Day 586

Monday, Tuesday.

1402.1493
The effect of AGN feedback on the halo mass function
Cui, Borgani, Murante

Investigate baryon effects on HMF based on sims, with emphasis on the role played by AGN feedback.  Find halos with FoF and SO algorithms.  New code PIAO (Python spherical overdensity).  SO identified at Delta_c=2500, 500, 200, and masses computed within 3 corresponding radii.  Confirm that hydrodynamical sims based on radiative cooling, SF and SN feedback (CSF) produce MF higher than from collisionless simulations [baryonic contraction?].  In contrast, the effect of AGN feedback is that to suppressing the HMFs to a level even below that of DM sims, for both FoF and SO haloes [the other way around.  And how does this compare to observations?].  Find that the ratio between the HMF in the AGN and in the DM simulations is ~0.8, almost independent of the mass, when estimated at overdensity Delta_c=500, a difference that increases at higher overdensity Delta_c=2500, with no significant redshift dependence for these ratios.  Verify that the decrease of the HMF in the AGN simulation witnesses that mass reduction is induced by the sudden expulsion of displacement of gas induced by AGN energy feedback.  Provide fitting functions to describe halo mass variations at different over densities.  Demonstrate that, using these fitting functions, recover the DM halo MF starting from that of hydrodynamical simulations, with a residual random scatter <5% for halo mass larger than 1e13 Msun/h [some model fitting of the effect of AGN on HMF compared to DM-only sim].

1402.1498
The surprising inefficiency of dwarf satellite quenching
Wheeler, …, Boylan-Kolchin, Bullock, et al

Study LCDM cosmo sim with NSA/SDSS catalog to facilitate select and interpretation [facilitate what?].  Show that fewer than 30% of dwarfs (M*~1e8.5-9.5 Msun) identified as satellites within massive host halos (Mhost~1e12.5-14 Msun) are quenched, in spite of the expectation from simulations that half of them should have been accreted more ethan 6 Gyr ago.  Conclude that whatever the action triggering environmental quenching of dwarf satellites, the process must be highly inefficient.  Investigate a series of simple, one-parameter quenching models in order [to] understand what is required to explain the low quenched fraction and conclude that either the quenching timescale is very long (>9.5 Gyr, a "slow starvation" scenario) or that the environmental trigger is not well matched to accretion within the virial volume.  Discuss these results in light of the fact that most of the low mass dwarf satellites in the Local Group are quenched, a seeming contradiction that could point to a characteristic mass scale for satellite quenching.  

1402.1623
An iterative method for the construction of N-body galaxy models in collisionless equilibrium
Yurin, Springel

A new iterative approach for the realization of equilibrium N-body systems for given density distributions.  Starting with some initial assignment of particle velocities, calculate their orbits using a flexible tree-based force algorithm in the stationary potential of the target density distribution.  The difference of the time-averaged density response produced by these orbits with respect to the initial density configuration is characterized through a merit function, and a stationary solution of the collision less Boltzmann equation is found by minimizing this merit function directly by iteratively adjusting the initial velocities.  Because the distribution function is in general not unique for a given density structure, augment the merit function with additional constraints that single out a desired target solution.  Do this for broad classes of axisymmetric density distributions by numerically solving the Jeans equations to obtain the second velocity moments, which are then imposed as further constraints in the optimization process.  The velocity adjustment is carried out with a stochastic process in which new velocities are randomly drawn from an approximate solution of the distribution function, but are kept only when they improved the fit.  Method converges rapidly and is flexible enough to allow the construction of solutions with third integrals of motion, including disk galaxies with realistic velocity distributions.  Demonstrate that the new method reproduces analytic distribution functions where they are known (e.g., Hernquist sphere) and that it yields very stable N-body realizations tofu a variety of examples of compound galaxy models, considerably improving on widely used moment-based approaches.

1402.1491
Secular damping of stellar bars in spinning dark matter halos
Long, Shlosman, Heller

Demonstrate that growth of stellar bars in spinning DM halos is heavily suppressed in the secular phase of evolution, using numerical simulations of isolated galaxies.  In a representative set of models, show that for values of the cosmological spin parameter lambda [?] > 0.03, bar growth (in strength in size) becomes increasingly quenched.  Furthermore, slowdown of bars is affected as well, including extent and shape of their boxy/peanut bulges. The essence of this effect lies in the modified angular momentum exchange between the disk and the halo facilitated by the bar.  For the first time demonstrated that a DM halo can emit and not purely absorb angular momentum [angular momentum transfer from halo to disk?].  Although the halo as a whole is not found to emit, the net transfer of angular momentum from the disk to the halo is significantly reduced or completely eliminated.  The paradigm shift implies that the accepted view that disks serve as sources of angular momentum and halos serve as sinks, must be revised.  Halos with lambda > 0.03 are expected to form a substantial fraction, based on lognormal distribution of lambda.  Dependence of secular bar evolution on halo spin, therefore, implies corollaries for the cosmological evolution of galactic disks.

1402.1760
Anisotroic expansion and SNIa: an open issue
Jiminez, Salzano, Lazkoz

Hemispherical comparison method to detect a possible anisotropic expansion: perform a complete analysis by MCMC methods and compare with previous studies where non-diagonal components of the covariance matrix were neglected.  Take advantage of the particular distribution of SNIa in the SNLS3 data set in which the observations were taken along 4 different directions.  Fit each direction independently and find consistent results at the 1 sigma level.  Although the likelihoods peak at relatively different values of Omega_m, the low number of data along each direction gives rise to large errors so that the likelihoods are sufficiently broad as to overlap within 1 sigma.  

1402.1764
Cold dark matter heats up
Pontzen, Governato

LCDM is being challenged by its apparent inability to explain the low density of DM measured at the center of cosmological systems (from faint dwarf galaxies to massive clusters containing tens of galaxies the size of the MW).  But before making conclusions one should carefully include the effect of gas and stars, which were historically seen as merely a passive component during the assembly of galaxies.  These can in fact significantly alter the DM component, through a coping based on rapid gravitational potential fluctuations.

1402.1952
Women in Italian astronomy
Metteucci, Gratton

>26% of Italian IAU members are women; at INAF, 32% of research staff, and 40% of technical/administrative staff, 30% of permanent research staff are women.  Only 17% in university are women.  Similar to USA or Germany, career prospects for Italian astronomers are clearly worse for women than for men: higher up, less fraction of women; similarly for papers with high citations.  Conclude that implicit sex discrimination factors dominate over explicit ones; discuss the possible connection between the typical career pattern and these factors.

1402.2036
Formation of disk galaxies in preheated media: a preventative feedback model
Lu, Mo, Wechsler

Introduce a SAM implementing a self-consistent treatment for the hot halo gas configuration and the assembly of central disks.  Using the model, explore a preventative feedback model, in which the circum-halo medium is assumed to be preheated up to a certain entropy level by early starbursts or other processes, and compare it with an ejective feedback model, in which baryons are first accreted into DM halos and subsequently ejected out by feedback.  The model demonstrates that when the medium is preheated to an entropy comparable to the halo virial entropy the baryon accretion can be largely reduced and delayed.  In addition, the preheated medium can establish an extended low density gaseous halo when it accretes into the DM halos, and result in a specific angular momentum of the cooling gas large enough to form central disks as extended as those observed [are these disks star-forming?].  Combined with simulated halo assembly histories, the preventative feedback model can reproduce remarkably well a number of observational scaling relations.  These include the cold baryon (stellar plus cold gas) mass fraction-halo mass relations, SFHs, disk size-stellar mass relation and its evolution, and the number density of low-mass galaxies as a function of redshift.  In contrast, the conventional ejective feedback model fails to reproduce these observational trends.  Using the model, demonstrate that the properties of disk galaxies are closely tied to the thermal state of hot halo gas and even possibly the circum-halo medium, which suggests that observational data for the disk properties and circum-galactic hot/warm medium may jointly provide interesting constraints for galaxy formation models.