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Growth of the structure in the Universe manifest as accretion flows of galaxies onto groups and clusters. Thus, the present day properties of groups and their member galaxies are influenced by the characteristics of this continuous infall pattern. Several works both theoretical, in numerical simulations, and in observations, study this process and provide useful steps for a better understanding of galaxy systems and their evolution. We aim at exploring the streaming flow of galaxies onto groups using observational peculiar velocity data. The effects of distance uncertainties are also analyzed as well as the relation between the infall pattern and group and environment properties.This work deals with analysis of peculiar velocity data and their projection on the direction to group centers, to determine the mean galaxy infall flow. We applied this analysis to the galaxies and groups extracted from the Cosmicflows-3 catalog. We also use mock catalogs derived from numerical simulations to explore the effects of distance uncertainties on the derivation of the galaxy velocity flow onto groups. We determine the infalling velocity field onto galaxy groups with cz < 0.033 using peculiar velocity data. We measure the mean infall velocity onto group samples of different mass range, and also explore the impact of the environment where the group reside. Well beyond the group virial radius, the surrounding large-scale galaxy overdensity may impose additional infalling streaming amplitudes in the range 200 to 400 km s$^{-1}$. Also, we find that groups in samples with a well controlled galaxy density environment show an increasing infalling velocity amplitude with group mass, consistent with the predictions of the linear model. These results from observational data are in excellent agreement with those derived from the mock catalogs.
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Galaxy clusters are expected to form hierarchically in a LCDM universe, growing primarily through mergers with lower mass clusters and the continual accretion of group-mass halos. Galaxy clusters assemble late, doubling their masses since z~0.5, and so the outer regions of clusters should be replete with infalling group-mass systems. We present an XMM-Newton survey to search for X-ray groups in the infall regions of 23 massive galaxy clusters at z~0.2, identifying 39 X-ray groups that have been spectroscopically confirmed to lie at the cluster redshift. These groups have mass estimates in the range 2x10^13-7x10^14Msun, and group-to-cluster mass ratios as low as 0.02. The comoving number density of X-ray groups in the infall regions is ~25x higher than that seen for isolated X-ray groups from the XXL survey. The average mass per cluster contained within these X-ray groups is 2.2x10^14Msun, or 19% of the mass within the primary cluster itself. We estimate that ~10^15Msun clusters increase their masses by 16% between z=0.223 and the present day due to the accretion of groups with M200>10^13.2Msun. This represents about half of the expected mass growth rate of clusters at these late epochs. The other half is likely to come from smooth accretion of matter not bound in halos. The mass function of the infalling X-ray groups appears significantly top-heavy with respect to that of field X-ray systems, consistent with expectations from numerical simulations, and the basic consequences of collapsed massive dark matter halos being biased tracers of the underlying large-scale density distribution.
We perform statistical analyses to study the infall of galaxies onto groups and clusters in the nearby Universe. The study is based on the UZC and SSRS2 group catalogs and peculiar velocity samples. We find a clear signature of infall of galaxies onto groups over a wide range of scales 5 h^{-1} Mpc<r<30 h^{-1} Mpc, with an infall amplitude on the order of a few hundred kilometers per second. We obtain a significant increase in the infall amplitude with group virial mass (M_{V}) and luminosity of group member galaxies (L_{g}). Groups with M_{V}<10^{13} M_{odot} show infall velocities V_{infall} simeq 150 km s^{-1} whereas for M_{V}>10^{13} M_{odot} a larger infall is observed, V_{infall} simeq 200 km s^{-1}. Similarly, we find that galaxies surrounding groups with L_{g}<10^{15} L_{odot} have V_{infall} simeq 100 km s^{-1}, whereas for L_{g}>10^{15} L_{odot} groups, the amplitude of the galaxy infall can be as large as V_{infall} simeq 250 km s^{-1}. The observational results are compared with the results obtained from mock group and galaxy samples constructed from numerical simulations, which include galaxy formation through semianalytical models. We obtain a general agreement between the results from the mock catalogs and the observations. The infall of galaxies onto groups is suitably reproduced in the simulations and, as in the observations, larger virial mass and luminosity groups exhibit the largest galaxy infall amplitudes. We derive estimates of the integrated mass overdensities associated with groups by applying linear theory to the infall velocities after correcting for the effects of distance uncertainties obtained using the mock catalogs. The resulting overdensities are consistent with a power law with delta sim 1 at r sim 10 h^{-1}Mpc.
Two aspects of filamentary molecular cloud evolution are addressed: (1) Exploring analytically the role of the environment for the evolution of filaments demonstrates that considering them in isolation (i.e. just addressing the fragmentation stability) will result in unphysical conclusions about the filaments properties. Accretion can also explain the observed decorrelation between FWHM and peak column density. (2) Free-fall accretion onto finite filaments can lead to the characteristic fans of infrared-dark clouds around star-forming regions. The fans may form due to tidal forces mostly arising at the ends of the filaments, consistent with numerical models and earlier analytical studies.
Ongoing accretion of low-metallicity gas onto the disk is a natural prediction of semi-analytical Galactic chemical evolution models. This star formation fuel ameliorates the overproduction of metal-poor G- and K-dwarfs in the solar neighbourhood which otherwise plague so-called ``closed-box models of Galaxy evolution. Do High-Velocity Clouds (HVCs) represent the source of this necessary fuel? We know that HVCs provide an important clue as to the processes governing galaxy formation and evolution - what is less clear is whether their role lies more closely aligned with cosmology (as relics of the Local Groups formation) or star formation (as tidal debris from nearby disrupted dwarfs, or the waste byproducts of disk supernova-driven winds). I provide a summary of recent speculations as to the origins of HVCs, and highlight several future projects which will lead to a deeper understanding of the role they play in galaxy evolution.
In an extension of Fischera & Martin (2012a) and Heitsch (2013), two aspects of the evolution of externally pressurized, hydrostatic filaments are discussed. (a) The free-fall accretion of gas onto such a filament will lead to filament parameters (specifically, FWHM--column density relations) inconsistent with the observations of Arzoumanian et al. (2011), except for two cases: For low-mass, isothermal filaments, agreement is found as in the analysis by Fischera & Martin (2012b). Magnetized cases, for which the field scales weakly with the density as $Bpropto n^{1/2}$, also reproduce observed parameters. (b) Realistically, the filaments will be embedded not only in gas of non-zero pressure, but also of non-zero density. Thus, the appearance of sheet-embedded filaments is explored. Generating a grid of filament models and comparing the resulting column density ratios and profile shapes with observations suggests that the three-dimensional filament profiles are intrinsically flatter than isothermal, beyond projection and evolution effects.
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