This is the astro-ph blog of the Theoretical Modelling of Cosmic Structures group (TMoX) at the Max-Planck-Institute for Extraterrestrial Physics. We are an independent Max-Planck Research Group focusing on the various aspects in the formation and evolution of galaxies. Part of our focus is on the formation and evolution of early-type galaxies, super-massive black holes, the formation of the first structures in the universe and the enrichment history of the Universe. We are theoreticians using analytic modelling as well as numerical simulations in our work.

The CosmologyCake blog is dedicated to the discussion of research papers and current developments. We will regularly post interesting papers and comment on them. Feel free to leave your comments as well. We encourage authors of discussed papers to post replies if they wish to. Our aim is to provide a platform to discuss recent astro-ph papers within a wider audience. Please feel free to send papers you would like to be discussed to us at

6 December 2012

Two epochs of globular cluster formation from deep fields luminosity functions: Implications for reionization and the Milky Way satellites

Authors: Katz & Ricotti
Link: here

In this paper the contribution of old globular clusters to reionization is discussed. Observations of globular clusters in the Milky Way (and a few extra-galactic ones) reveal two distinct categories: metal-poor and metal-rich. Age determinations of these two populations are highly uncertain (errors of 1 Gyr) and there is a significant spread in ages (1 Gyr for the metal-poor and 6 Gyr for the metal-rich population). However, the age gap between metal-rich and metal-poor globular clusters is greater than the age range within each population, suggesting that there are two distinct epochs of globular cluster formation. Likely 55% (worst case: 20%) of the globular clusters in the Milky Way have formed at z>4. The authors try to constrain the formation rate of globular clusters at hight redshift by computing synthesised luminosity functions.

The following assumptions are made on globular cluster formation:
1) At the time of formation, the globular clusters were 9.1 times more massive than at the time they were observed in the Milky Way
2) The globular cluster initial mass function is lognormal or a power law
3) From the frequency of globular clusters in galaxies in the local Universe, the mass density and the formation rate can be inferred
The unknown is the fraction of todays globular clusters that formed at high redshift

The most massive globular clusters can be observed with HST up to redshift 6, and with JWST up to redshift 7.5. At z>0.01 globular clusters are unresolved and their light will dominate a high-z dwarf galaxy for about 10 Myr after their formation.

The authors constrain the fraction of globular clusters forming as a function of redshift using the luminosity function and continuum slope. They use two approaches. First assuming only one globular forms per halo, which requires the least amount of assumptions but is unrealistic. Second, allowing for multiple globular clusters per halo, assuming a linear relationship between the halo mass and globular cluster mass, a Press-Schechter mass function and a minimum halo mass in which star formation takes place (independent of redshift).

With these upper limits on the formation rate of globular clusters the authors conclude there are two distinct formation epochs of globular clusters: near z~2.5 and around the epoch of reionisation. Although the authors can only give upper limits, they argue that these upper limits are close to the actual values. These formation rates of globular clusters during reionisation imply that if the escape fraction is close to 1, the number of ionising photons produced is sufficient for reionisation.

24 September 2012

The Impacts of Ultraviolet Radiation Feedback on Galaxies during the Epoch of Reionisation

Authors: Hasegawa, K. & Semelin, B.
Paper: here

The goal of this paper is to assess the effectiveness of feedback from UV sources in the early Universe on high-redshift, low mass galaxies. They use a coupled radiative transfer and hydro SPH simulation to study the UV feedback in a cosmological simulation of 5 comoving Mpc. This code has high spatial and mass resolution, and so is able to include sources internal to a given galaxy, which while not the first code to do this, is one of the improvements over many other reionization simulations. This increased spatial resolution in the RT code, as well as the increased mass resolution (though a small volume because of this), allows them to follow the effects of the UV feedback on star formation between z = 6 - 15.

They find that including the UV feedback (photo-ionization of HI, HeI, HeII; photo-dissociation of H2 and H2+; photo-detachment of H-; self-shielding of H2 above a given column density) suppresses star formation, which is expected. However, they argue that the decrease in SF is larger than other simulations. The causes of this suppression depends on halo mass. For low mass halos (<10^9 Msun) the baryons are evacuated, leaving few baryons left to form stars. For higher mass halos, there is little mass loss, but the gas density is smoothed out, and the H2 molecule nearly destroyed. As their star formation algorithm is a strong function of density and temperature, this causes the drop of star formation in their simulation.

This work suffers from resolution effects, which the authors themselves note. The global star formation history has not converged for the runs with radiative transfer. This is partially due to resolving the lower mass galaxies, but also due to the fact that with increased resolution, stars form earlier, and hence the feedback effects start earlier. Thus, further studies with higher resolution are necessary.

Another problem with this work relates to the star formation algorithm. First, they have a very low density threshold required for star formation: a gas particle must have rho > 200*rho_ave, which includes basically every gas particle inside a halo. Second, they do not include any metal feedback from the supernovas in their box. The metals will cool the gas, bringing more gas below the temperature cut (T < 5000K) for star formation. Without the metals, only the gas which is strongly self-shielded against H2 dissociation can cool enough to form stars. Thus, further studies with metals are also necessary, and the authors note they will include this in future work.

25 June 2012

Galaxy disks do not need to survive in the L-CDM paradigm

Full title: Galaxy disks do not need to survive in the L-CDM paradigm: the galaxy merger rate out to z~1.5 from morpho-kinematic data

Authors: Puech, M., et al
Paper: here

The main result of the paper is the derivation of the time evolution of the galaxy merger rate using a sample of intermediate mass (M_stellar=10^10-10^11 M_sun) emission-line galaxies for which they have both kinematic (IMAGES survey) and morphological (from HST) data. These galaxies are chosen because they are the most likely progenitors of local spirals (see justifications in the paper). A large fraction of these galaxies are not relaxed morphologically and/or kinematically and the authors demonstrate that the disturbances are most likely triggered by major mergers.

The authors then match as many as possible of these observed, disturbed galaxies to simulated major mergers by comparing the real and simulated velocity fields and morphology etc. This allows them to associate a time since the merger began with each observation and they group them into “pre-fusion”, “post-fusion” (the main starburst period) and “relaxation” phases. They plot the observed SFR (normalised by gas mass) versus the time since the start of merger to illustrate that the IMAGES galaxies sample all the phases of an average merger well. They can then derive a merger rate from: rate = ngal in merger phase/time period for this phase, and a corresponding redshift for this rate: z= average start time of mergers in this phase. This gives them merger rates of 5.5, 10.1, 11.1 %/Gyr at z=0.72, 1.12, 1.55 respectively. They show that these merger rate values are in agreement with those from the Hopkins et al. 2009 semi-empirical model to within a factor of 2-3.

The authors draw some additional conclusions, including:
· Given the high gas fractions they infer for the major mergers occurring at z>0.6, a significant disc should be able to reform by z=0.
· The merger rate from the semi-empirical model is not in conflict with the fraction of bulgeless galaxies observed locally.
· Since cold streams have been shown to be suppressed below z~1.5, gas-rich major mergers could potentially take over as a channel for massive, thin disk formation at this redshift.
· Overall: there is no disc survival problem, but a period in which discs could/should be rebuilt at z<1.5.

15 June 2012

CANDELS: The contribution of the observed galaxy population to cosmic reinisation

Authors: Finkelstein, S. et al (2012)
Paper: here

Summary: In this paper, the contribution of the observed galaxies at redshifts 6, 7 and 8 to the reionisation of the IGM is studied. Taking only the observed galaxy population, an escape fraction of 30% is necessary to sustain reionisation at z=6, while if the luminosity function is integrated down to fainter galaxies, an escape fraction of 10% is sufficient. At redshifts 7 and 8 the observed galaxies do not produce enough ionising photons and fainter galaxies are needed to complete reionisation. The inferred volume ionised fraction from the ionising emissivity of these galaxy populations is consistent with observations of quasar spectra and Lyman-alpha spectroscopy, but inconsistent with the WMAP measurements of the Thomson scattering optical depth.

Birth of a Galaxy II: The Role of Radiation Pressure

Authors: Wise, J. et al (2012)
Paper: here

Summary: Using the AMR code ENZO, coupled with a ray-tracing radiative transfer solver, they study how radiation pressure affects the star formation history and growth of an early low-mass galaxy. They run three models, one with only primordial cooling (base model), one which also includes metal cooling (MC model), and one which includes metal cooling, momentum transfer from ionizing radiation, and an H2 dissociating radiation background (RP model). They then study how the metal cooling and radiation pressure affect the internal gas dynamics, the chemo-thermal state of the ISM, the ejecta and outflows, and the star formation history in the three models.

They find that the MC model overcools the gas, which traps the metals in a small region and creates an extremely metal rich dwarf galaxy, which is at odds with observed local dwarfs. When including the RP, however, the SFR is reduced, and the RP adds turbulence, and mixes the SNe ejecta throughout the ISM, preventing over-cooling. This model creates a galaxy which matches the local dwarf mass-metallicity relation.

7 June 2012

Monster Black Holes

2 letters:
McConnell et al. 2011
"Two ten-billion-solar-mass black holes at the centres of giant elliptical galaxies"
Paper: here

The authors report on the two new BHs found in the nearby universe, that are now the most massive BHs known to us. They explain their model which they used to infer the mass of these objects and talk about the underlying uncertainty. As a result of their high mass, they are outliers on the Mbh-sigma relation and hence, interesting to the BH-galaxy coevolution question.

Cappellari 2011
"Astrophysics: Monster black holes"
Paper: here

The implication of the above paper on the dry-merger-driven growth scenario for BHs is explored. The author argues that the two outliers on the Mbh-sigma relation could be a result of coalescing BHs, keeping the velocity dispersion on the resultant galaxy the same, but growing the central BH to double its initial mass. A speculative yet interesting insight into the BH-galaxy coevolution.

Star Formation in Galaxy Mergers with Realistic Models of Stellar Feedback & the Interstellar Medium

Authors: Hopkins et al. 2012
Paper: here

The authors discuss the results of high-resolution hydrodynamic simulations of galaxy mergers. They employ a sophisticated implementation of stellar feedback coupled to a `realistic' modelling of the multiphase ISM. They show that most of the stellar mass that ends up in the bulge is produced in situ, contrary to other recent simulations. The reason is the efficient disruption of GMC by stellar feedback which supplies more gas to the bulge than in other models. They compare their simulations to models assuming an effective equation of state.

1 June 2012

Stellar Disks in Aquarius Dark Matter Haloes

Authors: DeBuhr et al (2012)
Paper: here

The paper (Stellar Disks in Aquarius Dark Matter Haloes) shows results for stellar-bar formation in 4 Milky-Way-size dark matter haloes of the Aquarius project. The find that (i) the presence of a stellar component in the center of the haloes induces inner-halo contraction; (ii) 3/4 of the disks form a central bar; (iii) almost all of them experience a buckling instability with a boost of the stellar vertical velocity dispersion; (iv) when mass is reduced by at least a factor 2 no bar is formed; (v) re-orientation of the central part of the disk and halo determines warp formation.

There is not much new: just usual stellar dynamics analysis on the new Aquarius haloes.

Similar results about warps and bars might be reproduced by different phenomena (e.g. infalling satellite galaxies; dark-matter substructures; cold gas clumps; etc.)

31 May 2012

Galactic star formation and accretion histories from matching galaxies to dark matter haloes

Authors: Moster et al (2012)
Paper: here

The authors perform a 'multi-epoch abundance matching' model (MEAM) for 0<z<4. Subhaloes are taken from the Millennium I and II simulations, and satellites are forced to have the same stellar mass as centrals at the time of infall (i.e. self-consistent treatment, but with no SF for satellites). The results are then combined with merger trees extracted from the dark matter simulations to predict the average assembly histories of galaxies, separated into star formation within the galaxies (in-situ) and accretion of stars (ex-situ).

Results: The peak star formation efficiency decreases with redshift from 23% at z=0 to 9% at z=4 while the corresponding halo mass increases from 10^11.8 Msun to 10^12.5 Msun. The star formation rate of central galaxies peaks at a redshift which depends on halo mass; for massive haloes this peak is at early cosmic times while for low-mass galaxies the peak has not been reached yet. In haloes similar to that of the Milky-Way about half of the central stellar mass is assembled after z=0.7. In low-mass haloes, the accretion of satellites contributes little to the assembly of their central galaxies, while in massive haloes more than half of the central stellar mass is formed ex-situ with significant accretion of satellites at z<2.

The sizes, masses and specific star-formation rates of massive galaxies

Authors: McLure et al, 2012
Text: here

The authors in this paper present results from recent observations based on a mass complete (M > 6*10^10 M_sun) high redshift (z~1.4) sample of galaxies. The focus of the paper is on the sizes of these galaxies and their correlation with other physical properties such as specific star formation (sSFR,) stellar ages and morphology of the galaxies. The paper shows convincingly that galaxies at z~1.4 are more compact then the local counterparts by a factor 1.6 - 2.4 depending on their sSFR. Based on simple toy models they then go on to claim, that the size-evolution is dominated by minor mergers.

A very interesting result of this work is that morphology and sSFR do not correlate strongly. The authors conclude based on this that two separate process must be responsible for the morphological transformation and the shutting down of the SFR.

The latter conclusion is still somewhat debated in the community.

24 May 2012

Runaway Stars and the Escape of Ionizing Radiation from High-Redshift Galaxies

Authors: Conroy & Kratter
Paper: here

In this paper, the effect of runaway stars on the escape fraction of ionising radiation of galaxies around redshift 10 (so during the epoch of reionisation) is investigated. Runaway stars are stars with high velocities that are able to migrate from the dense environment in which they were formed. This would significantly increase the fraction of photons produced by these stars that is able to escape into the IGM. The two main formation channels are through dynamical ejections from young stellar clusters and through the explosion of companion star. The first channel is most important, since the stars that produce most ionising photons are short-lived. The fraction of massive stars that are runaways is highly uncertain, ranging from 10% to 50%. The authors adopt a value of 30%.

To assess the enhancement of the escape fraction due to runaway stars the authors adopt a simple analytic model for the galaxies, similar to Ricotti & Shull (2000). The two models for the gas distribution are a spherical (exponential) number density profile and an isothermal disc profile. The escape fraction is computed by considering the distance and line-of-sight column density from the position of the star to the virial radius, integrated over all solid angles. The distribution of the stars follows that of the gas.

With this simple model, the authors find that for galaxies at redshift 10, the escape fraction can be higher up to a factor of 7 when runaway stars are included. The reason is that the non-runaway stars have escape fractions of less than 10% due to the dense environment of the galaxy. The enhancement peaks around a halo mass of 1e8 solar masses, because runaways travel a fixed distance inside the galaxy and higher mass galaxies have larger radii.

The authors discuss the effects of various assumptions in their model, by changing the redshift, the dust attenuation, the minimum and maximum stellar mass, the runaway velocity, the ionising luminosity function and the gas density profile. The latter had the most significant effect, with enhancement scaling different with halo mass for spherical and disc-like galaxies.

This study provides a rough first estimate of what the effect of runaway stars on the escape fraction could be, but, as the authors show, the significant dependence on the gas distribution makes the conclusions not very robust.

11 May 2012

PISN at the Epoch of Reionization

Authors: Pan, Kasen, & Loeb
Link: here

This paper discusses the observability of PISN, Core Collapse, and Type Ia Supernovae with JWST. They use light curves and spectra of the explosions using a time dependent radiative transfer code for a wide range of Pop III stellar masses and evolutionary states. One key point they argue based on the luminosity of various explosions is that for detecting large numbers of PISN, a wide rather than deep survey will be better with JWST. In addition, spectra of the explosions will help determine the stellar progenitor and it's evolutionary state prior to the explosion.

After obtaining the spectra, they attempt to quantify how many SN are produced by using a very simple analytic argument for the SFR in the early universe. The SFR is calibrated to ensure the Universe is reionized by z=6. They have two classes of models: one where only Pop III stars reionize and one where only Pop II stars provide the reionizing photons. Two different IMF's are assumed (Salpeter and a flat IMF) for the Pop III stars, and a Salpeter IMF is used for the Pop II case. With this constraint, they make predictions for the detectability of PISN and CCSN, and find that it would be possible with JWST to distinguish between the two Pop III IMF models used here.

The radius of baryonic collapse in disc galaxy formation

Authors: Kassin et al.
Link: here.

The authors propose a simple solution for the angular momentum catastrophe: measure the DM specific angular momentum (j) at the radius of baryonic collapse, defined as R_BC in their work, which is smaller than the R_vir and hence has a higher concentration of DM encompassed in it. Assuming the standard picture of galaxy formation where the baryons collapse from inside to outside in a DM halo, they are able to make a reasonable case for their R_BC parameter. The measurement of j at an inner radius leads to an agreement in the baryonic and DM j which they are able to constrain from observational data. Although they only run a DM-only simulation, certain assumptions about the j of the observed spiral galaxies (relating their circular velocity to the angular momentum) allow them to relate their simulated DM haloes and observed galaxies.

How long does it take to form a molecular cloud?

Authors: Clark, Glover, Klessen & Bonnell
Link: here

The authors use a 3D hydrodynamical code (GADGET2) coupled with a chemical network (including modeling of dust and the interstellar radiation field), to self-consistently study the formation of molecular clouds at the interface between colliding flows of the warm neutral medium. It has already been shown that this formation mechanism can produce bound gas clouds in 3D simulations, but no one has yet followed the chemistry (coupled with 3D hydro) in order to establish how much of this gas could be molecular. In particular, the authors wish to establish how much CO would be produced in this scenario, since it is this which is observed, not H2.

The authors have two simulations, one fast flow (with Mach number, M=2.62) and one slow flow (with M=1.22), which both start with all gas at 5000K, 1 atm/cm^3 and solar metallicity. The fast flow case results in star formation around 10 Myr earlier than the slow flow case and there appears to be a different mechanism at work in the formation of protostellar cores (modeled by sink particles here). In the fast flow case there is a greater mass of very cold (<30K, the temperature at which we observe most CO emission), very dense (>10^4 atm/cc, the typical density of protostellar cores) gas and both H2 and CO formation occur earlier in time than in the slow flow case. However, note that the time delay between the development of H2 regions and the onset of star formation is shorter for the fast flow.

The most interesting results, however, lie in the similarities in the cloud/star formation in these two simulations. In both cases some small regions develop completely molecular hydrogen early on, and well in advance of the onset of star formation (although the exact time delay is different in the two simulations). Even more striking is the fact that, in both simulations, the fraction of CO only becomes significant 1-2 Myrs before the onset of SF i.e. the actual time delay is about the same, despite the different physical conditions. At this point there is a very rapid increase in the mass of CO, taking it quickly from undetectable to observable levels. This supports models in which there are “dark” molecular clouds; clouds containing large reservoirs of H2 that we cannot observe due to their lack of CO.

Systematic variation of the stellar IMF in early-type galaxies

Authors: Cappellari et al.
Link: here

The authors present results on the shape of the initial mass function of stars in a complete sample of early-type galaxies observed with the integral-field spectroscopy. The novel aspect is that for the first time they manage to disentangle the contribution from dark matter and a changing IMF by detailed dynamical modeling in combination with stellar population modeling. The main result is that they find evidence for an IMF shape that is either top-heavy or bottom-heavy. This is an interesting and important result contributing to the general discussion on a changing IMF in galaxies.

16 April 2012

The Formation of the First Cosmic Structures and the Physics of the z ~ 20 Universe

Authors: O'Leary and McQuinn
Link: here

This paper focuses on three aspects of research into simulations of the early Universe. First, they properly (almost -- see first comment) solve the equations of linear growth of baryons and dark matter. They include four major additions to previous solutions. 1. The baryons and dark matter are solved separately, allowing the impact of pressure on the baryons to be properly included. 2. Radiation is included in the calculation of the initial densities and velocities. 3. The mean temperature (and fluctuations in T) and electron density are self-consistently included. 4. They include a (homogeneous) bulk velocity between the dark matter and baryons. The overall effect of these changes is best shown in Figure 15, where it is clear that for k = 30-2000 Mpc^-1, there is a significant difference in the growth factor of dark matter at z = 30. Previous IC generators (including previous works which included a velocity difference between DM and baryons) yielded too much power in this range.

The authors then took their improved IC generator and ran simulations using an AMR code (ENZO) and an SPH code (GADGET3). They provide detailed comparisons between the codes, as well as changing various aspects of the simulation (e.g. starting redshift, box size, & bulk flow velocity). These are best seen in Figures 4, 5, & 6). There is little difference between the two codes, with two small exceptions. In GADGET with no bulk flow, there is particle coupling between the DM and baryon particles which add spurious power (Fig 5). In Enzo with AMR, there is a small increase in power in the baryons due to the AMR grid size.

Lastly, they compare the formation of the first structures in a universe with a non-zero bulk flow to that of typical first-structure formation. The basic conclusion is two-fold. First, the effect of a bulk flow depends strongly on environment -- whether or not the halos is forming in filaments aligned or transverse with the bulk flow. Second, the wind means smaller total gas masses, less dense gas in halos, and less gas available for star formation. In a companion paper, the authors plan to discuss the impact a bulk flow will have on the 21cm line absorption and the growth of Lyman-Werner and other radiation backgrounds.

1 March 2012

A Density Independent Formulation of SHP

Authors: Saitoh & Makino
Link: here

A new SPH scheme is presented in this paper. The scheme is based on using the total internal energy density as smoothed quantity. This allows for a formulation of conservation equations that are independent from the gas density. The advantage is that pressure is naturally treated as a smoothed quantity, solving the well known problems of SPH in resolving contact discontinuity. The new implementation has been successful in several standard tests.

27 February 2012

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.

24 February 2012

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.

20 January 2012

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.

13 January 2012

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.