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Register a new userThe Faint End of the Luminosity Function in the Core of the Coma Cluster
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We present optical measurements of the faint end of the luminosity function in the core of the Coma cluster. Dwarf galaxies are detected down to a limiting magnitude of R approx. 25.75 in images taken with the Hubble Space Telescope. This represents the faintest determination of the Coma luminosity function to date. With the assumption that errors due to cosmic variance are small, evidence is found for a steep faint end slope with alpha < -2. Such a value is expected in theories in which reionization and other feedback processes are dependent on density.
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Our previous study of the faint end (R$leq$21.5) of the galaxy luminosity function (GLF) was based on spectroscopic data in a small region near the Coma cluster center. In this previous study Adami et al. (1998) suggested, with moderate statistical significance, that the number of galaxies actually belonging to the cluster was much smaller than expected. This led us to increase our spectroscopic sample. Here, we have improved the statistical significance of the results of the Coma GLF faint end study (R$leq$22.5) by using a sample of 85 redshifts. This includes both new spectroscopic data and a literature compilation. The relatively small number of faint galaxies belonging to Coma that was suggested by Adami et al. (1998) and Secker et al. (1998) has been confirmed with these new observations. We also confirm that the color-magnitude relation is not well suited for finding the galaxies inside the Coma cluster core, close to the center at magnitudes fainter than R$sim$19. We show that there is an enhancement in the Coma line of sight of field galaxies compared to classical field counts. This can be explained by the contribution of groups and of a distant $zsim 0.5$ cluster along the line of sight. The result is that the Coma GLF appears to turn-over or at least to become flat for the faint galaxies. We suggest that this is due to environmental effects.
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.
I review recent measurements of the faint end of the galaxy luminosity function in galaxy clusters. Evidence is presented that the luminosity function of galaxies in the central parts of clusters is remarkably constant between clusters and that this luminosity function is steep at bright and faint magnitudes and shallow in-between. The curvature is highly significant -- neither a power-law nor a Schechter function is consistent with the data. At no magnitude does alpha=-1 fit the data well. The faintest galaxies in all clusters that have been studied are dwarf spheroidal galaxies.
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.
The Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) is constructing a wide-field map of the resolved stellar populations in the extended halos of these two nearby, prominent galaxies. We present new Magellan/Megacam imaging of a $sim3$ deg$^2$ area around Centaurus A (Cen A), which filled in much of our coverage to its south, leaving a nearly complete halo map out to a projected radius of $sim$150 kpc and allowing us to identify two new resolved dwarf galaxies. We have additionally obtained deep Hubble Space Telescope (HST) optical imaging of eleven out of the thirteen candidate dwarf galaxies identified around Cen A and presented in Crnojevic et al. (2016): seven are confirmed to be satellites of Cen A, while four are found to be background galaxies. We derive accurate distances, structural parameters, luminosities and photometric metallicities for the seven candidates confirmed by our HST/ACS imaging. We further study the stellar population along the $sim$60 kpc long (in projection) stream associated with Dw3, which likely had an initial brightness of $M_{V}$$sim$$-$15 and shows evidence for a metallicity gradient along its length. Using the total sample of eleven dwarf satellites discovered by the PISCeS survey, as well as thirteen brighter previously known satellites of Cen A, we present a revised galaxy luminosity function for the Cen A group down to a limiting magnitude of $M_Vsim-8$, which has a slope of $-1.14pm0.17$, comparable to that seen in the Local Group and in other nearby groups of galaxies.
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