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Register a new userThe Faint End of the Luminosity Function and Low Surface Brightness Galaxies
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SHELS (Smithsonian Hectospec Lensing Survey) is a dense redshift survey covering a 4 square degree region to a limiting R = 20.6. In the construction of the galaxy catalog and in the acquisition of spectroscopic targets, we paid careful attention to the survey completeness for lower surface brightness dwarf galaxies. Thus, although the survey covers a small area, it is a robust basis for computation of the slope of the faint end of the galaxy luminosity function to a limiting M_R = -13.3 + 5logh. We calculate the faint end slope in the R-band for the subset of SHELS galaxies with redshif ts in the range 0.02 <= z < 0.1, SHELS_{0.1}. This sample contains 532 galaxies with R< 20.6 and with a median surface brightness within the half light radius of SB_{50,R} = 21.82 mag arcsec^{-2}. We used this sample to make one of the few direct measurements of the dependence of the faint end of the galaxy luminosity function on surface brightness. For the sample as a whole the faint end slope, alpha = -1.31 +/- 0.04, is consistent with both the Blanton et al. (2005b) analysis of the SDSS and the Liu et al. (2008) analysis of the COSMOS field. This consistency is impressive given the very different approaches of th ese three surveys. A magnitude limited sample of 135 galaxies with optical spectroscopic reds hifts with mean half-light surface brightness, SB_{50,R} >= 22.5 mag arcsec^{-2} is unique to SHELS_{0.1}. The faint end slope is alpha_{22.5} = -1.52+/- 0.16. SHELS_{0.1} shows that lower surface brightness objects dominate the faint end slope of the l uminosity function in the field, underscoring the importance of surface brightness limits in evaluating measurements of the faint end slope and its evolution.
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We measure the faint end slope of the galaxy luminosity function (LF) for cluster galaxies at 1<z<1.5 using Spitzer IRAC data. We investigate whether this slope, alpha, differs from that of the field LF at these redshifts, and with the cluster LF at low redshifts. The latter is of particular interest as low-luminosity galaxies are expected to undergo significant evolution. We use seven high-redshift spectroscopically confirmed galaxy clusters drawn from the IRAC Shallow Cluster Survey to measure the cluster galaxy LF down to depths of M* + 3 (3.6 microns) and M* + 2.5 (4.5 microns). The summed LF at our median cluster redshift (z=1.35) is well fit by a Schechter distribution with alpha[3.6] = -0.97 +/- 0.14 and alpha[4.5] = -0.91 +/- 0.28, consistent with a flat faint end slope and is in agreement with measurements of the field LF in similar bands at these redshifts. A comparison to alpha in low-redshift clusters finds no statistically significant evidence of evolution. Combined with past studies which show that M* is passively evolving out to z~1.3, this means that the shape of the cluster LF is largely in place by z~1.3. This suggests that the processes that govern the build up of the mass of low-mass cluster galaxies have no net effect on the faint end slope of the cluster LF at z<1.3.
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We present and discuss optical measurements of the faint end of the galaxy luminosity function down to M_R = -10 in five different local environments of varying galaxy density and morphological content. The environments we studied, in order of decreasing galaxy density, are the Virgo Cluster, the NGC 1407 Group, the Coma I Group, the Leo Group and the NGC 1023 Group. Our results come from a deep wide-angle survey with the NAOJ Subaru 8 m Telescope on Mauna Kea and are sensitive down to very faint surface-brightness levels. Galaxies were identified as group or cluster members on the basis of their surface brightness and morphology. The faintest galaxies in our sample have R ~ 22.5. There were thousands of fainter galaxies but we cannot distinguish cluster members from background galaxies at these faint limits so do not attempt to determine a luminosity function fainter than M_R = -10. In all cases, there are far fewer dwarfs than the numbers of low mass halos anticipated by cold dark matter theory. The mean logarithmic slope of the luminosity function between M_R = -18 and M_R = -10 is alpha ~ -1.2, far shallower than the cold dark matter mass function slope of alpha ~ -1.8. We would therefore need to be missing about 90 per cent of the dwarfs at the faint end of our sample in all the environments we study to achieve consistency with CDM theory.
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