The Growth of Galaxy Stellar Mass Within Dark Matter Halos

We study the evolution of stellar mass in galaxies as a function of host halo mass, using the "MPA" and "Durham" semi-analytic models, implemented on the Millennium Run simulation. The results from both models are similar. We find that about 45% of the stellar mass in central galaxies in present-day halos less massive than ~10^{12} Msun/h is already in place at z~1. This fraction increases to ~65% for more massive halos. The peak of star formation efficiency shifts toward lower mass halos from z~1 to z~0. The stellar mass in low-mass halos grows mostly by star formation since z~1, while in high-mass halos most of the stellar mass is assembled by mergers. These trends are clear indications of "halo downsizing". We compare our findings to the results of the phenomenological method developed by Zheng, Coil & Zehavi (2007). The theoretical predictions are in qualitative agreement with these results, however there are large discrepancies. The most significant one concerns the amount of stars already in place in the progenitor galaxies at z~1, which is about a factor of two larger in both semi-analytic models. We also use the semi-analytic catalogs to test different assumptions made in that work, and illustrate the importance of smooth accretion of dark matter when estimating the mergers contribution. We demonstrate that methods studying galaxy evolution from the galaxy-halo connection are powerful in constraining theoretical models and can guide future efforts of modeling galaxy evolution. Conversely, semi-analytic models serve an important role in improving such methods.Comment: 13 pages, 8 figures, submitted to Ap

Quantifying the coherent outflows of galaxies around voids in the SDSS DR7

We report the detection, with a high level of confidence, of coherent outflows around voids found in the seventh data release of the Sloan Digital Sky Survey (SDSS DR7). In particular, we developed a robust statistical test to quantify the strength of redshift-space distortions (RSD) associated with extended coherent velocity fields. We consistently find that the vector that joins void centers with galaxies that lie in shells around them is more likely to be perpendicular to the line-of-sight than parallel to it. This effect is clear evidence for the existence of outflows in the vicinity of voids. We show that the RSD exist for a wide range of void radius and shell thickness, but they are more evident in the largest voids in our sample. For instance, we find that the for galaxies located in shells within 2 h^-1 Mpc from the edge of voids larger than 15 h^-1 Mpc deviates 3.81sigma from uniformity. The measurements presented in this work provide useful information to constrain cosmological parameters, in particular Omega_m and Sigma_8.Comment: 4 pages, 1 figure, to appear in A&A Letter

The Properties of Galaxies in Voids

We present a comparison of the properties of galaxies in the most underdense regions of the Universe, where the galaxy number density is less than 10% of the mean density, with galaxies from more typical regions. We have compiled a sample of galaxies in 46 large nearby voids that were identified using the Sloan Digital Sky Survey DR4, which provides the largest coverage of the sky. We study the u-r color distribution, morphology, specific star formation rate, and radial number density profiles for a total of 495 galaxies fainter than M_r=-20.4 +5logh located inside the voids and compare these properties with a control sample of field galaxies. We show that there is an excess of blue galaxies inside the voids. However, inspecting the properties of blue and red galaxies separately, we find that galaxy properties such as color distribution, bulge-to-total ratios, and concentrations are remarkably similar between the void and overall sample. The void galaxies also show the same specific star formation rate at fixed color as the control galaxies. We compare our results with the predictions of cosmological simulations of galaxy formation using the Millennium Run semi-analytic galaxy catalog. We show that the properties of the simulated galaxies in large voids are in reasonably good agreement with those found in similar environments in the real Universe. To summarize, in spite of the fact that galaxies in voids live in the least dense large-scale environment, this environment makes very little impact on properties of galaxies.Comment: 11 pages, 15 figures, Submitted to MNRA

Colloid Carcinoma of Gallbladder - Incidental Finding of a Rare Entity

Majority gall bladder carcinomas are conventional adenocarcinomas of pancreatobiliary type. Mucinous carcinomas in gall bladder constitutes 2.5% of all gallbladder carcinomas. Pure mucinous (colloid carcinomas) are even rarer. Its rarity deserves this case being highlighted. Less than twenty-five cases of mucinous carcinomas of the gallbladder have been reported so far. A 46-year-old female presented with recurrent right upper quadrant pain along with nausea and vomiting for last one year. Liver function tests & kidney function tests were within range. USG Abdomen revealed an enlarged gallbladder measuring 8x5 cm with no stones. No other abnormalities were found on USG. A simple cholecystectomy was performed for chronic cholecystitis. Gross examination of specimen showed glistening, globular, soft and cystic gall bladder filled with mucoid tenacious yellowish material. There was no discernible mass lesion. Microscopic examination revealed pools of extracellular mucinous material dissecting muscular wall containing few signet ring cells. A diagnosis of colloid or pure mucinous adenocarcinoma of the gallbladder was made. Pure mucinous carcinomas as seen in exocrine glands like breast, pancreas and skin are very uncommon in gall bladder. Mucinous variant (>50% stromal mucin) are most of the time admixed with conventional type. Mucinous carcinomas most of the time are large and advanced at the time of diagnosis and are aggressive in nature than conventional type. This case is presented owing to its extreme rarity

Properties of voids in the Local Volume

Current explanation of the overabundance of dark matter subhalos in the Local Group (LG) indicates that there maybe a limit on mass of a halo, which can host a galaxy. This idea can be tested using voids in the distribution of galaxies: at some level small voids should not contain any (even dwarf) galaxies. We use observational samples complete to M_B=-12 with distances less than 8 Mpc to construct the void function (VF): the distribution of sizes of voids empty of any galaxies. There are ~ 30 voids with sizes ranging from 1 to 5 Mpc. We also study the distribution of dark matter halos in very high resolution simulations of the LCDM model. The theoretical VF matches the observations remarkably well only if we use halos with circular velocities larger than 45 +/- 10 km/s. This agrees with the Local Group predictions. Small voids look quite similar to heir giant cousins: the density has a minimum at the center of a void and it increases as we get closer to the border. Thus, both the Local Group data and the nearby voids indicate that isolated halos below 45 +/- 10 km/s must not host galaxies and that small (few Mpc) voids are truly dark.Comment: 5 pages 1 figure. To appear in proceedings of the conference "Galaxies in the Local Volume", Sydney, 8 to 13 July 200

A hierarchy of voids: More ado about nothing

We extend earlier work on the problem of estimating the void-volume function -- the abundance and evolution of large voids which grow gravitationally in an expanding universe -- in two ways. The first removes an ambiguity about how the void-in-cloud process, which erases small voids, should be incorporated into the excursion set approach. The main technical change here is to think of voids within a fully Eulerian, rather than purely Lagrangian, framework. The second accounts for correlations between different spatial scales in the initial conditions. We provide numerical and analytical arguments showing how and why both changes modify the predicted abundances substantially. In particular, we show that the predicted importance of the void-in-cloud process depends strongly on whether or not one accounts for correlations between scales. With our new formulation, the void-in-cloud process dramatically reduces the predicted abundances of voids if such correlations are ignored, but only matters for the smallest voids in the more realistic case in which the spatial correlations are included.Comment: 9 pages, 3 figures; v2 -- improved Eulerian void-assignment algorithm, new figures (including LCDM walks) and clarified discussion. Conclusions regarding walks with correlated steps unchanged. Accepted in MNRA

Detailed theoretical predictions of the outskirts of dark matter halos

In the present work we describe the formalism necessary to derive the properties of dark matter halos beyond two virial radius using the spherical collapse model (without shell crossing), and provide the framework for the theoretical prediction presented in Prada et al. (2005). We show in detail how to obtain within this model the probability distribution for the spherically-averaged enclosed density at any radii P(delta,r). Using this probability distribution, we compute the most probable and mean density profiles, which turns out to differ considerably from each other. We also show how to obtain the typical profile, as well as the probability distribution and mean profile for the spherically averaged radial velocity. Two probability distributions are obtained: a first one is derived using a simple assumption, that is, if Q is the virial radius in Lagrangian coordinates, then the enclosed linear contrast delta_l(q,Q) must satisfy the condition that delta_l(q=Q) = delta_vir, where delta_vir is the linear density contrast within the virial radius Rvir at the moment of virialization. Then we introduce an additional constraint to obtain a more accurate P(delta,r) which reproduces to a higher degree of precision the distribution of the spherically averaged enclosed density found in the simulations. This new constraint is delta_l(q,Q) < delta_vir for all q > Q, which means that there are no radii larger than Rvir where the density contrast is larger than that used to define the virial radius. Finally, we compare in detail our theoretical predictions for the probability distributions with the results found in the simulations.Comment: 12 pages, 8 figures, 1 table, replaced to match the published versio

How far do they go? The outer structure of dark matter halos

We study the density profiles of collapsed galaxy-size dark matter halos with masses 1e11-5e12 Msun focusing mostly on the halo outer regions from the formal virial radius Rvir up to 5-7Rvir. We find that isolated halos in this mass range extend well beyond Rvir exhibiting all properties of virialized objects up to 2-3Rvir: relatively smooth density profiles and no systematic infall velocities. The dark matter halos in this mass range do not grow as one naively may expect through a steady accretion of satellites, i.e., on average there is no mass infall. This is strikingly different from more massive halos, which have large infall velocities outside of the virial radius. We provide accurate fit for the density profile of these galaxy-size halos. For a wide range (0.01-2)Rvir of radii the halo density profiles are fit with the approximation rho=rho_s exp(-2n[x^{1/n}-1])+rho_m, where x=r/r_s, rho_m is the mean matter density of the Universe, and the index n is in the range n=6-7.5. These profiles do not show a sudden change of behavior beyond the virial radius. For larger radii we combine the statistics of the initial fluctuations with the spherical collapse model to obtain predictions for the mean and most probable density profiles for halos of several masses. The model give excellent results beyond 2-3 formal virial radii.Comment: 15 pages, 10 figures, submitted to Ap