per area than normal galaxies.Because of their lower contrast with the night sky they arehard to find, and hence their contribution to the generalgalaxy population has long been underestimated.Galaxies with low surface brightnesses are hard tofind because the night sky is never black. Scatteredsunlight coming from dust inside and outside ouratmosphere, air-glow, street lights, faint Galactic andextra-Galactic sources all illuminate the sky. Thisforeground and background glare acts as a filter that onlylets us see a part of the total galaxy population.Compare this with the example of a person sittinginside a brightly lit room at night. This person will havetrouble seeing what is happening outside because of thebright reflections of the room in the windows. Only thebrightest objects (e.g. headlights of cars) will be easilyvisible. Headlights of bicycles, or pedestrians will be veryhard to make out, even though they may dominate thetraffic outside. The bright reflections thus filter out largeparts of the total population.In more practical terms, selection effects caused bythe brightness of the night sky have made astronomersonly give attention to those galaxies that were most easilyobserved. The underlying reason for these selectioneffects is that galaxy catalogs are usually created usingcertain isophotal diameter and/or magnitude limits (withan additional implicit surface brightness limit). Thebrightness of the night sky ensures that galaxies of acertain fixed luminosity look largest, and are thus mosteasily seen, if their central surface brightness has someintermediate ‘optimum’ value. If they have higher surfacebrightnesses they look too compact and resemble stars;with lower surface brightnesses they are difficult todistinguish from the night sky.Diameter- or magnitude-limited catalogs thus mainlycontain galaxies with a narrow range in central surfacebrightnesses. They will be biased and incomplete forgalaxies with central surface brightnesses other than the‘optimum’ value.In the following, galaxies that are not affectedby selection effects and that have an optimum valuefor detection are called ‘high surface brightness’ (HSB)galaxies. For all practical purposes these can be equatedto the well-knownSPIRAL GALAXIES that define the Hubblesequence. The dimmer galaxies that are affected byselection effects are called LSB galaxies. Examples areshown in figure 1.Selection effectsThe best way to get a feeling for the severity ofselection effects is by discussing a practical example.We will discuss five different types of model galaxies,which approximately span the currently known range instructural properties ofDISK GALAXIES. All are assumed to bepure exponential disks, with central surface brightnessesFigure 1. An HSB galaxy (NGC 2403, left panel) and an LSBgalaxy (UGC 128, right panel) with identical luminosities,shown at the same physical scale, and observed with the sameinstrument. As the light of the LSB galaxy is much more spreadout it is harder to distinguish from the glare of the night sky.µ0and scale lengths h as given in table 1. Four of thesegalaxies (A, B, C and D) have identical luminosities MB=−21. The fifth one, E, is equal in size to galaxy B, but 2.5mag or a factor of 10 fainter. It is furthermore assumed thatall five types are spread through space homogeneouslyand in equal numbers. In each volume element there aretherefore an equal number of different types of galaxies.We will now ‘observe’ these galaxies and create aCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Low Surface Brightness GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSTable 1. Contribution of various galaxy types in diameter-limited catalog.TypeaMBbhcµ0dDmaxeFractionA −21.0 0.5 16.0 60 0.09 Compact HSBB −21.0 5.0 21.0 125 0.72 Normal HSBC −21.0 25.0 24.5 76 0.18 Big LSBD −21.0 50.0 26.0 — 0.00 Giant LSBE −18.5 5.0 23.5 30 0.01 Normal LSBaType of galaxy.bAbsolute magnitude.cScale length in kpc.dCentral surface brightness in mag arcsec−2.eMaximum distancein Mpc where galaxy is included in catalog.fFraction of catalogedgalaxies that will be of this type.diameter-limited catalog. The condition for inclusion inthe catalog will be that the diameter of the 25 mag arcsec−2isophote D25must be larger than 1 arcmin. It is obviousthat galaxies at large distances will look smaller than thosenearby. Therefore, as we examine each type of galaxy andmove it toincreasingly largerdistances, its D25will at somepoint become less than 1 arcmin, and consequently it willno longer be included in the catalog. The distance wheregalaxies of a certain type drop out of the catalog is givenin table 1: galaxy A will drop out of the sample at 60 Mpc;galaxy B will be in the sample out to distances of 125 Mpc;galaxy C out to 76 Mpc. Galaxy D has no D25diameterand will never be taken up in the catalog. Finally, galaxyE will be seen only out to 30 Mpc.Galaxies of type Bcorrespond tonormal HSBgalaxies,while type E represents typical LSB galaxies. Type C isa big LSB galaxy. Types A and D are in practice rare.Cataloged HSB galaxies (such as B) can be detected ina volume 72 times larger than the volume in which LSBgalaxies (such as E) can be detected. Assuming thatall types of galaxies are spread equally through space,the corresponding volumes cataloged galaxies occupyshow that the catalog will be severely dominated byHSB galaxies of type B and will lead to the conclusionthat almost three-quarters of the total galaxy populationconsists of type B. Only an apparently insignificant 1% ofthe galaxies in the catalog are of type E. Furthermore, wewould not even know that giant galaxies, such as type D,existed. On the basis of these results we would concludethat three-quarters of the observed galaxy population hada constant central surface brightness.To correct for