1,720,997 research outputs found
The Ages of Elliptical Galaxies from Mid-Infrared Emission
No full textThe mid-infrared (10–20 m) luminosity of elliptical galaxies is dominated by the integrated emission from
circumstellar dust in red giant stars. As a single stellar population evolves, the rate of dusty mass loss from red giant
stars decreases with time, so the mid-infrared luminosity should also decline with stellar age. To seek such a
correlation, we have used archival Infrared Space Observatory (ISO) observations to determine surface brightness
profiles and central fluxes at 15 m in 17 early-type galaxies for which stellar ages have been determined from
optical spectral indices. The radial surface brightness distributions at 15 m generally follow the stellar de Vaucouleurs
profile, as expected. We find that the surface brightness ratio 15 m=I band is systematically higher in
elliptical galaxies with ages P5 Gyr and in galaxies that exhibit evidence of recent mergers. Within the accuracy
of our observations, 15 m=I band shows no age dependence for ages k5 Gyr. The corresponding flux ratios
F15 m=FI band within apertures scaled to the effective radius (Re=8) are proportional to the 15 m=I band ratios at
larger galactic radii, indicating that no 15 m emission is detected from central dust clouds visible in optical images
in some of our sample galaxies. Emission at 15 m is observed in noncentral massive clouds of dust and cold gas in
NGC 1316, an elliptical galaxy that is thought to have had a recent merger. Recent Spitzer Space Telescope data also
indicate the presence of polycyclic aromatic hydrocarbon (PAH ) emission at 8 m. Several ellipticals have extended
regions of 15 m emission that have no obvious counterparts at other frequencies
Absence of Dwarf Galaxies at High Redshifts: Evidence from a Galaxy Group
No full textThe galaxy group NGC 5044 consists of a luminous giant elliptical galaxy surrounded by a cluster of 160
low-luminosity and dwarf galaxies, mostly of early type. The cumulative projected radial distribution of dwarf
galaxies in the NGC 5044 group, unlike distributions of more luminous galaxies in rich clusters, does not follow
a projected dark matter (NFW ) profile. A deficiency or absence of low-luminosity galaxies is apparent in
NGC 5044 within about 350 kpc. Most of the dwarf galaxies in NGC 5044 entered the virial radius at redshifts
z P2:5(af =0:25), where af 1⁄4 1=(1þ zf ) is the epoch of group formation, and very few entered during redshifts
z k2:5(af =0:25). The peculiar, non-NFW shape of the projected cumulative dwarf galaxy distribution in NGC
5044 within 350 kpc resembles the characteristic cumulative distribution of dark subhalos that are also known to
be relatively young. Dynamical friction is unlikely to explain the apparent lack of group member galaxies at small
radii in NGC 5044
Ultralow Iron Abundances in the Distant Hot Gas in Galaxy Groups
No full textA new XMM observation of the outer regions of the galaxy group NGC 5044 indicates hot gas iron abundances
of only between rvir and 0.4rvir ZFe /Z, ∼ 0.15 r p 0.2 . While the total baryon mass within the virial radius may be
close to the cosmic mean value observed in rich clusters, the ratio of total iron mass to optical light in NGC 5044
is about 3 times lower than that in rich clusters. The remarkably low iron abundance over a large volume of the
intergroup gas in the outer regions of NGC 5044 cannot be easily understood in terms of the outflow of enriched
gas in a group wind during its early history or by the long-term enrichment by the group member galaxies that
currently occupy this region. It is possible that the stars in NGC 5044 did not produce iron with the same efficiency
as in clusters, or that the iron resides in nonluminous clouds or stars, or that the entropy of the iron-enriched gas
created in early galactic starburst winds was too high to penetrate the group gas of lower entrop
Weighing Super-Massive Black Holes with X-Ray Gas
No full textWe present a new approach to measure the mass of a super-massive black hole (SMBH) located at the center of a giant elliptical galaxy. This method applies the well-known technique of using the hot, X-ray emitting plasma as a tracer of the large-scale gravitational potential of a giant galaxy (or galaxy cluster) and extends it far down into the central region of a galaxy using high-resolution X-ray data from the Chandra X-ray Observatory. We report the first detection of a SMBH using this method in the Virgo elliptical galaxy, NGC 4649, and present results of preliminary detections in 3 other systems. In addition to providing interesting constraints on the black-hole masses, we show that the stellar mass-to-light ratios of the galaxies computed from this approach agree very well with the prediction from stellar population synthesis models, thus providing strong support for the underlying assumptions of the method (e.g., hydrostatic equilibrium)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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