The clustering of galaxies as a function of their photometrically-estimated atomic gas content

Authors: Li, C., et al.

The paper (link) introduces a new estimate for the mass of HI gas, based on a few photometric quantities: stellar mass, color, surface density, and color gradient. This mass estimate is then being applied to a large sample from SDSS, to study the clustering dependence on the HI fraction. The results are compared to recent SAMs from the Munich group, showing that the models disagree with the observations mostly at low stellar masses.

The origin of disks and spheroids in simulated galaxies.

Sales et al. 2011 (MNRAS Submitted)

In this paper (link) the authors review the possible mechanisms to form disks in simulated galaxies and argue that the main mechanism responsible for disk formation is hot cooling gas. This is against previous works who found cold streams to be responsible for disk formation.

The effect of intergalactic helium on hydrogen reionisation: implications for the sources of ionising photons at z > 6

This paper (link) improves on existing reionisation simulations in two ways: by including helium in the radiative transfer simulations that are done in post-processing and by modeling the ionising emissivities in such a way that they are consistent with the observational constraints on the Thomson scattering optical depth and hydrogen photo-ionisation rate at z<6. The evolution of the volume fraction of ionised hydrogen is not significantly impacted by the inclusion of helium (and is in fact reproduced very well with a simple semi-analytic model), except for a small delay in reionisation when helium is included. The impact of helium on the temperature evolution is larger: at lower redshifts the volume averaged temperature of the IGM is higher. Comparing the simulation results with measurements of the IGM temperature shows that reionisation is mainly driven by sources with a soft spectrum. Contribution from mini-quasars or Pop III stars has to be small at redshifts 6 < z < 9. A significant number of ionising photons is produced by faint, low-mass galaxies.

MaGICC Disks: Matching Observed Galaxy Relationships Over a Wide Stellar Mass Range

In this paper (link) the authors demonstrate that using their feedback model (note this is just stellar feedback : supernovae + heating from massive stars) in cosmological zoom simulations results in galaxies that match observed relations between a wide range of properties. A key aspect of this result is that the simulations also reproduce the way these relations scale with stellar mass, suggesting that their feedback model has a realistic efficiency over 2 orders of magnitude in stellar mass. The authors claim that it is the amount of outflow they get in their models that allows them to get the correct mass dependence. They also show that this level of outflow is supported by OVI absorption line observations (which can indicate the radial extent of metal rich gas that has been ejected from a galaxy).

Growth of early SMBH and high redshift Eddington Ratio Distributions

This paper (link) uses a cosmological hydro simulation to explore the Edd. Ratios (ER) of the black holes that they seed in their simulation. A BH of mass 10^5 SM is seeded in haloes over 10^10 SM. They allow for Bondi-Hoyle Accretion with a modified parameter for the gas density close to the BH and compare it with Edd. Accretion to get the ERs. Various feedback effects are accounted for and they also plot the ER carefully so as to avoid any inherent trends. They find that the ER depends mostly on the the density of the gas being accreted.

They fit a universal log-normal distribution to the Edd. Ratio and compare it with observations. The Edd. Ratios they find are mostly < 1 implying sub-Eddnington accretion for the massive BH seeds in their work.

The cosmic web and the orientation of angular momenta

This paper (link) uses a dark matter only simulation to see if a collapsed, virialized halo remembers the cosmic web out of which it formed. The new work in this paper uses the eigenvalues of the velocity shear tensor to determine if a halo is in a knot, filament, sheet, or void. Then, they measure the angle between the halo angular momentum and the eigenvectors, and the subhalo orbital angular momentum and the eigenvectors. Interestingly, the internal angular momentum correlates with the eigenvectors in sheets and filaments, implying that even virialized halos retain memory of the initial conditions. The orbital angular momentum in substructures is aligned in knots, filaments, and sheets, implying that halo accretion is not isotropic. This work has impact in understanding the spatial distribution of satellites around host galaxies, as is seen in the Milky Way and the SDSS sample.

Which galaxy property is the best indicator of its host dark matter halo properties?

This paper (link) discusses an interesting new observational trend: It is found that the clustering of massive galaxies depends more strongly on the central velocity dispersion (s) than on stellar mass (m). This fact is emphasized by measuring w_p for subsamples of a given s (or m) at various bins in m (or s). The authors claim that this observational evidence supports the idea that the halo mass is more tightly related to s, than to m. In addition, the paper includes results that show the clustering dependence on surface density, color, and dynamical mass estimates.

Disruption of a Proto-Planetary Disk by the Black Hole at the Milky Way Center

This (link) is a very interesting paper trying to explain the properties of the gas cloud close to Sgr A* using back of the envelope calculations. Though the model will not be the final word in the study of this gas cloud, it provides interesting prediction, especially on its density evolution, without the use of any numerical simulations. It will be interesting to see how it will hold up with respect to detailed numerical simulations that are underway or already run (e.g. http://arxiv.org/abs/1201.1414B)

Adaptive gravitational softening in GADGET

Authors: Francesca Iannuzzi and Klaus Dolag

Link to paper

The authors describe the implementation of adaptive gravitational softening in GADGET. The softening lenght is a function of the local density, as for standard SPH. A correction to the formulation of the Lagrangian is necessary to reduce noise and preserve the translational invariance.

Applied to cosmological simulations, the enhanced resolution at high densities shows in the increase of particle clustering at small scales, amplitude of the correlation function, and inner profile of massive haloes. This allows to recover the results of higer resolution simulations at much lower computational costs.

On the effects of microphysical grain properties on the yields of carbonaceous dust from type II SNe

http://arxiv.org/abs/1105.4631
The paper studies the carbonaceous dust yields and properties from SN II, by theoretically exploring the parameter space {shape, sticking coefficient}.
The authors do not attempt to explain the discrepancies of some order of magnitude with the observed data, but try to constrain the effects of the different model parameters (based on nucleation theory).
They find that grain condensation times and grain size distributions are affected by the choice of different parameters, and, e.g.,
1. - larger sticking coefficients imply larger grains;
2. - more spherical grains have larger dimensions;
3. - larger sticking coefficient imply shorter condensation times (grains form earlier).
However, the total amount of dust prodiced (i.e. the dust yield) is quite independent from them.

The effects of a hot gaseous halo in galaxy major mergers

Authors: Benjamin P. Moster, Andrea V. Maccio', Rachel S. Somerville, Thorsten Naab, Thomas J. Cox

This paper presents the first attempt to simulate a major galaxy merger, including a component of hot gas. Galaxy properties including SFR, burst efficiency and B/T evolution are presented for mergers with and without a hot halo and with or without winds. These are compared with the equivalent simulation of an isolated galaxy.

The most interesting results are that, perhaps unsurprisingly, winds suppress the efficiency of the merger-induced starburst, causing the enhanced SFR to be suppressed but last longer, and that a hot gas halo leads to an enhanced SFR in both isolated and merger cases, but this leads to a reduced difference between the two. More interestingly perhaps, cooling from the hot halo is suppressed after the merger due to its increased temperature and spin-up (the latter effect may relate to the unusual coplanar merger geometry). With the enhanced "background" SFR from the cooling of the hot halo, and the winds suppressing any strong star burst, the strong dependence of burst efficiency on the progenitor cold gas fraction (as widely reported by Hopkins, Cox et al.) is reduced, or even negated (though it must be said, the gas fraction is defined at the beginning of the simulation, not directly before the merger).

The paper makes an important step in the direction of a multi-phase treatment of galaxy mergers. However the restriction to one (unusual) orbital configuration means these results cannot be used to provide any empirical relation describing the results of mergers. Indeed, the complexity of including winds and hot gas cooling in simulations, and dependence on the prescription used, implies that such empirical relations will be difficult to establish for sensitive parameters (e.g. star formation). On the other hand, the final bulge mass, and to a certain extent the internal dynamics of the merger remnant, appear to be fairly robust, and dependent primarily on the already formed stars in the progenitor galaxies with only a secondary dependence on the merger gas physics.

The First Massive Black Hole Seeds and Their Hosts

Link to Paper
Authors: Jillian Bellovary, Marta Volonteri, Fabio Governato, Sijing Shen, Thomas Quinn, James Wadsley

The paper attempts to investigate the haloes in which massive seed black holes (MBH) can form by looking at local properties of the halo. The MBH are put in by using an efficiency factor on the same prescription that is used to make stars. In the study, MBH refers to 10^4 SM black holes which can be considered as direct collapse remnants or Pop III remnants that grew over time to reach this mass via mergers or accretion. Accretion onto black holes that are populated in this fashion is not accounted for. The aim is to track the formation histories of MBH seeds in haloes and make predictions for future surveys on the occupation fraction of MBH in various type of galaxies.

Hierarchical formation of bulgeless galaxies II: Redistribution of angular momentum via galactic fountains

Link to paper here.
Authors: C.B. Brook, G.Stinson, B.K. Gibson, R. Roškar, J. Wadsley, T. Quinn

In this paper, the second in a suite, the authors are using cosmological zoom simulations to investigate the possibility of forming a bulgeless disc galaxy. Starting with initial conditions from the McMaster Unbiased Galaxy Simulations (MUGS) project, their programme is running disc galaxy re-simulations spanning 4 order of magnitude in stellar mass. After showing in their first paper that ejection of low angular momentum material enables the formation of bulgeless dwarf disc galaxies, this works introduces the mechanism of redistribution of momentum by galactic fountain as a possible solution to the angular momentum problem for intermediate mass disc galaxies.
After presenting the evolution of the main properties of the simulated object (especially the formation of a secular pseudo-bulge via bar/disc instabilities), they study in detail the evolution of the "bulge gas" (BG) since the last major merging epoch. They find that, after expelling most of this BG by efficient SNe activity during the starburst phase, by z=0, 71% of the BG has been brought back to the disk to form stars. The authors show that during the re-accretion of the BG via the galactic fountain, the material has gained angular momentum which prevent from accumulating gas to the centre of the galaxy, and hence prevent the formation of a too massive bulge.

Galaxy and Mass Assembly (GAMA): The star formation rate dependence of the stellar initial mass function

Link to paper on ADS here

Observations of galaxies found in the Galaxy and Mass Assembly (GAMA) survey are presented and analyzed. It is argued that the observations suggest a high-mass IMF slope which evolves with star formation rate. For galaxies with larger star formation rates, the trend is for flatter IMF slope, meaning a larger portion of high mass stars than in less actively star forming galaxies. If correct, the implications of this finding are potentially far-reaching.

Recoiling Black Holes in Merging Galaxies...

Link to paper here.
Authors: Laura Blecha, Thomas J. Cox, Abraham Loeb, Lars Hernquist

This proceeding paper present a parameter study of the effect of Gravitational-Wave (GW) recoil of merging supermassive black holes (SMBH). Using a suite of merger SPH simulations, the authors identify systematic trends in the behavior of recoiling BHs. The main results from this study are the following: i) as a function of the escape velocity, kicked BHs may remain confine in gas-rich galaxy merger. ii) recoil events reduce the lifetime of bright AGN, which can still be detectable either as kinematical or spatial offsets. iii) recoiling BHs may be up to 5 time less massive than their stationary counterparts. iv) the displacement of AGN feedback by a recoil causes higher central star formation, hence extending the starburst phase.

An Evolving Stellar Initial Mass Function and the Gamma-ray Burst Redshift Distribution

Link to paper here.
Authors: Wang & Dai

Whether or not the gamma-ray burst rate tracks the overall star formation rate in the universe has been a topic of much interest and debate. In particular, with the ever-growing sample of GRBs at high redshift, it appears that the GRB rate rises faster with redshift than the star formation rate. It has been suggested that this is due to GRB progenitors being of preferentially low metallicity, but some observations show GRBs exploding in metal-rich galaxies so this may not provide a clear explanation for the dicrepancy. In this paper, the authors show that an IMF, such as that suggested by Dave (2010), which becomes more top-heavy at higher redshifts can explain the discrepancy between the star formation rate and the GRB rate.

Relativistic jet feedback in evolving galaxies

http://arxiv.org/abs/1012.1092
Authors: Alexander Y. Wagner, Geoffrey V. Bicknell

The actual physics of radio-mode AGN feedback is still largely mysterious and so far can be treated in cosmological simulations only by subgrid models. The authors examine the interaction of young radio sources with a fractal two-phase ISM (1 kpc, up to ~10⁵ years) by high resolution simulations with a physical jet implementation. They discuss effects of enhanced or inhibited star formation as well as the momentum and energy budget.

PCA for Halo properties

http://arxiv.org/abs/1103.1641
Authors: Ramin A. Skibba, Andrea V. Maccio'

The authors study the correlations between the structural parameters of dark matter haloes using Principal Component Analysis (PCA). They find three important components: halo mass, concentration, and halo 'relaxedness'.

Stellar black holes at the dawn of the universe

http://arxiv.org/abs/1102.1891v1

Authors: Mirabel, Dijkstra, Laurent, Loeb, Pritchard


The authors propose that feedback from accreting black holes in high-mass X-ray binaries are an important and so-far overlooked source in the process of reionisation. They present an estimate of the total number of ionising photons produced by these sources and discuss some implications for reionisation models.

Disentangling galaxy environment and host halo mass

http://arxiv.org/abs/1103.0547
Authors: Marcel R. Haas (1 and 2), Joop Schaye (1), Akila Jeeson-Daniel

A lot of indicators of galaxy environment used in the (observational) literature are strong indicators of the mass of the halo in which the galaxy resides, the 'internal' environment. These authors give methods to compute a halo mass independent parameter to quantify the 'external' environment of a galaxy.