Tuesday. Wednesday. Thursday. Friday.
1309.3271
How to combine correlated data sets -- A Bayesian hyperparameter matrix method
Ma, Berndsen
Construct a statistical method for performing the joint analyses of multiple correlated astronomical data sets, in which the weights of data sets are determined by their own statistical properties. This method is a generalization of the hyperparameter method constructed by Lahav00 and Hobson02 which was designed to combine independent data sets. The hyperparameter matrix method presented here includes the relevant weights of multiple data sets and mutual correlations, and when the hyperparameters are marginalized over, the parameters of interest are recovered. Define a new "element-wise" product, which greatly simplifies the likelihood function with hyperparameter matrix. Rigorously prove the simplified formula of the joint likelihood and show that it recovers the original hyperparameter method in the limit of no covariance between data sets. Then illustrate the method by applying a classic model of fitting a straight line to two sets of data. Show that the hyperparameter matrix method can detect unaccounted systematics or underestimated errors in the data sets. Additionally, the ratio of Bayes' factors provides a distinct indicator of the necessity of including hyperparameters. The example shows that the likelihood constructed for joint analyses of correlated data sets can be widely applied to many astrophysical systems.
1309.3276
Galaxy masses: a review
Courteau, ... Hoekstra, ... Treu
Galaxy masses play a fundamental role in the understanding of structure formation models. This review addresses the variety and reliability of mass estimators that pertain to stars, gas, and DM. The different sections on masses from stellar populations, dynamical masses of gas-rich and gas-poor galaxies, with some attention paid to our MW, and masses from WL and SL methods, all provide review material on galaxy masses in a self-consistent manner.
1309.3281
Validation of the equilibrium model for galaxy evolution to z~3 through molecular gas and dust observations of lensed star-forming galaxies
Saintonge ... Genzel, ... et al
Combine interferometer and Herschel measurements to study the dust and gas contents of high-z SF galaxies. Present new observations for a sample of 17 lensed galaxies at z=1.4-3.1, which allow direct probe of the cold ISM of normal SF galaxies with stellar masses of 1e10 Msun, a regime otherwise not (yet) accessible by individual detections in Herschel and molecular gas studies. The lensed galaxies are combined with reference samples of sub-millimeter and normal z~1-2 SF galaxies with similar FIR photometry to study the gas and dust properties of galaxies in the SFR-M*- redshift parameter space. The mean gas depletion timescale of MS galaxies at z>2 is measured to be only ~450 Myr, a factor of ~1.5 (~5) shorter than at z=1 (z=0), in agreement with a (1+z)^-1 scaling. The mean gas mass fraction [fraction with respect to what?] at z=2.8 is 40pm15% (44% after incompleteness correction), suggesting a flattening or even a reversal of the trend of increasing gas fractions with redshift recently observed up to z~2. The depletion timescale and gas fractions of the z>2 normal SF galaxies can be explained under the "equilibrium model" for galaxy evolution, in which the gas reservoir of galaxies is the primary driver of the redshift evolution of sSFR. Due to their high SF efficiencies and low metallicities, the z>2 lensed galaxies have warm dust [low metallicity means inefficient cooling, I suppose] despite being located on the SF main sequence. At fixed metallicity, they also have a gas-to-dust ratio 1.7 times larger than observed locally when using the same standard techniques, suggesting that applying the local calibration of the relation between gas-to-dust ratio and metallicity to infer the molecular gas mass of high redshift galaxies may lead to systematic differences with CO-based estimates.
1309.3350
The cross-power spectrum between 21cm emission and galaxies in hierarchical galaxy formation models
Park, Kim, Wyithe, Lacey
The correlation between 21cm fluctuations and galaxies is sensitive to the astrophysical properties of the galaxies that drove reionization. Thus, detailed measurements of the cross-PS and its evolution could provide a powerful measurement both of the properties of early galaxies and the process of reionization. In this paper, study the evolution of the cross-power spectrum between 21cm emission and galaxies using a model which combines the hierarchical galaxy formation model GALFORM implemented within Millennium-II dark matter sims, with a semi-numerical scheme to describe the resulting ionization structure. Find that inclusion of different feedback processes changes the cross-power spectrum shape and amplitude. In particular, the feature in the cross-power spectrum corresponding to the size of ionized regions is significantly affected by SNe feedback. Calculate predicted observational uncertainties of the cross-correlation coefficient based on the specifications of the Murchison Widefield Array (MWA) combined with galaxy surveys of varying area and depth. Find that the cross-power spectrum could be detected over several square degrees of galaxy survey with galaxy redshift errors less than 0.1.
1309.3391
CosMO - a cosmic muon observer experiment for students
Franke et al
What are cosmic particles and where do they come from? With the CosMO (Cosmic Muon Observer) experiment, students can autonomously study these particles. They can perform their own hands-on experiments to become familiar with modern scientific working methods and to obtain a direct insight into astroparticle physics. In this contribution, present the experimental setup and possible measurements. The detector consists of 3 scintillator boxes. Events are triggered and read out by a data acquisition board developed for the QuarkNet Project. With a Python program running on a netbook under Linux, The trigger and data taking conditions can be defined. The program displays the particle rates in real-time and stores the data for offline analysis. Possible student experiments are the measurement of cosmic particle rates dependent on the zenith angle, the distribution of geometrical size of particle showers, and the lifetime of muons. Twenty CosMO detectors have been built at DESY. They are used within the German outreach network Netzwork Teilchenwelt at 15 astroparticle-research institutes and universities for project work with students.
1309.3430
The stellar abundances for galactic archaeology (SAGA) database III - analysis of enrichment histories for elements and two modes of star formation during the early evolution of Milky Way
Yamada et al
Study the enrichment histories for nine elements: C; alpha elements Mg, Si, Ca and Ti; Sc; three iron-peak elements Co, Ni and Zn----by using a large number of stellar data, collected by the SAGA. Find statistically significant changes, or breaks, or the mean abundance ratios to iron at three metallicities of [Fe/H] = -1.8, -2.2, and -3.3. Across the first one, the mean abundance ratios decrease with metallicity by similar extents for all the elements with sufficient data. Across the latter two, downward trends with the metallicity are also detected but for limited elements, C, Co, An and possibly Sc, and for two of Co and Zn, respectively. The breaks define 4 stellar populations with different abundance patterns which are dominant in each meatllicity range divided by the breaks, Pop IIa, IIb, IIc, and IId in order of increasing metallicity. Also explore their spatial distributions wit hteh spectroscopic distances to demonstrate the Pops IIa and IIb spread over the Galactic halo while Pops IIc and IId are observed near the Galactic plane. In particular, Pop IIc stars emerge around [Fe/H] = -2.6 and coexist with Pop IIb stars, segregated by the spatial distributions. Results reveal two distinct modes of SF during the early stages of Galaxy formation, which are associated with the variations of IMF and the spatial distribution of remnant low-mass stars. For the two lower-metallicity populations, the enhancements of Zn and Co indicate a high-mass and top-heavy IMF together with the statistics on the carbon-enhanced stars [cool!]. Discuss the relevance to the kinematically resolved structures of the Galactic halo and the possible sites of these populations within the framework of hierarchical structure formation scenario.
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