We have investigated the mass-metallicity (M-Z ) relation using galaxies at 0:4 < z < 1:0 from the Gemini Deep Deep Survey (GDDS) and Canada-France Redshift Survey (CFRS). Deep K-and z 0 -band photometry allowed us to measure stellar masses for 69 galaxies. From a subsample of 56 galaxies, for which metallicity of the interstellar medium is also measured, we identified a strong correlation between mass and metallicity for the first time in the distant universe. This was possible because of the larger baseline spanned by the sample in terms of metallicity (a factor of 7) and mass (a factor of 400) than in previous works. This correlation is much stronger and tighter than the luminosity-metallicity relation, confirming that stellar mass is a more meaningful physical parameter than luminosity. We find clear evidence for temporal evolution in the M-Z relation in the sense that at a given mass, a galaxy at z $ 0:7 tends to have lower metallicity than a local galaxy of similar mass. We use the z $ 0:1 Sloan Digital Sky Survey M-Z relation and a small sample of z $ 2:3 Lyman break galaxies with known mass and metallicity to propose an empirical redshift-dependent M-Z relation. According to this relation the stellar mass and metallicity in small galaxies evolve for a longer time than they do in massive galaxies. This relation predicts that the generally metal-poor damped Ly galaxies have stellar masses of the order of 10 8:8 M (with a dispersion of 0.7 dex) all the way from z $ 0:2 to 4. The observed redshift evolution of the M-Z relation can be reproduced remarkably well by a simple closed-box model in which the key assumption is an e-folding time for star formation that is higher or, in other words, a period of star formation that lasts longer in less massive galaxies than in more massive galaxies. Such a picture supports the downsizing scenario for galaxy formation.

We report the discovery of 47 low surface brightness objects in deep images of a 3 • × 3 • field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array. The objects have central surface brightness µ(g, 0) ranging from 24 -26 mag arcsec −2 and effective radii r eff = 3 ′′ -10 ′′ , as measured from archival Canada France Hawaii Telescope images. From their spatial distribution we infer that most or all of the objects are galaxies in the Coma cluster. This relatively large distance is surprising as it implies that the galaxies are very large: with r eff = 1.5 kpc -4.6 kpc their sizes are similar to those of L * galaxies even though their median stellar mass is only ∼ 6 × 10 7 M ⊙ . The galaxies are relatively red and round, with g − i = 0.8 and b/a = 0.74. One of the 47 galaxies is fortuitously covered by a deep Hubble Space Telescope ACS observation. The ACS imaging shows a large spheroidal object with a central surface brightness µ 475 = 25.8 mag arcsec −2 , a Sersic index n = 0.6, and an effective radius of 7 ′′ , corresponding to 3.4 kpc at the distance of Coma. The galaxy is not resolved into stars, consistent with expectations for a Coma cluster object. We speculate that these "ultra-diffuse galaxies" (UDGs) may have lost their gas supply at early times, possibly resulting in very high dark matter fractions.

To re-examine the rich cluster $\Omega$ value the CNOC Cluster Survey has observed 16 high X-ray luminosity clusters in the redshift range 0.17 to 0.55, obtaining approximately 2600 velocities in their fields. Directly adding all the K and evolution corrected $r$ band light to $M_r(0)=-18.5$, about $0.2L_\ast$, and correcting for the light below the limit, the average mass-to-light ratio of the clusters is $283\pm27h\msun/\lsun$ and the average mass per galaxy is $3.5\pm0.4\times10^{12}h^{-1}\msun$. The clusters are consistent with having a universal $M_v/L$ value (within the errors of about 20\%) independent of their velocity dispersion, mean color of their galaxies, blue galaxy content, redshift, or mean interior density. Using field galaxies within the same data set, with the same corrections, we find that the closure mass-to-light, $\rho_c/j$, is $1160\pm130h\msun/\lsun$ and the closure mass per galaxy, $\rho_c/\phi(>0.2L_\ast)$, is $13.2\pm1.9\times10^{12}h^{-1}\msun$. Under the assumptions that the galaxies are distributed like the mass and that the galaxy luminosities and numbers are statistically conserved, which these data indirectly support, $\Omega_0=0.20\pm0.04\pm0.09$ where the errors are, respectively, the $1\sigma$ internal and an estimate of the $1\sigma$ systematic error resulting from the luminosity normalization.Comment: 34 page Latex document (no figures) requiring AAS macros. Postscript document (or uufile) availble at http://manaslu.astro.utoronto.ca/~carlberg/cnoc/general.htm

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We have analyzed the redshift-dependent fraction of galactic bars over 0:2 < z < 0:84 in 2157 luminous face-on spiral galaxies from the COSMOS 2 deg 2 field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z $ 0:84 this fraction drops to about 20%. Over this redshift range the fraction of strong bars (SBs) drops from about 30% to under 10%. It is clear that when the universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z $ 0:84, whereas for the low-mass, blue spirals it declines significantly with redshift beyond z ¼ 0:3. There is also a slight preference for bars in bulge-dominated systems at high redshifts that may be an important clue toward the coevolution of bars, bulges, and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate.

In this paper we present a new statistic for quantifying galaxy morphology based on measurements of the Gini coefficient of galaxy light distributions. This statistic is easy to measure and is commonly used in econometrics to measure how wealth is distributed in human populations. When applied to galaxy images, the Gini coefficient provides a quantitative measure of the inequality with which a galaxy's light is distributed amongst its constituent pixels. We measure the Gini coefficient of local galaxies in the Early Data Release of the Sloan Digital Sky Survey and demonstrate that this quantity is closely correlated with measurements of central concentration, but with significant scatter. This scatter is almost entirely due to variations in the mean surface brightness of galaxies. By exploring the distribution of galaxies in the three-dimensional parameter space defined by the Gini coefficient, central concentration, and mean surface brightness, we show that all nearby galaxies lie on a well-defined two-dimensional surface (a slightly warped plane) embedded within a three-dimensional parameter space. By associating each galaxy sample with the equation of this plane, we can encode the morphological composition of the entire SDSS g * -band sample using the following three numbers: {22.451, 5.366, 7.010}. The i * -band sample is encoded as: {22.149, 5.373, and 7.627}.

Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M/M has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 10 solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2 × 10 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 10 solar masses. This implies that the ratio M/M is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.