Day 486

Tuesday.

1308.1669
Confronting simulations of optically thick gas in massive haloes with observations at z=2-3
Fumagalli, Hennawi, Prochaska, Kasen, Dekel, Cervino, Primack

Predict distribution of neutral hydrogen around 21 galaxies in the halo mass range M_vir~3e11-4e12 Msun at z~2, with high-res hydro sims.  Covering fraction of optically-thick gas interior to the virial radius varies between f_c~0.5-0.2, with significant scatter among haloes.  Contrary to recent claims, both the mass fraction of cold (T<=3e4 K) gas within the virial radius and the covering fraction of optically-thick absorbers are found to be only weakly dependent on halo mass, even above the critical values for the formation of stable virial shocks.  Massive simulated haloes with M_vir >= 1e12 Msun underpredict the covering fraction of optically-thick gas observed in the environs of quasar host galaxies by a large factor.  The reasons for this discrepancy, possibly related to the treatment of feedback and hydrodynamic instability in simulations or to the fact that quasars may represent a special phase in the life of a galaxy, remain unclear.  Conversely, do not find statistically significant difference between the predicted covering fraction and observations in the lower mass haloes M_vir >= 5e11 Msun hosting Lyman break galaxies.  However, current samples of quasar-galaxy pairs are too small for conclusive comparisons, limiting the ability to test current theories for cold-gas accretion.  To overcome this limitation, propose an alternative method for mapping the distribution of optically-thick gas around distant galaxies based on the small-scale auto-correlation function of optically-thick gas clouds measured in the foreground of close quasar paris.  With numerical models, show that this new observable provides statistical information on the size of the CGM, its covering factor, and the underlying dark haloes hosting Lyman Limit systems at z~2-3, without the need to compile large samples of galaxy-quasar pairs.


1308.1675
The contribution of haloes with different mass ratios to the overall growth of cluster-sized haloes
Lemze, et al

New observational test for a key prediction of the LCDM: the contributions of mergers with different halo-to-main-cluster mass ratios to cluster-sized halo growth [?].  Perform test by dynamically analyzing seven galaxy clusters, spanning the redshift range 0.13<z_c<0.45 and caustic mass range 0.4-1.5e15 Msun/h_0.73, with an average of 293 spectroscopically-confirmed bound galaxies to each cluster.  The large radial coverage (a few virial radii), which covers the whole infall regions, with a high number of spectroscopically identified galaxies enables this new study.  For each cluster, identify bound galaxies.  Out of these galaxies, identify infalling [how?] and accreted haloes [a sub-halo within a cluster? ...or a neighboring halo?] and estimate their masses and their dynamical states.  Using the estimated masses, derive the contribution of different mass ratios to cluster-sized halo growth.  For mass ratios between ~0.2 and ~0.7, find a ~1 sigma agreement with LCDM expectations based on the Millennium simulations I and II [statistics on 7 clusters??].  At low mass ratios <0.2, derived contribution is underestimated since the detection efficiency decreases at low masses, ~2e14 Msun/h_73.  At large mass ratios, >~0.7, do not detect haloes probably because the sample (chosen to be quite X-ray related) is biased against large mass ratios.  Therefore, at large mass ratios, the derived contribution is also underestimated.

1308.1692
CLASH: a census of magnified star-forming galaxies at z~6-8
Bradley, Zitrin, Coe, ... Broadhurst, et al

18 lensing clusters, HST 16-band observations of CLASH for z~6-8 galaxies.  Discovery of 206, 45, 13 LBG candidates at z~6, 7, and 8 respectively, identified from purely photometric redshift selections.  This large sample, representing nearly an order of magnitude increasei n the number of magnified SF galaxies at z~6-8 presented to date, is unique in that there are observations in 4 WVC3/UVIS UV, 7 ACS/WFC optical and all 5 WFC3/IR broad-band filters, which enable very accurate photometric redshift selections.  Construct detailed lensing models for all 18 clusters (although some are preliminary) to estimate object magnifications and to identify 2 new multiply-lenzed z>~6 candidates.  The median magnifications for these 18 clusters are 4, 4, and 5 for the z~6,7, and 8 samples, respectively, over an average area of 4.5 arcmin^2 per cluster.  Compare observed number counts with expectations based on convolving "blank" field UV luminosity functions through cluster lens models, and find agreement down to ~27 mag, where it begins to suffer fro significant incompleteness.  In all 3 redshift bins, find a higher number density at brighter observed magnitudes than the field predictions, in excellent agreement with the lensed expectations and clearly demonstrating the enhanced efficiency of lensing clusters over field surveys.  Lensing clusters appear to be a powerful tool in the discovery and study of high-z galaxies and allow for the first glimpse of faint galaxies beyond the reach of the deepest HST legacy field surveys, a technique that will continue to be exploited with the upcoming ultradeep Hubble Frontier Fields campaign.

1308.1703
The distribution of dark matter in the milky way's disk
Kuhlen, Pillepich, Guedes, Madau

Analysis of the effects of dissipational baryonic physics on the local DM distribution at the location of the Sun, with an emphasis on the consequences for direct detection experiments. Find that 2 distinct processes lead to a 30% enhancement of DM in the disk plane: the accretion and disruption of satellites resulting in a DM component with net angular momentum and the contraction of baryons pulling DM into the disk plane without forcing it to co-rotate [interesting].  The co-rotating dark disk in Eris [what is Eris? the sim?] is less massive than what has been suggested by previous work, contributing only 9% of the local DM density.  The speed distribution in Eris is broadened and shifted to higher speed compared to its DM-only twin simulation ErisDark.  At high speed f(v) falls more steeply in Eris than in ErisDark or the Standard Halo Model (SHM), easing the tension between recent results from the CDMS-II and XENON100 experiments.  The non-Maxwellian aspects of f(v) are still present, but much less pronounced in Eris than in DM-only runs.  The weak dark disk increases the time-averaged scattering rate by only a few percent a low recoil energies.  On the high velocity tail, however, the increase in typical speeds due to baryonic contraction results in strongly enhanced mean scattering rates compared to ErisDark, although still suppressed compared to the SHM.  Similar trends are seen regarding the amplitude of the annual modulations, while the modulated fraction in increased compared to the SHM and decreased compared to ErisDark.

1308.1873
Formulation of non-steady-state dust formation process in astrophysical environments
Nozawa, Kozasa

The non-steady-state formation of small clusters and the growth of grains accompanied by chemical reactions are formulated under the consideration that the collision of key gas species (key molecule) controls the kinetics of dust formation process.  The formula allows evaluation of the size distribution and condensation efficiency of dust formed in astrophysical environments.  Apply the formulation to the formation of C and NgSiO3 grains in the ejecta of supernovae, as an example, to investigate how the non-steady effect influences the formation process, condensation efficiency f_con, and average radius a_ave of newly formed grains in comparison with the results calculated with the steady-state nucleation rate.  Show that the steady-state nucleation rate is a good approximation if the collision timescale of key molecule tau_coll is much smaller than the timescale tau_sat with which the supersaturation ratio increases; otherwise the effect of the non-steady state becomes remarkable, leading to a lower f_con and a larger a_ave.  Examining the results of calculations, reveals that the steady-state nucleation rate is applicable if the cooling gas satisfies Lambda = tau_sat/tau_coll > 30 during the formation of dust, and find that f_con and a_ave are uniquely determined by Lambda_on at the onset time t_on of dust formation  The approximation formulae for f_con and a_ave as a function of Lambda_on could be useful in estimating the mass and typical size of newly formed grains from observed or model-predicted physical properties not only in SNe ejecta but also in mass-loss winds from evolved stars.

1308.1908
A new approach to developing interactive software modules through graduate education
Sanders, Faesi, Goodman

Discuss a set of 15 new interactive, educational, online software modules developed by Harvard University graduate students to demonstrate various concepts related to astronomy and physics.  Their achievement demonstrates that only SW tools for education and outreach on specialized topics can be produced while simultaneously fulfilling project-based learning objectives.  Describe a set of technologies suitable for module development and present in detail 4 examples of modules developed by the students.  Offer recommendations for incorporating educational SW development within a graduate curriculum and conclude by discussing the relevance of this novel approach to new online learning environments like edX.

1308.1953
THe effect of gravitational focusing on annual modulation
Lee, Lisanti, Peter, Safdi

The scattering rate at DM direct-detection experiments should modulate annually due to the Earth's orbit around the Sun.  The rate is typically thought to be extremized around June 1, when the relative velocity of the Earth with respect to the DM wind is maximal.  Point out that gravitational focusing can alter this modulation phase.  Unbound DM particles are focused by the Sun's gravitational potential, affecting their phase-space density in the lab frame.  Gravitational focusing can result in a significant overall shift in the annual-modulation phase, which is most relevant for DM with low scattering speeds.  The induced phase shift for light O(10) GeV DM may also be significant, depending on the threshold energy of the experiment.

1308.1958
The rich globular cluster system of Abell 1689 and the radial dependence of the globular cluster formation efficiency
Alamo-Martinez et al

Study the rich GC system in the center of the massive cluster of galaxies Abell 1789 (z=0.18), one of the most powerful gravitational lenses known.  HST magnitude I_814=29 mag reached, in 28 orbits, with >90% completeness and sample the brightest ~5% of the GC system.  Assuming the well-known Gaussian form of the GCLF, estimate a total population of N(GC_total) = 162,850 GCs within a projected radius of 400 kpc.  As many as half may comprise as intracluster component.  Even with the sizable uncertainties, which mainly result from the uncertain GCLF parameters, this is by far the largest GC system studied to date.  The specific frequency S_N [number of GC's per unit galaxy luminosity] is high, but not uncommon for central galaxies in massive clusters, rising from S_N~5 near the center to ~12 at large radii.  Passive galaxy fading would increase S_N by ~20% at z=0.  Construct the radial mass profiles of the GCs, stars, intracluster gas, and lensing-derived total mass, and compare the mass fractions as a function of radius.  The estimated mass in GCs, M(GC_total)=3.9e10 Msun, is comparable to ~80% of the total stellar mass of the MW.  The shape of the GC mass profile appears intermediate between those of the stellar mass and total cluster mass.  Despite the extreme nature of this system, the ratios of the GC mass to the baryonic and total masses, and thus the GC formation efficiency, are typical of those in other rich clusters when comparing at the same physical radii.  The GC formation efficiency is not constant, but varies with radius, in a matter that appears similar for different clusters; speculate on the reasons for this similarity in profile.

1308.2030
A predicted new population of UV-faint galaxies at z>4
Wyithe, Loeb, Oesch

Show: a bursty model of high redshift SF explains several puzzling observations of the high-z galaxy population.  Begin by pointing out that the observed sSFR requires a duty-cycle of ~10%, which is much lower than found in many hydro-dynamical simulations.  This value follows directly from the fact that the observed SFR in galaxies integrated over a Hubble time exceeds the observed stellar mass by an order of magnitude.  Use the large observed sSFR which includes merger driven SF regulated by SNe feedback.  This model reproduces the SFR density function and the stellar mass function of galaxies at 4<z<7.  A prediction of the model is that the sSFR does not evolve very rapidly with either mass or redshift, in agreement with observation.  This is in contrast to results from hydrodynamical sims where the SF closely follows the accretion rate, and so increases strongly towards high z.  The bursty SF model naturally explains the observation that there is not enough stellar mass at z~2-4 to account for all of the SF observed, without invoking properties like an evolving IMF of stars.  The finding of a duty cycle that is ~10% implies that there should be ten times the number of known galaxies at fixed stellar mass that have not yet been detected through standard UV selection at high z.  Therefore predict the existence of a large undetected population of UV-faint galaxies that accounts for most of the stellar mass density at z~4-8.