1,257 research outputs found
Evolution of the Near-Infrared Tully-Fisher Relation: Constraints on the Relationship Between the Stellar and Total Masses of Disk Galaxies since z=1
Using a combination of Keck spectroscopy and near-infrared imaging, we
investigate the K-band and stellar mass Tully-Fisher relation for 101 disk
galaxies at 0.2 < z < 1.2, with the goal of placing the first observational
constraints on the assembly history of halo and stellar mass. Our main result
is a lack of evolution in either the K-band or stellar mass Tully-Fisher
relation from z = 0 - 1.2. Furthermore, although our sample is not
statistically complete, we consider it suitable for an initial investigation of
how the fraction of total mass that has condensed into stars is distributed
with both redshift and total halo mass. We calculate stellar masses from
optical and near-infrared photometry and total masses from maximum rotational
velocities and disk scale lengths, utilizing a range of model relationships
derived analytically and from simulations. We find that the stellar/total mass
distribution and stellar-mass Tully-Fisher relation for z > 0.7 disks is
similar to that at lower redshift, suggesting that baryonic mass is accreted by
disks along with dark matter at z < 1, and that disk galaxy formation at z < 1
is hierarchical in nature. We briefly discuss the evolutionary trends expected
in conventional structure formation models and the implications of extending
such a study to much larger samples.Comment: ApJ, in press, 9 page
Why do some CEOs become celebrities while others donât?
Celebrity benefits chief executives personally: increased pay, more opportunities to join boards, and protection from dismissal. But their firms do not enjoy similarly positive outcomes. These CEOs often demonstrate higher levels of complacency, risk-taking, and hubris. But what leads them to attain celebrity in the first place? Jeffrey B. Lovelace, Jonathan N. Bundy, Tim Pollock, and Donald Hambrick write that a CEOâs personal attributes, a firmâs non-conforming actions, and a CEOâs use of self-promotion tactics play important roles in attracting high levels of positive media attention â with implications for individuals, firms, and society
Dislocation density and graphitization of diamond crystals
Two sets of diamond specimens compressed at 2 GPa at temperatures varying between 1060 K and 1760 K were prepared; one in which graphitization was promoted by the presence of water and another in which graphitization of diamond was practically absent. X-ray diffraction peak profiles of both sets were analyzed for the microstructure by using the modified Williamson-Hall method and by fitting the Fourier coefficients of the measured profiles by theoretical functions for crystallite size and lattice strain. The procedures determined mean size and size distribution of crystallites as well as the density and the character of the dislocations. The same experimental conditions resulted in different microstructures for the two sets of samples. They were explained in terms of hydrostatic conditions present in the graphitized samples
The Bimodal Galaxy Stellar Mass Function in the COSMOS Survey to z~1: A Steep Faint End and a New Galaxy Dichotomy
We present a new analysis of stellar mass functions (MF) in the COSMOS field
to fainter limits than has been previously probed to z~1. Neither the total nor
the passive or star-forming MF can be well fit with a single Schechter function
once one probes below 3e9 Msun. We observe a dip or plateau at masses ~1e10
Msun, and an upturn towards a steep faint-end slope of -1.7 at lower mass at
any z<1. This bimodal nature of the MF is not solely a result of the blue/red
dichotomy. The blue MF is by itself bimodal at z~1. This suggests a new
dichotomy in galaxy formation that predates the appearance of the red sequence.
We propose two interpretations for this bimodality. If the gas fraction
increases towards lower mass, galaxies with M_baryon~1e10 Msun would shift to
lower stellar masses, creating the observed dip. This would indicate a change
in star formation efficiency, perhaps linked to supernovae feedback becoming
much more efficient. Therefore, we investigate whether the dip is present in
the baryonic (stars+gas) MF. Alternatively, the dip could be created by an
enhancement of the galaxy assembly rate at ~1e11 Msun, a phenomenon that
naturally arises if the baryon fraction peaks at M_halo ~1e12 Msun. In this
scenario, galaxies occupying the bump around M* would be identified with
central galaxies and the second fainter component having a steep faint-end
slope with satellites. While the dip is apparent in the total MF at any z, it
appears to shift from the blue to red population, likely as a result of
transforming high-mass blue galaxies into red ones. At the same time, we detect
a drastic upturn in the number of low-mass red galaxies. Their increase with
time reflects a decrease in the number of blue systems and so we tentatively
associate them with satellite dwarf galaxies that have undergone quenching.Comment: 16 pages, 10 figures, accepted for publication in Ap
Electronic Transport in a Three-dimensional Network of 1-D Bismuth Quantum Wires
The resistance R of a high density network of 6 nm diameter Bi wires in
porous Vycor glass is studied in order to observe its expected semiconductor
behavior. R increases from 300 K down to 0.3 K. Below 4 K, where R varies
approximately as ln(1/T), the order-of-magnitude of the resistance rise, as
well as the behavior of the magnetoresistance are consistent with localization
and electron-electron interaction theories of a one-dimensional disordered
conductor in the presence of strong spin-orbit scattering. We show that this
behaviour and the surface-enhanced carrier density may mask the proposed
semimetal-to-semiconductor transition for quantum Bi wires.Comment: 19 pages total, 4 figures; accepted for publication in Phys. Rev.
Star Formation in AEGIS Field Galaxies since z=1.1 : The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-Forming Galaxies
We analyze star formation (SF) as a function of stellar mass (M*) and
redshift z in the All Wavelength Extended Groth Strip International Survey
(AEGIS). For 2905 field galaxies, complete to 10^10(10^10.8) Msun at z<0.7(1),
with Keck spectroscopic redshifts out to z=1.1, we compile SF rates (SFR) from
emission lines, GALEX, and Spitzer MIPS 24 micron photometry, optical-NIR M*
measurements, and HST morphologies. Galaxies with reliable signs of SF form a
distinct "main sequence (MS)", with a limited range of SFR at a given M* and z
(1 sigma < +-0.3 dex), and log(SFR) approximately proportional to log(M*). The
range of log(SFR) remains constant to z>1, while the MS as a whole moves to
higher SFR as z increases. The range of SFR along the MS constrains the
amplitude of episodic variations of SF, and the effect of mergers on SFR.
Typical galaxies spend ~67(95)% of their lifetime since z=1 within a factor of
<~ 2(4) of their average SFR at a given M* and z. The dominant mode of the
evolution of SF since z~1 is apparently a gradual decline of the average SFR in
most individual galaxies, not a decreasing frequency of starburst episodes, or
a decreasing factor by which SFR are enhanced in starbursts. LIRGs at z~1 seem
to mostly reflect the high SFR typical for massive galaxies at that epoch. The
smooth MS may reflect that the same set of few physical processes governs star
formation prior to additional quenching processes. A gradual process like gas
exhaustion may play a dominant role.Comment: 5 pages, 1 figure, emulateapj; ApJ Letters, accepted; AEGIS special
issue; proof-level corrections added; title change
Land beneficiaries as game farmers: conservation, land reform and the invention of the 'community game farm' in KwaZulu-Natal
Scholarship on post-apartheid land reform includes research on land claims made to formal protected areas, such as national parks and state game reserves. Little attention has however, been paid to the question of land restitution claims on private lands, on which a range of nominally âconservation-friendlyâ land-uses (including commercial hunting) have taken place. This article traces the emergence of the âcommunity game farmâ as a product of land reform processes affecting freehold land in the midlands of KwaZulu-Natal province, South Africa. Two groups of land beneficiaries who were granted title to former privately owned game farms used for leisure hunting are studied in detail. The article shows that a range of state and private actors, as well as traditional authorities, have worked to ensure the continuation of the land under conservation or game farming after transfer. The central argument is that in this process, a generic narrative is imposed which works to conflate or deny the distinct historical identities of the beneficiary groups. The article raises questions about the real efficacy of land restitution in this context, as well as the appropriateness of a community-based conservation narrative when applied in the context of small farms such as those considered here.International Bibliography of Social Science
The Build-Up of the Hubble Sequence in the COSMOS Field
We use ~8,600 >5e10 Msol COSMOS galaxies to study how the morphological mix
of massive ellipticals, bulge-dominated disks, intermediate-bulge disks,
bulge-less disks and irregular galaxies evolves from z=0.2 to z=1. The
morphological evolution depends strongly on mass. At M>3e11 Msol, no evolution
is detected in the morphological mix: ellipticals dominate since z=1, and the
Hubble sequence has quantitatively settled down by this epoch. At the 1e11 Msol
mass scale, little evolution is detected, which can be entirely explained with
major mergers. Most of the morphological evolution from z=1 to z=0.2 takes
place at masses 5e10 - 1e11 Msol, where: (i) The fraction of spirals
substantially drops and the contribution of early-types increases. This
increase is mostly produced by the growth of bulge-dominated disks, which vary
their contribution from ~10% at z=1 to >30% at z=0.2 (cf. the elliptical
fraction grows from ~15% to ~20%). Thus, at these masses, transformations from
late- to early-types result in disk-less elliptical morphologies with a
statistical frequency of only 30% - 40%. Otherwise, the processes which are
responsible for the transformations either retain or produce a non-negligible
disk component. (ii) The bulge-less disk galaxies, which contribute ~15% to the
intermediate-mass galaxy population at z=1, virtually disappear by z=0.2. The
merger rate since z=1 is too low to account for the disappearance of these
massive bulge-less disks, which most likely grow a bulge via secular evolution.Comment: 5 pages, 3 figures, submitted to ApJ
Unpacking the âEmergent Farmerâ Concept in Agrarian Reform:Evidence from Livestock Farmers in South Africa
South Africa has historically perpetuated a dual system of freehold commercial and communal subsistence farming. To bridge these extremes, agrarian reform policies have encouraged the creation of a class of âemergentâ, commercially oriented farmers. However, these policies consider âemergentâ farmers as a homogeneous group of land reform beneficiaries, with limited appreciation of the class differences between them, and do little to support the rise of a âmiddleâ group of producers able to bridge that gap. This article uses a case study of livestock farmers in Eastern Cape Province to critique the âemergent farmerâ concept. The authors identify three broad categories of farmers within the emergent livestock sector: a large group who, despite having accessed private farms, remain effectively subsistence farmers; a smaller group of small/medium-scale commercial producers who have communal farming origins and most closely approximate to âemergentâ farmers; and an elite group of large-scale, fully commercialized farmers, whose emergence has been facilitated primarily by access to capital and a desire to invest in alternative business ventures. On this basis the authors suggest that current agrarian reform policies need considerable refocusing if they are to effectively facilitate the emergence of a âmiddleâ group of smallholder commercial farmers from communal systems
Galaxy Collisions - Dawn of a New Era
The study of colliding galaxies has progressed rapidly in the last few years,
driven by observations with powerful new ground and space-based instruments.
These instruments have used for detailed studies of specific nearby systems,
statistical studies of large samples of relatively nearby systems, and
increasingly large samples of high redshift systems. Following a brief summary
of the historical context, this review attempts to integrate these studies to
address the following key issues. What role do collisions play in galaxy
evolution, and how can recently discovered processes like downsizing resolve
some apparently contradictory results of high redshift studies? What is the
role of environment in galaxy collisions? How is star formation and nuclear
activity orchestrated by the large scale dynamics, before and during merger?
Are novel modes of star formation involved? What are we to make of the
association of ultraluminous X-ray sources with colliding galaxies? To what do
degree do mergers and feedback trigger long-term secular effects? How far can
we push the archaeology of individual systems to determine the nature of
precursor systems and the precise effect of the interaction? Tentative answers
to many of these questions have been suggested, and the prospects for answering
most of them in the next few decades are good.Comment: 44 pages, 9 figures, review article in press for Astrophysics Update
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