Day 639

Saturday.

1401.5466
The Zeldovich approximation
White

Zeldovich approximation for the growth of large-scale structure, which remains one of the most successful and insightful analytic models of structure formation.  Use the Z approximation to compute the 2-pt function of the matter and biased tracers, and compare to the results of N-body simulations and other Lagrangian perturbation theories.  Show that Lagrangian perturbation theories converge well and that the Z approximation provides a good fit to the N-body results except for the quadrupole moment of the halo correlation function.  Extend the calculation of halo bias to 3rd order and also consider non-local biasing schemes, none of which remove the discrepancy.  Argue that a part of the discrepancy owes to an incorrect prediction of inter-halo velocity correlations.  Use the Z approximation to compute the ingredients of the Gaussian streaming model and show that this hybrid method provides a good fit to clustering of haloes in z space down to scales of tens of Mpc.

1401.5467
First results from the Dragonfly Telephoto Array: the apparent lack of a stellar halo in the massive spiral galaxy M101
Van Dokkum, Abraham, Merritt

A new telescope concept to study the low surface brightness outskirts of the spiral galaxy M101.  The radial surface brightness profile is measured down to mu_g~32 mag/arcsec^2, a depth that approaches the sensitivity of star count studies in the Local Group.  Convert surface brightness to surface mass density using the radial g-r color profile. The mass density profile shows no significant upturn at large radius and is well-approximated by a simple bulge + disk model out to R=70 kpc, corresponding to 18 disk scale lengths.  Fitting a bulge+dusk+halo model find that the best-fitting halo mass M_halo ~ 1.7e8Msun.  The total stellar mass of M101 is M_tot*, ~5.3e10Msun, and infer that the halo mass fraction f_halo = Mhalo/Mtot*~0.003 [? the mass of the "halo stars"?].  This mass fraction is lower than that of the MW (f_halo~0.02) and M31 (f_halo~0.04).  All three galaxies fall below the f_halo - Mtot* relation predicted by recent cosmological sims that trace the light of disrupted satellites, with M101's halo mass a factor of ~10 below the median expectation.  However, the predicted scatter in this relation is large, and more galaxies are needed to better quantify this possible tension with galaxy formation models.  Dragonfly is well suited for this project: as integrated-light surface brightness is independent of distance, large numbers of galaxies can be studied in a uniform way.

1401.5472
Simultaneous modeling of the stellar and dust emission in distant galaxies: implications for star formation rate measurements
Utomo, Kriek, Labbe, Conroy, Fumagalli

Use NUV to MIR composite SEDs to simultaneously model the attenuated stellar and dust emission of 0.5<z<2.0 galaxies.  These composite SEDs were previously constructed from the photometric catalogs of the NEWFIRM medium-band survey, by stacking the observed photometry of galaxies that have similar rest-fram NUV-to-NIR SEDs.  In this work, include a stacked MIPS 24 um measurement for each SED type to extend the SEDs to rest-frame MIR wavelengths.  Consistent with previous studies, the observed MIR emission for most SED types is higher than expected from only the attenuated stellar emission.  Fit the NUV-to-MIR composite SEDs by the Flexible Stellar Population Synthesis (SPS) models, which include both stellar and dust emission.  Compare the best-fit SFRs to the SFRs based on simple UV+IR estimators.  The UV and IR luminosities overestimate SFRs - compared to the model SFRs - by more than ~1 dex for quiescent galaxies, while for the highest SF galaxies in the sample the two SFRs are broadly consistent.  The difference in sSFRs also shows a gradually increasing trend with declining sSFR, implying that quiescent galaxies have even lower sSFRs than previously found.  Contributions from evolved stellar populations to both the UV and the MIR SEDs most likely explain the discrepancy.  Based on this work, conclude that SFRs should be determined from modeling the attenuated stellar and dust emission simultaneously, instead of employing simple UV+IR-based SFR estimators.

1401.5473
Ultra low surface brightness imaging with the Dragonfly telephoto array
Abraham, van Dokkum

Describe the Dragonfly Telephoto Array, a robotic imaging system optimized for the detection of extended ultra low surface brightness structures.  The array consistent of 8 Canon 400mm f/2.8 telephoto lenses coupled to eight science-grade commercial CCD cameras.  The lenses are mounted on a common framework and are co-aligned to image simultaneously the same position on the sky.   The system provides an imaging capability equivalent to a 0.4m aperture f/1.0 refractor with a 2.6deg x 1.9deg FoV.  The system has no obstructions in the light path, optimized baffling, and internal optical surfaces coated with a new generation of antireflection coatings based on sub-wavelength nano structures.  As a result, the array's PSF has a factor of ~10 less scattered light at large radii than well-baffled reflecting telescopes.  The DTA is capable of imaging extended structures to surface brightness levels below 30 mag/arcsec2 in 10h integrations (without binning or FG SF removal).  This is considerably deeper than the surface brightness limit of any existing wide-field telescope.  At present no systematic errors limiting the usefulness of much longer integration times has been identified.  With longer integrations (50-100h), FG star removal and modest binning the DTA is capable of probing structures with surface brightnesses below 32 mag/arcsec2.  Detection of structures at these surface brightness levels may hold the key to solving the missing substructure and missing satellite problems of inventional hierarchical galaxy formation models.  The DTA is therefore executing a fully-automated multi-year imaging survey of a complete sample of nearby galaxies in order to undertake the first census of ultra-faint substructures in the nearby Universe.

1401.5578
Shaping the dust mass - star-formation rate relation
Hjorth, Gall, Michałowski

There is a remarkably tight relation between the observationally inferred dust masses and SFRs of SDSS galaxies, Mdust ~ SFR^1.11.  Extend the relation to the high end and show that it bends over at very large SFRs (dust masses are lower than predicted for given SFR).  Identify several distinct evolutionary processes in the diagram: (1) a star-bursting phase in which dust builds up rapidly at early times.  The maximum attainable dust mass in this process is the case of the bend-over of the relation.  A high dust-foramtion efficiency, a bottom-light IMF, and negligible SN shock dust destruction are required to produce sufficiently high dust masses.  (2) A quiescent SF phase in which the subsequent parallel decline in dust mass and SFR gives rise to the Mdust-SFR relation, through astration [?] and dust destruction.  The dust-go-gas ratio is approximately constant along the relation.  Show that the power-law slpe of the Mdust-SFR relation is inversely proportional to the global Schmidt-Kennicutt law exponent (~0.9) in simple chemical evolution models.  (3) A quenching phase which causes SF to drop while the dust mass stays roughly constant or drops proportionally.  Combined with merging, these processes, as well as the range in total baryonic mass, give rise to a complex population of the diagram which adds significant scatter to the original Mdust-SFR relation.  (4) At very high z, a population of galaxies located significantly below the local relation is predicted.