Munch: Monday, May 7, 2007

                               


 

 WHERE: 6TH FLOOR CONFERENCE ROOM
WHEN  : 12:30, MONDAY

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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       Munch Archive

 arXiv:0705.0343 [ps, pdf, other] : Title: The bispectrum of galaxies from high-redshift galaxy surveys: primordial non-Gaussianity and non-linear galaxy bias
Authors: Emiliano Sefusatti, Eiichiro Komatsu
Comments: 16 pages, 8 figures
The greatest challenge in the interpretation of galaxy clustering data from any surveys is galaxy bias. Using a simple Fisher matrix analysis, we show that the bispectrum provides an excellent determination of linear and non-linear bias parameters of intermediate and high-z galaxies, when all measurable triangle configurations down to mildly non-linear scales, where perturbation theory is still valid, are included. The bispectrum is also a powerful probe of primordial non-Gaussianity. The planned galaxy surveys at z>2 should yield constraints on non-Gaussian parameters, f_{NL}^{loc.} and f_{NL}^{eq.}, that are comparable to, or even better than, those from CMB experiments. We study how these constraints improve with volume, redshift range, as well as the number density of galaxies. Finally, we show that a halo occupation distribution may be used to improve these constraints further by lifting degeneracies between gravity, bias, and primordial non-Gaussianity.

arXiv:0705.0521 [ps, pdf, other] : Title: Dark matter and the first stars: a new phase of stellar evolution  (suggested by Pasquale)
Authors: Douglas Spolyar, Katherine Freese, Paolo Gondolo
Comments: 5 pages, 2 figures
A mechanism is identified whereby dark matter (DM) in protostellar halos dramatically alters the current theoretical framework for the formation of the first stars. Heat from neutralino DM annihilation is shown to overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase. A "dark star'' may result: a giant ($\gtrsim 1$ AU) hydrogen-helium star powered by DM annihilation instead of nuclear fusion, and detectable via annihilation products (gamma-rays, neutrinos, antimatter) possibly in combination with hydrogen lines.

arXiv:0705.0020 [ps, pdf, other] : Title: The Mass Function of Active Black Holes in the Local Universe
Authors: Jenny E. Greene (Princeton), Luis C. Ho (Carnegie Observatories)
Comments: to appear in ApJ; 19 pages, 13 figures
We present the first measurement of the black hole (BH) mass function for broad-line active galaxies in the local Universe. Using the ~9000 broad-line active galaxies from the Fourth Data Release of the Sloan Digital Sky Survey, we construct a broad-line luminosity function that agrees very well with the local soft X-ray luminosity function. Using standard virial relations, we then convert observed broad-line luminosities and widths into BH masses. A mass function constructed in this way has the unique capability to probe the mass region <10^6 M_sun, which, while insignificant in terms of total BH mass density, nevertheless may place important constraints on the mass distribution of seed BHs in the early Universe. The characteristic local active BH has a mass of ~10^7 M_sun radiating at 10% of the Eddington rate. The active fraction is a strong function of BH mass; at both higher and lower masses the active mass function falls more steeply than one would infer from the distribution of bulge luminosity. The deficit of local massive radiating BHs is a well-known phenomenon, while we present the first robust measurement of a decline in the space density of active BHs at low mass.

arXiv:0705.0142 [ps, pdf, other] : Title: A Modest Proposal for the Astronomical Community
Authors: Krzysztof Zbigniew Stanek
Comments: 7 pages, no figures, one link;
Inspired by a recent astro-ph posting, I propose a creation of an Alternative History astro-ph archive (althistastro-ph). Such an archive would serve as a final resting place for the various telescope (and possibly other) proposals that were not successful. As we all know, from both submitting proposals and also from serving on various time allocation committees, many excellent proposals ``do not make it''. Creating such an AltHist archive would serve many goals, including venting the frustration of the authors and also providing possible amusement for the readers. These are worthy goals, but they alone would not warrant creating such an archive. The truly useful role of AltHistAstro-ph archive would be to match astronomers with unappreciated ideas with other astronomers with underutilized resources, hopefully leading in some cases to resurrection of old proposals and resulting publications in the regular astro-ph archive. Given the possible danger of a low signal-to-noise and possible confusion, a creation of a separate archive seems like a good idea, although it should be noted that low signal-to-noise is achieved on astro-ph quite often already. Finally, I include my own excellent, but rejected (twice), HST proposal, as an example of a potential AltHistAstro-ph posting.

arXiv:0705.0153 [ps, pdf, other] : Title: A Generic Test of Modified Gravity Models which Emulate Dark Matter
Authors: E. O. Kahya, R. P. Woodard (University of Florida)
Comments: 4 pages, uses RevTex, no figures
We propose a generic test for models in which gravity is modified to do away with dark matter. These models tend to have gravitons couple to a different metric than ordinary matter. A strong test of such models comes from comparing the arrival time of the gravitational wave pulse from a cosmological event such as a supernova with the arrival times of the associated pulses of neutrinos and photons. For SN 1987a we show that the gravity wave would have arrived 5.3 days after the neutrino pulse.

 arXiv:0705.0166 [ps, pdf, other] : Title: Dark energy constraints from cosmic shear power spectra: impact of intrinsic alignments on photometric redshift requirements  (suggested by Josh)
Authors: Sarah Bridle (University College London), Lindsay King (IoA, Cambridge)
Comments: 14 pages and 9 figures. Invited original contribution submitted to "Gravitational Lensing" Focus Issue of the New Journal of Physics
Cosmic shear constrains cosmology by exploiting the apparent alignments of pairs of galaxies due to gravitational lensing by intervening mass clumps. However galaxies may become (intrinsically) aligned with each other, and with nearby mass clumps, during their formation. This effect needs to be disentangled from the cosmic shear signal to place constraints on cosmology. We use the linear intrinsic alignment model as a base and compare it to an alternative model and data. If intrinsic alignments are ignored then the dark energy equation of state is biased by ~50 per cent. We examine how the number of tomographic redshift bins affects uncertainties on cosmological parameters and find that when intrinsic alignments are included two or more times as many bins are required to obtain 80 per cent of the available information. We investigate how the degradation in the dark energy figure of merit depends on the photometric redshift scatter. Previous studies have shown that lensing does not place stringent requirements on the photometric redshift uncertainty, so long as the uncertainty is well known. However, if intrinsic alignments are included the requirements become a factor of three tighter. These results are quite insensitive to the fraction of catastrophic outliers, assuming that this fraction is well known. We show the effect of uncertainties in photometric redshift bias and scatter. Finally we quantify how priors on the intrinsic alignment model would improve dark energy constraints.

arXiv:0705.0165 [ps, pdf, other] : Title: Is Modified Gravity Required by Observations? An Empirical Consistency Test of Dark Energy Models (suggested by Josh)
Authors: Sheng Wang (Brookhaven; Columbia), Lam Hui (Columbia; ISCAP), Morgan May (Brookhaven), Zoltan Haiman (Columbia)
Comments: 7 pages, 3 figures, submitted to Phys. Rev. D
We apply the technique of parameter-splitting to existing cosmological data sets, to check for a generic failure of dark energy models. Given a dark energy parameter, such as the energy density Omega_Lambda or equation of state w, we split it into two meta-parameters with one controlling geometrical distances, and the other controlling the growth of structure. Observational data spanning Type Ia Supernovae, the cosmic microwave background (CMB), galaxy clustering, and weak gravitational lensing statistics are fit without requiring the two meta-parameters to be equal. This technique checks for inconsistency between different data sets, as well as for internal inconsistency within any one data set (e.g., CMB or lensing statistics) that is sensitive to both geometry and growth. We find that the cosmological constant model is consistent with current data. Theories of modified gravity generally predict a relation between growth and geometry that is different from that of general relativity. Parameter-splitting can be viewed as a crude way to parametrize the space of such theories. Our analysis of current data already appears to put sharp limits on these theories: assuming a flat universe, current data constrain the difference Omega_Lambda(geom) - Omega_Lambda(grow) to be -0.0044 +/- 0.0058 (68% C.L.); allowing the equation of state w to vary, the difference w(geom) - w(grow) is constrained to be 0.37 +/- 0.37 (68% C.L.). Interestingly, the region w(grow) > w(geom), which should be generically favored by theories that slow structure formation relative to general relativity, is quite restricted by data already. We find w(grow) < -0.80 at 2 sigma.

arXiv:0705.0163 [ps, pdf, other] : Title: Probing dark energy with cluster counts and cosmic shear power spectra: including the full covariance (suggested by Josh)
Authors: Masahiro Takada (Tohoku Univ., Japan), Sarah Bridle (University College London)
Comments: 30 pages, 13 figures, invited original contribution to gravitational lensing focus issue, New Journal of Physics
(Abridged) Combining cosmic shear power spectra and cluster counts is powerful to improve cosmological parameter constraints and/or test inherent systematics. However they probe the same cosmic mass density field, if the two are drawn from the same survey region, and therefore the combination may be less powerful than first thought. We investigate their cross-covariance based on the halo model approach, where the cross-covariance arises from the three-point correlations of the underlying mass density field as a result of non-linear gravitational clustering in structure formation. Taking into account the cross-covariance as well as non-Gaussian errors on the lensing power spectrum covariance, we find a significant cross-correlation between the lensing power spectrum signals at multipoles l~1000 and the cluster counts containing halos with masses M > 10^14 M_sun. Including the cross-covariance for the combined measurement degrades and in some cases improves the total signal-to-noise by up to plus or minus 20% relative to when the two are independent. For cosmological parameter determination, the cross-covariance has a smaller effect as a result of working in a multi-dimensional parameter space, implying that the two observables can be considered independent to a good approximation. We also found that cluster counts using lensing selected mass peaks are more complementary to lensing tomography than mass-selected cluster counts. Using lensing selected clusters with a realistic usable detection threshold (S/N~6 for a ground-based survey) the uncertainty on each dark energy parameter is roughly halved by combining cluster counts and lensing power spectra, relative to using power spectra alone.

arXiv:0705.0354 [ps, pdf, other] : Title: Constraining dark energy via baryon acoustic oscillations in the (an)isotropic light-cone power spectrum
Authors: Christian Wagner, Volker Müller, Matthias Steinmetz
Comments: submitted to A&A, 11 pages, 12 figures
The measurement of the scale of the baryon acoustic oscillations (BAO) in the galaxy power spectrum as a function of redshift is a promising method to constrain the equation-of-state parameter of the dark energy w. In order to measure precisely the scale of the BAO a huge volume has to be surveyed. We test whether light-cone effects become important and whether the scaling relations used to compensate for a wrong reference cosmology are accurate enough in this case. We compare two different fitting methods to extract the scale of the BAO. Further, we analyze the advantage of using the two-dimensional anisotropic power spectrum. Finally, we estimate the uncertainty with which an effectively constant w can be measured with proposed surveys around redshifts of z=3 and z=1, respectively. We find that light-cone effects for the simulated survey are negligible and that the simple scaling relations used to correct for the cosmological distortions work well even for such large survey volumes. The two different fitting approaches deliver consistent results and both should be considered further. The analysis of the two-dimensional anisotropic power spectra allows independent determination of the apparent scale of BAO perpendicular and parallel to the line of sight but it does not significantly lower the uncertainty of an effectively constant w. Nevertheless, for less constrained models of w independent measurements of the apparent scale of BAO perpendicular and parallel to the line of sight are essential. We estimate that with planned surveys around z=3 and z=1 one will be able to measure an effectively constant w with sigma_w ~ 4% in both cases.