Day 396

Wednesday.

1303.6279
Recurring flares from supermassive black hole binaries: implications for tidal disruption candidates and OJ 287
Tanaka (Taka!)

Possible: accreting SMBH binaries with sub-parsec separations produce luminous, periodically recurring outbursts that interrupt periods of relative quiescence.  Simulations of binaries embedded in prograde accretion disks motivates this: (i) the formation of a central, low-density cavity, and (ii) the leakage of circumbinary gas into this cavity, occurring once per orbit, via discrete streams on nearly radial trajectories.  (i) diminishes AGN optical/UV flux compared to single SMBHs, while the second is likely to trigger periodic fluctuations in the emergent flux.  Toy model: a leaked stream crosses its own orbit and shocks, converting its bulk KE to heat, resulting in a hot, optically thick flow that is quickly accreted and produces a flare with an AGN-like spectrum that peaks in the UV and ranges from optical to soft X-ray.  The preceding quiescence causes the flare to be mistaken from the tidal disruption of a star.  For typical binary periods of years to decades, the event rate in an individual system can be much higher than predicted for stellar tidal disruptions, but infrequent enough to hinder tests of periodicity.  The flares proposed here can be produced by very massive (>1e8 Msun) SMBHs that would not tidally disrupt solar-type stars.  They could be discovered serendipitously in the future by observatories such as LSST or eROSITA.  Apply the model to the active galaxy OJ 287, whose production of periodic optical flares has long fueled speculation that it hosts a SMBH binary.

1303.6281
Absorption signatures of warm-hot gas at low redshift: NeVIII
Tepper-Garcia, Richter, Schaye

Strong NeVIII absorbers are robust probes of shock-heated diffuse gas.

1303.6283
Galactic accretion and the outer structure of galaxies in the CDM model
Cooper, D'Souza, Kauffmann, Wang, Boylan-Kolchin, Guo, Frenk, White

Combine SAM with the particle-tagging technique to predict galaxy surface brightness profiles in a representative sample of ~1900 massive DM haloes (1e12-14 Msun) from Millennium II LCDM N-body sim.  Focus on outer regions of galaxies consisting of stars accreted in mergers.  These simulations cover scales from the stellar haloes of MW-like galaxies to the 'cD envelopes' of groups and clusters, and resolve low SB substructure such as tidal streams.  Find that the surface density of accreted stellar mass around the central galaxies of DM haloes is well described by a Sersic profile, the radial scale and amplitude of which vary systematically with halo mass (M_200).  The total stellar mass surface density profile breaks at the radius where accreted stars start to dominate over stars formed in the galaxy itself.  This break disappears with increasing M_200 because accreted stars contribute more of the total mass of galaxies, and is less distinct when the same galaxies are averaged in bins of stellar mass, because of scatter in the relation between stellar mass and halo mass.  To test our model, derive average stellar mass surface density profiles for massive galaxies at z~0.08 by stacking SDSS images.  Model agrees well with these stacked profiles and with other data from the literature, but can be more rigorously tested by future surveys that extend the analysis of the outer structure of galaxies to fainter isophotes.  Conclude that it is likely that the structure of the spheroidal components of galaxies is largely determined by collisionless merging during their hierarchical assembly.

1303.6285
Nonlinear color-metallicity relations of globular clusters.  IV.  Testing the nonlinearity scenario for color bimodality via HST/WFC3 u-band photometry of M84 (NGC 4374)
Yoon et al

Globular cluster (GC) color distributions in most massive galaxies are bimodal; viewed as presence of merely two GC subsystems with distinct metallicities, forming backbone of various galaxy formation theories.  Recent studies however show that color-metallicity relations (CMRs) often used to derive GC metallicities are in fact inflected.  Such inflection can create bimodal color distributions if the underlying GC metallicity spread is simply broad as expected from the hierarchical merging paradigm of galaxy formation.  In order to test the NL-CMR scenario for GC color bimodality, the u-band photometry is proposed, because the u-related CMRs are theoretically predicted to be least inflected and most distinctive among commonly used optical CMRs.  Present HST WFC3 F336W (u-band) photometry of the GC system in M84, a giant elliptical in the Virgo cluster.  Combine u-band with existing g and z data, find that the u-z and u-g color distributions are different from the g-z distribution in a very systematic manner and remarkably consistent with model predictions based on the NL-CMR hypothesis.  Results lend further confidence to validity of the NL-CMR scenario as an explanation for GC color bimodality.  There are some GC systems showing bimodal spectroscopic metallicity, and in such systems, the inflected CMRs often create stronger bimodality in the color domain.

1303.6286
What controls star formation in the central 500 pc of the Galaxy?
Kruijssen et al

The SFR in the central molecular zone (CMZ, the central 500 pc) of the MW is lower by a factor of >10 than expected for the substantial amount of dense gas it contains, which challenges current SF theories.  Quantify which physical mechanism could be causing this observations.  On scales larger than the disc scale height, the low SFR is found to be consistent with episodic SF due to secular instabilities [what are they?] or variations of the gas inflow along the Galactic bar.  The CMZ is marginally Toomre-stable [condition for differentially rotating disk to be stable] when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating that the condensation of self-gravitating clouds may be limited.  On small scales, find that the SFR in the CMZ is consistent with an elevated critical density for SF due to the high turbulent pressure - potentially aided by weak magnetic effects and an underproduction of massive stars due to a bottom-heavy IMF.  The existence of a universal density threshold for SF is ruled out, as well as the importance of the HI-H_2 phase transition of hydrogen, the tidal field, the B-field, radiation pressure, and CR heating.  Propose observational and numerical tests to distinguish between the remaining candidate SF inhibitors, in which ALMA play a key role.  Conclude by proposing a self-consistent cycle of SF in the CMZ, in which the plausible SF inhibitors are combined.  Ubiquity suggests that the perception of a lowered central SFR should be a common phenomenon in other galaxies.  Discuss the implications for galactic SF and SMBH growth, including a prediction that the recently reported bimodality of SF in high-z galaxies may emanate from a difference in the gas inflow rates.

1303.6287
Dark matter halo properties from Galaxy-galaxy lensing
Brimioulle, Seitz, Lerchster, Bender, Snigula

CFHTLS, 124 deg sq in ugriz, photoz scatter of Delta z/(1+z)=0.033, outlier rate of 2% for i<=22.5, and extract galaxy shapes down to i=24.0.  Lens: 0.05<z<=1, and source 0.05<z_s<2.  Fit 3 different galaxy halo profiles to the lensing signal: SIS, truncated isothermal sphere (BBS), and NFW.  Derive velocity dispersions, luminosity scaling relations, separate red and blue lenses, as well as the combined sample.

1303.6396
Cold gas in the inner regions of intermediate redshift clusters
Jablonka et al

CO observations of LIRGs inside the virial radii of 2 intermediate redshift clusters (z~0.4-0.5).  Detect 3 galaxies at high S/N (5-10 sigma), and provide robust estiamtes of their CO luminosities.  Find evidence that at fixed LIR (or fixed stellar mass), the frequency of high L'CO galaxies is lower in cluster than in the field, suggesting environmental depletion of the reservoir of cold gas.  Level of SF activity in a galaxy is primarily linked to the amount of cold gas.  In clusters (just as in the field), the conversion between gas and stars seems universal. ... find evidence of a decrease in CO towards cluster centers.

1303.6506
Large-scale inhomogeneities of the intracluster medium: improving mass estimates using the observed azimuthal scatter
Roncarelli, Ettori, et al

Hydro-sims of 62 galaxy clusters and groups: study ICM of inhomogeneities.  Intorduce the concept of residual clumsiness, C_R, that quantifies the LS inhomogeneity of the ICM.  Can be robustly defined, study dependence on radius, mass and dynamical state of the halo.  Observe that it introduces an overestimate in the determination of the density profile from the X-ray emission, which translates into a systematic overestimate of 5(12)% in the measurement of M_gas at R_200 for relaxed (perturbed) cluster sample.  ..other biases.. Residual clumpiness of the ICM is not directly observable, so study its correlation with azimuthal scatter in the X-ray SB of the halo and in the y-parameter profiles.  Find that their correlation is highly significant (0.6-0.7), allowing to define a function that connects the 2 quantities.  Obtain that correcting the observed gas density profiles using the azimuthal scatter eliminates the bias in the measurement of M_gas for relaxed objects, which becomes 0pm2% up to 2R_200, and reduces it by a factor of 3 for perturbed ones.  Method allows to eliminate the systematics on the measurements of M_he and f_gas, although a significant halo to halo scatter remains.

1303.6550
High precision simulations of weak lensing effect on cosmic microwave background polarization
Fabbian, Stompor

As the title says.  Code publicly available.

1303.6564
Reconstructing the lensing mass in the universe from photometric catalogue data
Collett, Marshall, Auger, Hilbert, Suyu, Greene, Treu, Fassnacht, Koopmans, Bradac, Blandford

High precision cosmological distance measurements towards individual objects such as time delay gravitational lenses or type Ia SNe are affected by WL perturbations by galaxies and groups along the line of sight  In time delay gravitational lenses, "external convergence" kappa can dominate the uncertainty in the inferred distances and hence cosmological parameters.  Attempt to reconstruct kappa along LoS using a simple halo model.  Use mock catalogues from MS, and calibrate and compare reconstructed P(kappa) to ray-traced true kappa, including observational uncertainties.  Find that reconstruction of kappa improves precision of ~50% over galaxy number ocunts.  Find that the lowest-kappa LoS have the best constrained P(kappa).  Find: selecting the thrid of the systems with the highest precision kappa estimates gives a sample of unbiased time delay distance measurements with just ~1% uncertainty due to LoS external convergence effects.  Photometric data sufficient to pre-select the best constrained LoS, and can be done before investment in light-curve monitoring.  Conversely, show that selecting LoS with high external shear could induce biases of up to 1% in time delay distance [??what's different from low shear LoS??]; this could introduce ~2% biases on the time-delay distance if ignored.  Suggest areas for improvement of the analysis framework.

1303.6583
A lower growth rate from recent redshift space distortions than expected from Planck
Macaulay, Wehus, Eriksen

Results from 6dFGS, BOSS, LRG, WIggleZ and VIPERS: the RSD measurements are consistently less than the values expected from Planck, and the relative scatter between the RSD meausrements is lower than expected.  

1303.6588
Cluster lensing profiles derived from a redshift enhancement of magnified BOSS-SUrvey galaxies
Coupon, Broadhurst, Umetsu

Detection of z-depth enhancement of BG galaxies magnified by FG clusters.  A clear trend of increasing mean redshift towards the cluster centers is found (use 300k BOSS redshifts, 5-15k SDSS clusters).  Similar but noisier results for (158) X-ray clusters.  The radial form of this z enhancement is well fitted by richness-to-mass weighted composite NFW profile with an effective mass ranging between M_200~1.4-1.8 e14 Msun for the optical cluster sample, and 5e14 Msun for X-ray.  Lensing detection helps to establish the credibility of SDSS cluster surveys; provides a normalization for their respective mass-richness relations.  An independent means of deriving the masses of cluster samples for examining the cosmological evolution, and provides a relatively clean consistency check of WL measurements, free from systematic limitations of shear calibration.