Day 769

Wednesday.

1410.5425
The impact of feedback on cosmological gas accretion
Nelson, Genel, Vogelsberger, Springel, Sijacki, Torrey, Hernquist

Compare cosmo hydro sims with and without feedback, both with the moving mesh code AREPO.  The feedback runs implement the full physics model of the Illustrious simulation project, including SF driven galactic winds and energetic feedback from SMBH.  Explore (a) the accretion rate of material contributing to the net growth of galaxies and originating directly from the IGM, finding that feedback strongly suppresses the raw, as well as the net, inflow of this "smooth mode" gas at all z, regardless of the temperature history of newly acquired gas.  (b) At the viral radius the temperature and radial flux of inflowing gas is largely unaffected at z=2.  However, the spherical covering fraction of inflowing gas at 0.25 rvir decreases substantially, from more than 80% to less than 50%, while the rates of both inflow and outflow increase, indicative of recycling across this boundary.  (c) The fractional contribution of smooth accretion to the total accretion rate is lower in the simulation with feedback, by roughly a factor of two across all redshifts.  Moreover, the smooth component of gas with a cold temperature history, is entirely suppressed in the feedback run at z<1.  (d) The amount of time taken by gas to cross from the viral radius to the galaxy - the "halo transit time" - increases in the presence of feedback by a factor of ~2-3, and is notably independent of halo mass.  Discuss the possible implications of this invariance for theoretical models of hot halo gas cooling. 

1410.5428
Extending the $L_{\mathrm{X}}-T$ relation from clusters to groups-Impact of cool core nature, AGN feedback, and selection effects
Bharadwaj, Reiprich, Lovisari, Eckmiller

Aim to investigate the bolometric L_X-T relation for galaxy groups, and study the impact of gas cooling, feedback from SMBHs, and selection effects on it.  With a sample of 26 galaxy groups, obtain the best-fit L_X-T relation for 5 different cases depending on the ICM core properties and central AGN radio emission, and determined the slopes, normalizations, intrinsic and statistical scatters for both temperature and luminosity.  Simulations were undertaken to correct for selection effects (e.g. Malmquist bias) and the bias corrected relations for groups and clusters were compared.  The slope of the bias corrected L_X-T relation is marginally steeper but consistent with clusters (~3).  Groups with a central cooling time less than 1Gyr (SCC groups) show indications of having the steepest slope and the highest normalization.  For the groups, the bias corrected intrinsic scatter in L_X is larger than the observed scatter for most cases, which is reported here for the first time.  Lastly, see indications that the groups with an extended central radio sources have a much steeper slope than those groups which have a CRS with only core emission.  Additionally, also see indications that the more powerful radio AGN are preferentially located in NSCC groups rather than SCC groups.

1410.5429
First space-based microlens parallax measurement of an isolated star: Spitzer Observations of OGLE-2014-BLG-0939
Yee, ... Gould, et al

From the striking differences in the light curve as seen from Earth and from Spitzer (~1AU to the West), infer a projected velocity v_helio,pro ~240 km/s, which strongly favors a lens in the Galactic Disk with mass M=0.23pm0.07 Msun and distance D_L=3.1pm0.4 kpc.  An ensemble of such measurements drawn from the ongoing program could be used to measure the single-lens mass function including dark objects, and also is necessary for measuring the Galactic distribution of planets since the ensemble reflects the underlying Galactic distribution of micro lenses.  Study the application of the many ideas to break the four-fold degeneracy first predicted by Refsdal 50 years ago.  Find that this degeneracy is clearly broken, but by two unanticipated mechanisms.  

1410.5438
New constraints on $\sigma_8$ from a joint analysis of stacked gravitational lensing and clustering of galaxy clusters
Sereno, et al

Joint analysis of clustering and stacked gravitational lensing of galaxy clusters in large surveys can constrain the formation and evolution of structures and the cosmological parameters.  On scales outside a few viral radii, the halo bias b is linear and the lensing signal is dominated by the correlated distribution of matter around galaxy clusters.  Discuss a method to measure the PS amplitude sigma_8 and b based on a minimal modeling.  Considered a sample of ~120k clusters photometrically selected from SDSS in 0.1<z<0.6.  The auto-correlation was studied through the 2-pt function of a subsample of ~70k clusters; the matter-halo correlation was derived from the WL signal of the subsample of ~1200 clusters from CFHTLS.  Obtained a direct measurement of b, which increases with mass in agreement with prediction of the LCDM paradigm.  Assuming Omega_M=0.3, found sigma_8=0.78pm0.17.  Used the same clusters for measuring both lensing and clustering, and the estimate of Sigma_8 did require neither the mass-richness relation, nor the knowledge of the selection function, nor the modeling of b. With an additional theoretical prior on the bias, obtain sigma_8=0.80pm0.10.

1410.5446
Masked areas in shear peak statistics: a forward modeling approach
Bard, Kratochvil, Dawson

The statistics of shear peaks have been shown to provide valuable cosmological info beyond the PS, and will be an important constant of models of cosmology with the large survey areas provided by forthcoming astronomical surveys.  Surveys include masked areas due to bright stars, bad pixels etc, which must be accounted for in producing constraints on cosmology from shear maps.  Advocate a forward-modeling approach, where the impact of masking (and other survey artifacts) are accounted for in the theoretical prediction of cosmo params, rather than removed from survey data.  Use masks based on the DLS, and explore the impact of up to 37% of the survey area being masked on LSST and DES-scale surveys.  By reconstructing maps of aperture mass, the masking effect is smoothed out, resulting in up to 14% smaller statistical uncertainties compared to simply reducing the survey area by the masked area.  Show that, even in the presence of large survey masks, the bias in cosmo parameter estimation produced in the forward-modeling process is ~1%, dominated by bias caused by limited simulation volume.  Also explore how this potential bias scales with survey area and find that small survey areas are more significantly impacted by the differences in cosmological structure in the data and simulated volumes, due to cosmic variance.

1410.5470
The formation of low-mass helium white dwarfs orbiting pulsars: evolution of low-mass X-ray binaries below the bifurcation period
Istrate, Tauris, Langer

Millisecond pulsars (MSPs) are generally believed to be old neutron stars (NSs) which have been spun up to high rotation rates via accretion of matter from a companion star in a LMXB.  This scenario has been strongly supported by various pieces of observational evidence.  However, many details of this recycling scenario remain to be understood.  Investigate binary evolution in close LMXBs to sturdy the formation of radio MSPs with low-mass He WD companions in tight binaries with orbital periods P_orb=2-9 hr.  In particular, examine i) if such observed systems can be reproduced from theoretical modeling using standard prescriptions of orbital angular momentum losses (i.e. with respect to the nature and the strength of magnetic breaking), ii) if the computations of the Roche-lobe detachments can match the observed orbital periods, and iii) if the correlation between WD mass and orbital period (M_WD, P_orb) is valid for systems with P_orb<2days.  Numercial calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in ~400 close LMXB systems with different initial values of donor star mass, NS mass, orbital period and the so-called gamma-index of magnetic breaking.  Subsequently, follow the orbital and the interior evolution of the detached low-mass (proto) HE WDs, including stages with residual shell H burning.  Find that a severe fine-tuning is necessary to reproduce the observed MSPs in tight binaries with He WD companions of mass <0.20 Msun, which suggests that something needs to be modified or is missing in the standard input physics of LMXB modeling.  Results from previous independent studies support this conclusion.  Demonstrate that the theoretically calculated (M_WD, P_orb)-relation is in general also valid for systems with P_orb<2days, although with a large scatter in He WD masses between 0.15-0.20 Msun.  The results of the thermal evolution of the (proto) He WDs are reported in a follow-up paper (Paper II).

1410.5471
The timescale of low-mass proto-helium white dwarf evolution
Istrate, Tauris, Langer, Antoniadis

A large number of low-mass (<0.20 Msun) He WDs have recently been discovered.  The majority of these are orbiting another WD or a millisecond pulsar (MSP) in a close binary system; a few examples are found to show pulsations or to have a main sequence star companion.  There appears to be discrepancies between current theoretical modeling of such low-mass He WDs and a number of key observed cases, indicating that their formation scenario remains to be fully understood.  Investigate the formation of detached proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs).  The prime focus is to examine the thermal evolution and the contraction phase towards the WD cooling track and investigate how this evolution depends on the WD mass.  Calculations are then compared to the most recent observational data.  Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in a large number of close-orbit LMXBs with different initial values of donor star mass, neutron star mass, orbital period and strength of magnetic braking.  Subsequently, follow the evolution of the detached low-mass proto-He WDs, including stages with residual shell hydrogen burning and vigorous flashes caused by unstable CNO burning.  Find that the time between Roche-lobe detachment until the low-mass proto-He WD reaches the WD cooling track is typically Delta t_proto = 0.5-2 Gyr, depending systematically on the WD mass and therefore on its luminosity.  The minimum WD mass for developing shell flashes is ~0.21 Msun for progenitor stars of mass M_2<1.5 Msun (and ~0.18 Msun for M_2=1.6 Msun) [why is shell flash important?].  The long timescale of low-mass proto-He WD evolution can explain a number of recent observations, including some MSP systems hosting He WD companions with very small surface gravities and high effective temperatures.  Find no evidence for Delta t_proto to depend on the occurrence of flashes and thus question the suggested dichotomy in thermal evolution of proto-WDs.