arXiv subjects

Michael J. Kurtz

Publications and source records attributed to Michael J. Kurtz.

At least 55 records · Page 3Linked to original sources

arXiv2012

Testing Weak Lensing Maps With Redshift Surveys: A Subaru Field

We use a dense redshift survey in the foreground of the Subaru GTO2deg^2 weak lensing field (centered at $α_{2000}$ = 16$^h04^m44^s$;$δ_{2000}$ =43^\circ11^{\prime}24^{\prime\prime}$) to assess the completeness and comment on the purity of massive halo identification in the weak lensing map. The redshift survey (published here) includes 4541 galaxies; 4405 are new redshifts measured with the Hectospec on the MMT. Among the weak lensing peaks with a signal-to-noise greater that 4.25, 2/3 correspond to individual massive systems; this result is essentially identical to the Geller et al. (2010) test of the Deep Lens Survey field F2. The Subaru map, based on images in substantially better seeing than the DLS, enables detection of less massive halos at fixed redshift as expected. We demonstrate that the procedure adopted by Miyazaki et al. (2007) for removing some contaminated peaks from the weak lensing map improves agreement between the lensing map and the redshift survey in the identification of candidate massive systems.

astro-ph.CO↗

arXiv2012

The Faint End of the Luminosity Function and Low Surface Brightness Galaxies

SHELS (Smithsonian Hectospec Lensing Survey) is a dense redshift survey covering a 4 square degree region to a limiting R = 20.6. In the construction of the galaxy catalog and in the acquisition of spectroscopic targets, we paid careful attention to the survey completeness for lower surface brightness dwarf galaxies. Thus, although the survey covers a small area, it is a robust basis for computation of the slope of the faint end of the galaxy luminosity function to a limiting M_R = -13.3 + 5logh. We calculate the faint end slope in the R-band for the subset of SHELS galaxies with redshif ts in the range 0.02 <= z < 0.1, SHELS_{0.1}. This sample contains 532 galaxies with R< 20.6 and with a median surface brightness within the half light radius of SB_{50,R} = 21.82 mag arcsec^{-2}. We used this sample to make one of the few direct measurements of the dependence of the faint end of the galaxy luminosity function on surface brightness. For the sample as a whole the faint end slope, alpha = -1.31 +/- 0.04, is consistent with both the Blanton et al. (2005b) analysis of the SDSS and the Liu et al. (2008) analysis of the COSMOS field. This consistency is impressive given the very different approaches of th ese three surveys. A magnitude limited sample of 135 galaxies with optical spectroscopic reds hifts with mean half-light surface brightness, SB_{50,R} >= 22.5 mag arcsec^{-2} is unique to SHELS_{0.1}. The faint end slope is alpha_{22.5} = -1.52+/- 0.16. SHELS_{0.1} shows that lower surface brightness objects dominate the faint end slope of the l uminosity function in the field, underscoring the importance of surface brightness limits in evaluating measurements of the faint end slope and its evolution.

astro-ph.CO↗

arXiv2012

CLASH: Precise New Constraints on the Mass Profile of Abell 2261

We precisely constrain the inner mass profile of Abell 2261 (z=0.225) for the first time and determine this cluster is not "over-concentrated" as found previously, implying a formation time in agreement with ΛCDM expectations. These results are based on strong lensing analyses of new 16-band HST imaging obtained as part of the Cluster Lensing and Supernova survey with Hubble (CLASH). Combining this with revised weak lensing analyses of Subaru wide field imaging with 5-band Subaru + KPNO photometry, we place tight new constraints on the halo virial mass M_vir = 2.2\pm0.2\times10^15 M\odot/h70 (within r \approx 3 Mpc/h70) and concentration c = 6.2 \pm 0.3 when assuming a spherical halo. This agrees broadly with average c(M,z) predictions from recent ΛCDM simulations which span 5 <~ <~ 8. Our most significant systematic uncertainty is halo elongation along the line of sight. To estimate this, we also derive a mass profile based on archival Chandra X-ray observations and find it to be ~35% lower than our lensing-derived profile at r2500 ~ 600 kpc. Agreement can be achieved by a halo elongated with a ~2:1 axis ratio along our line of sight. For this elongated halo model, we find M_vir = 1.7\pm0.2\times10^15 M\odot/h70 and c_vir = 4.6\pm0.2, placing rough lower limits on these values. The need for halo elongation can be partially obviated by non-thermal pressure support and, perhaps entirely, by systematic errors in the X-ray mass measurements. We estimate the effect of background structures based on MMT/Hectospec spectroscopic redshifts and find these tend to lower Mvir further by ~7% and increase cvir by ~5%.

astro-ph.CO↗

arXiv2011

Mapping the Universe: The 2010 Russell Lecture

Redshift surveys are a powerful tool of modern cosmology. We discuss two aspects of their power to map the distribution of mass and light in the universe: (1) measuring the mass distribution extending into the infall regions of rich clusters and (2) applying deep redshift surveys to the selection of clusters of galaxies and to the identification of very large structures (Great Walls). We preview the HectoMAP project, a redshift survey with median redshift z = 0.34 covering 50 square degrees to r= 21. We emphasize the importance and power of spectroscopy for exploring and understanding the nature and evolution of structure in the universe.

astro-ph.CO↗

arXiv2011

The ADS in the Information Age - Impact on Discovery

The SAO/NASA Astrophysics Data System (ADS) grew up with and has been riding the waves of the Information Age, closely monitoring and anticipating the needs of its end-users. By now, all professional astronomers are using the ADS on a daily basis, and a substantial fraction have been using it for their entire professional career. In addition to being an indispensable tool for professional scientists, the ADS also moved into the public domain, as a tool for science education. In this paper we will highlight and discuss some aspects indicative of the impact the ADS has had on research and the access to scholarly publications. The ADS is funded by NASA Grant NNX09AB39G

astro-ph.IM↗

arXiv2011

Usage Bibliometrics

Scholarly usage data provides unique opportunities to address the known shortcomings of citation analysis. However, the collection, processing and analysis of usage data remains an area of active research. This article provides a review of the state-of-the-art in usage-based informetric, i.e. the use of usage data to study the scholarly process.

cs.DL↗

arXiv2010

Empirical optical k-Corrections for redshifts <= 0.7

The Smithsonian Hectospec Lensing Survey (SHELS) is a magnitude limited spectroscopically complete survey for R<=21.0 covering 4 square degrees. SHELS provides a large sample (15,513) of flux calibrated spectra. The wavelength range covered by the spectra allows empirical determination of k-corrections for the g- and r-band from z=0 to ~0.68 and 0.33, respectively, based on large samples of spectra. We approximate the k-corrections using only two parameters in a standard way: Dn4000 and redshift. We use Dn4000 rather than the standard observed galaxy color because Dn4000 is a redshift independent tracer of the stellar population of the galaxy. Our approximations for the k-corrections using Dn4000 are as good as (or better than) those based on observed galaxy color (g-r) (sigma of the scatter is ~0.08 mag). The approximations for the k-corrections are available in an on-line calculator. Our results agree with previously determined analytical approximations from single stellar population (SSP) models fitted to multi-band optical and near-infrared photometry for galaxies with a known redshift. Galaxies with the smallest Dn4000-the galaxies with the youngest stellar populations-are always attenuated and/or contain contributions from older stellar populations. We use simple single SSP fits to the SHELS spectra to study the influence of emission lines on the k-correction. The effects of emission lines can be ignored for rest-frame equivalent widths <~ 100 A depending on required photometric accuracy. We also provide analytic approximations to the k-corrections determined from our model fits for z<=0.7 as a function of redshift and Dn4000 for ugriz and UBVRI (sigma of the scatter is typically ~0.10 mag). Again, the approximations using Dn4000 are as good (or better than) those based on a suitably chosen observed galaxy color. We provide all analytical approximations in an on-line calculator.

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arXiv2010

The Emerging Scholarly Brain

It is now a commonplace observation that human society is becoming a coherent super-organism, and that the information infrastructure forms its emerging brain. Perhaps, as the underlying technologies are likely to become billions of times more powerful than those we have today, we could say that we are now building the lizard brain for the future organism.

physics.soc-ph↗

arXiv2010

Finding Your Literature Match -- A Recommender System

The universe of potentially interesting, searchable literature is expanding continuously. Besides the normal expansion, there is an additional influx of literature because of interdisciplinary boundaries becoming more and more diffuse. Hence, the need for accurate, efficient and intelligent search tools is bigger than ever. Even with a sophisticated search engine, looking for information can still result in overwhelming results. An overload of information has the intrinsic danger of scaring visitors away, and any organization, for-profit or not-for-profit, in the business of providing scholarly information wants to capture and keep the attention of its target audience. Publishers and search engine engineers alike will benefit from a service that is able to provide visitors with recommendations that closely meet their interests. Providing visitors with special deals, new options and highlights may be interesting to a certain degree, but what makes more sense (especially from a commercial point of view) than to let visitors do most of the work by the mere action of making choices? Hiring psychics is not an option, so a technological solution is needed to recommend items that a visitor is likely to be looking for. In this presentation we will introduce such a solution and argue that it is practically feasible to incorporate this approach into a useful addition to any information retrieval system with enough usage.

cs.DL↗

arXiv2010

Towards a Resource-Centric Data Network for Astronomy

Over the past decade, astronomers have been using an increasingly larger number of web-based applications and archives to conduct their research. However, despite the early success in creating links across projects and data centers, the promise of a single integrated digital library environment supporting e-science in astronomy has proven elusive. While some of the issues hampering progress in this area are of technical nature, others are rooted in existing policies which should be re-analyzed if further rapid progress is to be made in this area. This paper describes a proposal that the NASA Astrophysics Data System project has put forth in order to improve its role as one of the primary discovery portals for astronomers, focusing on those aspects which could benefit from an increased level of involvement from the community, namely the effort to expose astronomy resources as linked data, and the harvesting of observational metadata.

astro-ph.IM↗

arXiv2010

Triggered Star Formation in Galaxy Pairs at z=0.08-0.38

We measure the strength, frequency, and timescale of tidally triggered star formation at redshift z=0.08-0.38 in a spectroscopically complete sample of galaxy pairs drawn from the magnitude-limited redshift survey of 9,825 Smithsonian Hectospec Lensing Survey (SHELS) galaxies with R<20.3. To examine the evidence for tidal triggering, we identify a volume-limited sample of major (|ΔM_R|<1.75, corresponding to mass ratio >1/5) pair galaxies with $M_R < -20.8 in the redshift range z=0.08-0.31. The size and completeness of the spectroscopic survey allows us to focus on regions of low local density. The spectrophotometric calibration enables the use of the 4000 Ang break (D_n4000), the Hαspecific star formation rate (SSFR_{Hα}), and population models to characterize the galaxies. We show that D_n4000 is a useful population classification tool; it closely tracks the identification of emission line galaxies. The sample of major pair galaxies in regions of low local density with low D_n4000 demonstrates the expected anti-correlation between pair-wise projected separation and a set of star formation indicators explored in previous studies. We measure the frequency of triggered star formation by comparing the SSFR_{Hα} in the volume-limited sample in regions of low local density: 32 +/-7% of the major pair galaxies have SSFR_{Hα} at least double the median rate of the unpaired field galaxies. Comparison of stellar population models for pair and for unpaired field galaxies implies a timescale for triggered star formation of ~300-400 Myr.

astro-ph.CO↗

arXiv2009

Using Multipartite Graphs for Recommendation and Discovery

The Smithsonian/NASA Astrophysics Data System exists at the nexus of a dense system of interacting and interlinked information networks. The syntactic and the semantic content of this multipartite graph structure can be combined to provide very specific research recommendations to the scientist/user.

astro-ph.IM↗

arXiv2009

SHELS: Testing Weak Lensing Maps with Redshift Surveys

Weak lensing surveys are emerging as an important tool for the construction of "mass selected" clusters of galaxies. We evaluate both the efficiency and completeness of a weak lensing selection by combining a dense, complete redshift survey, the Smithsonian Hectospec Lensing Survey (SHELS), with a weak lensing map from the Deep Lens Survey (DLS). SHELS includes 11,692 redshifts for galaxies with R < 20.6 in the four square degree DLS field; the survey is a solid basis for identifying massive clusters of galaxies with redshift z < 0.55. The range of sensitivity of the redshift survey is similar to the range for the DLS convergence map. Only four the twelve convergence peaks with signal-to-noise > 3.5 correspond to clusters of galaxies with M > 1.7 x 10^14 solar masses. Four of the eight massive clusters in SHELS are detected in the weak lensing map yielding a completeness of roughly 50%. We examine the seven known extended cluster x-ray sources in the DLS field: three can be detected in the weak lensing map, three should not be detected without boosting from superposed large-scale structure, and one is mysteriously undetected even though its optical properties suggest that it should produce a detectable lensing signal. Taken together, these results underscore the need for more extensive comparisons among different methods of massive cluster identification.

astro-ph.CO↗

arXiv2009

Evolution of the Halpha luminosity function

The Smithsonian Hectospec Lensing Survey (SHELS) is a window on the star formation history over the last 4 Gyr. SHELS is a spectroscopically complete survey for Rtot < 20.3 over 4 square degrees. We use the 10k spectra to select a sample of pure star forming galaxies based on their Halpha emission line. We use the spectroscopy to determine extinction corrections for individual galaxies and to remove active galaxies in order to reduce systematic uncertainties. We use the large volume of SHELS with the depth of a narrowband survey for Halpha galaxies at z ~ 0.24 to make a combined determination of the Halpha luminosity function at z ~ 0.24. The large area covered by SHELS yields a survey volume big enough to determine the bright end of the Halpha luminosity function from redshift 0.100 to 0.377 for an assumed fixed faint-end slope alpha = -1.20. The bright end evolves: the characteristic luminosity L* increases by 0.84 dex over this redshift range. Similarly, the star formation density increases by 0.11 dex. The fraction of galaxies with a close neighbor increases by a factor of 2-5 for L(Halpha) >~ L* in each of the redshift bins. We conclude that triggered star formation is an important influence for star forming galaxies with Halpha emission.

astro-ph.CO↗

arXiv2009

Worldwide Use and Impact of the NASA Astrophysics Data System Digital Library

By combining data from the text, citation, and reference databases with data from the ADS readership logs we have been able to create Second Order Bibliometric Operators, a customizable class of collaborative filters which permits substantially improved accuracy in literature queries. Using the ADS usage logs along with membership statistics from the International Astronomical Union and data on the population and gross domestic product (GDP) we develop an accurate model for world-wide basic research where the number of scientists in a country is proportional to the GDP of that country, and the amount of basic research done by a country is proportional to the number of scientists in that country times that country's per capita GDP. We introduce the concept of utility time to measure the impact of the ADS/URANIA and the electronic astronomical library on astronomical research. We find that in 2002 it amounted to the equivalent of 736 FTE researchers, or $250 Million, or the astronomical research done in France. Subject headings: digital libraries; bibliometrics; sociology of science; information retrieval

cs.DL↗

arXiv2009

The Bibliometric Properties of Article Readership Information

The NASA Astrophysics Data System (ADS), along with astronomy's journals and data centers (a collaboration dubbed URANIA), has developed a distributed on-line digital library which has become the dominant means by which astronomers search, access and read their technical literature. Digital libraries such as the NASA Astrophysics Data System permit the easy accumulation of a new type of bibliometric measure, the number of electronic accesses (``reads'') of individual articles. We explore various aspects of this new measure. We examine the obsolescence function as measured by actual reads, and show that it can be well fit by the sum of four exponentials with very different time constants. We compare the obsolescence function as measured by readership with the obsolescence function as measured by citations. We find that the citation function is proportional to the sum of two of the components of the readership function. This proves that the normative theory of citation is true in the mean. We further examine in detail the similarities and differences between the citation rate, the readership rate and the total citations for individual articles, and discuss some of the causes. Using the number of reads as a bibliometric measure for individuals, we introduce the read-cite diagram to provide a two-dimensional view of an individual's scientific productivity. We develop a simple model to account for an individual's reads and cites and use it to show that the position of a person in the read-cite diagram is a function of age, innate productivity, and work history. We show the age biases of both reads and cites, and develop two new bibliometric measures which have substantially less age bias than citations

cs.DL↗

arXiv2009

The Smithsonian/NASA Astrophysics Data System (ADS) Decennial Report

Eight years after the ADS first appeared the last decadal survey wrote: "NASA's initiative for the Astrophysics Data System has vastly increased the accessibility of the scientific literature for astronomers. NASA deserves credit for this valuable initiative and is urged to continue it." Here we summarize some of the changes concerning the ADS which have occurred in the past ten years, and we describe the current status of the ADS. We then point out two areas where the ADS is building an improved capability which could benefit from a policy statement of support in the ASTRO2010 report. These are: The Semantic Interlinking of Astronomy Observations and Datasets and The Indexing of the Full Text of Astronomy Research Publications.

astro-ph.IM↗

arXiv2008

Use of Astronomical Literature - A Report on Usage Patterns

In this paper we present a number of metrics for usage of the SAO/NASA Astrophysics Data System (ADS). Since the ADS is used by the entire astronomical community, these are indicative of how the astronomical literature is used. We will show how the use of the ADS has changed both quantitatively and qualitatively. We will also show that different types of users access the system in different ways. Finally, we show how use of the ADS has evolved over the years in various regions of the world. The ADS is funded by NASA Grant NNG06GG68G.

cs.DL↗