Probing Dark Matter Substructure in Lens Galaxies

We investigate the effects of numerous dark matter subhalos in a galaxy-sized halo on the events of strong lensing, to assess their presence as expected from the cold dark matter scenario. Lens galaxies are represented by a smooth ellipsoid in an external shear field and additional cold dark matter subhalos taken from Monte Carlo realizations which accord with recent N-body results. We also consider other possible perturbers, globular clusters and luminous dwarf satellites, for comparison. We then apply the models to the particular lens systems with four images, B1422+231 and PG1115+080, for which smooth lens models are unable to reproduce both the positions of the images and their radio flux ratios or dust-free optical flux ratios simultaneously. We show that the perturbations by both globular clusters and dwarf satellites are too small to change the flux ratios, whereas cold dark matter subhalos are most likely perturbers to reproduce the observed flux ratios in a statistically significant manner. This result suggests us the presence of numerous subhalos in lens galaxies, which is consistent with the results of cosmological N-body simulations.Comment: 19 pages, including 5 figures, ApJ in pres

The local space density of dwarf galaxies

We estimate the luminosity function of field galaxies over a range of ten magnitudes (-22 < M_{B_J} < -12 for H_0 = 100 km/s/Mpc) by counting the number of faint APM galaxies around Stromlo-APM redshift survey galaxies at known distance. The faint end of the luminosity function rises steeply at M_{B_J} \approx -15, implying that the space density of dwarf galaxies is at least two times larger than predicted by a Schechter function with flat faint-end slope. Such a high abundance of dwarf galaxies at low redshift can help explain the observed number counts and redshift distributions of faint galaxies without invoking exotic models for galaxy evolution.Comment: 20 pages, 5 included postscript figures, uses AAS LaTex macros. Accepted for publication in the Astrophysical Journal. Two figures and associated discussion added; results and conclusions unchange

The Properties of Poor Groups of Galaxies: III. The Galaxy Luminosity Function

We obtain R-band photometry for galaxies in six nearby poor groups for which we have spectroscopic data, including 328 new galaxy velocities. For the five groups with luminous X-ray halos, the composite group galaxy luminosity function (GLF) is fit adequately by a Schechter function with Mstar = -21.6 +/- 0.4 + 5log h and alpha = -1.3 +/- 0.1. We also find that (1) the ratio of dwarfs to giants is significantly larger for the five groups with luminous X-ray halos than for the one marginally X-ray detected group, (2) the composite GLF for the luminous X-ray groups is consistent in shape with that for rich clusters, (3) the composite group GLF rises more steeply at the faint end than that of the field, (4) the shape difference between the field and composite group GLF's results mostly from the population of non-emission line galaxies, whose dwarf-to-giant ratio is larger in the denser group environment than in the field, and (5) the non-emission line dwarfs are more concentrated about the group center than the non-emission line giants. This last result indicates that the dwarfs and giants occupy different orbits (i.e., have not mixed completely) and suggests that the populations formed at a different times. Our results show that the shape of the GLF varies with environment and that this variation is due primarily to an increase in the dwarf-to-giant ratio of quiescent galaxies in higher density regions, at least up to the densities characteristic of X-ray luminous poor groups. This behavior suggests that, in some environments, dwarfs are more biased than giants with respect to dark matter. This trend conflicts with the prediction of standard biased galaxy formation models. (Abridged)Comment: 36 pages, AASLaTeX with 8 figures. Table 1 also available at http://atropos.as.arizona.edu/aiz/papers/all_grp_lf_ascii.dat.final . To appear in Ap

The Dwarf Galaxy Population of the Dorado group down to Mv=-11

We present V and I CCD photometry of suspected low-surface brightness dwarf galaxies detected in a survey covering ~2.4 deg^2 around the central region of the Dorado group of galaxies. The low-surface brightness galaxies were chosen based on their sizes and magnitudes at the limiting isophote of 26.0V\mu. The selected galaxies have magnitudes brighter than V=20 (Mv=-11 for an assumed distance to the group of 17.2 Mpc), with central surface brightnesses \mu0>22.5 V mag/arcsec^2, scale lengths h>2'', and diameters > 14'' at the limiting isophote. Using these criteria, we identified 69 dwarf galaxy candidates. Four of them are large very low-surface brightness galaxies that were detected on a smoothed image, after masking high surface brightness objects. Monte Carlo simulations performed to estimate completeness, photometric uncertainties and to evaluate our ability to detect extended low-surface brightness galaxies show that the completeness fraction is, on average, > 80% for dwarf galaxies with $-17<M_{V}<-10.5$ and 22.5<\mu0<25.5 V mag/arcsec^2, for the range of sizes considered by us (D>14''). The V-I colors of the dwarf candidates vary from -0.3 to 2.3 with a peak on V-I=0.98, suggesting a range of different stellar populations in these galaxies. The projected surface density of the dwarf galaxies shows a concentration towards the group center similar in extent to that found around five X-ray groups and the elliptical galaxy NGC1132 studied by Mulchaey and Zabludoff (1999), suggesting that the dwarf galaxies in Dorado are probably physically associated with the overall potential well of the group.Comment: 32 pages, 16 postscript figures and 3 figures in GIF format, aastex v5.0. To appear in The Astronomical Journal, January 200

The Luminosity Function of Galaxies in Compact Groups

From R-band images of 39 Hickson compact groups (HCGs), we use galaxy counts to determine a luminosity function extending to M_R=-14.0, approximately two magnitudes deeper than previous compact group luminosity functions. We find that a single Schechter function is a poor fit to the data, so we fit a composite function consisting of separate Schechter functions for the bright and faint galaxies. The bright end is best fit with M^*=-21.6 and alpha=-0.52 and the faint end with M^*=-16.1 and alpha=-1.17. The decreasing bright end slope implies a deficit of intermediate luminosity galaxies in our sample of HCGs and the faint end slope is slightly steeper than that reported for earlier HCG luminosity functions. Furthermore, luminosity functions of subsets of our sample reveal more substantial dwarf populations for groups with x-ray halos, groups with tidal dwarf candidates, and groups with a dominant elliptical or lenticular galaxy. Collectively, these results support the hypothesis that within compact groups, the initial dwarf galaxy population is replenished by "subsequent generations" formed in the tidal debris of giant galaxy interactions.Comment: 26 pages, to be published in The Astrophysical Journal, 8 greyscale plates (figures 1 and 2) can be retrieved at http://www.astro.psu.edu/users/sdh/pubs.htm

Selling Ethics: Discourses of Responsibility in Tourism

10.1080/24694452.2016.1218266Annals of the American Association of Geographers1071218-23