543 research outputs found
The Gas Consumption History to z ~ 4
Using the observations of the star formation rate and HI densities to z ~ 4,
with measurements of the Molecular Gas Depletion Rate (MGDR) and local density
of H_2 at z = 0, we derive the history of the gas consumption by star formation
to z ~ 4. We find that closed-box models in which H_2 is not replenished by HI
require improbably large increases in rho(H_2) and a decrease in the MGDR with
lookback time that is inconsistent with observations. Allowing the H_2 used in
star formation to be replenished by HI does not alleviate the problem because
observations show that there is very little evolution of rho(HI) from z = 0 to
z = 4. We show that to be consistent with observational constraints, star
formation on cosmic timescales must be fueled by intergalactic ionized gas,
which may come from either accretion of gas through cold (but ionized) flows or
from ionized gas associated with accretion of dark matter halos. We constrain
the rate at which the extraglactic ionized gas must be converted into HI and
ultimately into H_2. The ionized gas inflow rate roughly traces the SFRD: about
1 - 2 x 10^8 M_sun Gyr^-1 Mpc^-3 from z ~ 1 - 4, decreasing by about an order
of magnitude from z=1 to z=0 with details depending largely on MGDR(t). All
models considered require the volume averaged density of rho(H_2) to increase
by a factor of 1.5 - 10 to z ~ 1.5 over the currently measured value. Because
the molecular gas must reside in galaxies, it implies that galaxies at high z
must, on average, be more molecule rich than they are at the present epoch,
which is consistent with observations. These quantitative results, derived
solely from observations, agree well with cosmological simulations.Comment: 11 pages, 6 figures. Accepted for publication in the Astrophysical
Journal
Confronting cold dark matter predictions with observed galaxy rotations
The rich statistics of galaxy rotations as captured by the velocity function (VF) provide invaluable constraints on galactic baryon physics and the nature of dark matter (DM). However, the comparison of observed galaxy rotations against cosmological models is prone to subtle caveats that can easily lead to misinterpretations. Our analysis reveals full statistical consistency between similar to 5000 galaxy rotations, observed in line-of-sight projection, and predictions based on the standard cosmological model (Lambda CDM) at the mass-resolution of the Millennium simulation (H I line-based circular velocities above similar to 50 km s(-1)). Explicitly, the H I linewidths in the H I Parkes All Sky Survey (HIPASS) are found to be consistent with those in S-3-SAX, a post-processed semi-analytic model for the Millennium simulation. Previously found anomalies in the VF can be plausibly attributed to (1) the mass-limit of the Millennium simulation, (2) confused sources in HIPASS, (3) inaccurate inclination measurements for optically faint sources, and (4) the non-detectability of gas-poor early-type galaxies. These issues can be bypassed by comparing observations and models using linewidth source counts rather than VFs. We investigate if and how well such source counts can constrain the temperature of DM
PKSB1740-517: An ALMA view of the cold gas feeding a distant interacting young radio galaxy
Cold neutral gas is a key ingredient for growing the stellar and central
black hole mass in galaxies throughout cosmic history. We have used the Atacama
Large Millimetre Array (ALMA) to detect a rare example of redshifted
CO(2-1) absorption in PKS B1740-517, a young (
yr) and luminous ( erg s ) radio
galaxy at that is undergoing a tidal interaction with at least one
lower-mass companion. The coincident HI 21-cm and molecular absorption have
very similar line profiles and reveal a reservoir of cold gas ( M), likely distributed in a disc or ring within
a few kiloparsecs of the nucleus. A separate HI component is kinematically
distinct and has a very narrow line width ( km
s), consistent with a single diffuse cloud of cold (
K) atomic gas. The CO(2-1) absorption is not associated with this
component, which suggests that the cloud is either much smaller than 100 pc
along our sight-line and/or located in low-metallicity gas that was possibly
tidally stripped from the companion. We argue that the gas reservoir in PKS
B1740-517 may have accreted onto the host galaxy 50 Myr before the young
radio AGN was triggered, but has only recently reached the nucleus. This is
consistent with the paradigm that powerful luminous radio galaxies are
triggered by minor mergers and interactions with low-mass satellites and
represent a brief, possibly recurrent, active phase in the life cycle of
massive early type galaxies.Comment: 15 pages, 7 figures, accepted for publication in MNRA
A Superluminous Supernova in High Surface Density Molecular Gas within the Bar of a Metal-rich Galaxy
We report the Atacama Large Millimeter/submillimeter Array observations of the metal-rich host galaxy of superluminous supernova (SLSN) PTF10tpz, a barred spiral galaxy at z = 0.03994. We find the CO(1–0) emission to be confined within the bar of the galaxy. The distribution and kinematics of molecular gas in the host galaxy resemble gas flows along two lanes running from the tips of the bar toward the galaxy center. These gas lanes end in a gaseous structure in the inner region of the galaxy, likely associated with an inner Lindblad resonance. The interaction between the large-scale gas flows in the bar and the gas in the inner region plausibly leads to the formation of massive molecular clouds and consequently massive clusters. This in turn can result in formation of massive stars, and thus the likely progenitor of the SLSN in a young, massive cluster. This picture is consistent with SLSN PTF10tpz being located near the intersection regions of the gas lanes and the inner structure. It is also supported by the high molecular gas surface densities that we find in the vicinity of the SLSN, surface densities that are comparable with those in interacting galaxies or starburst regions in nearby galaxies. Our findings therefore suggest in situ formation of massive stars due to the internal dynamics of the host galaxy and also lend support to high densities being favorable conditions for formation of SLSN progenitors
The role of Comprehension in Requirements and Implications for Use Case Descriptions
Within requirements engineering it is generally accepted that in writing specifications (or indeed any requirements phase document), one attempts to produce an artefact which will be simple to comprehend for the user. That is, whether the document is intended for customers to validate requirements, or engineers to understand what the design must deliver, comprehension is an important goal for the author. Indeed, advice on producing ‘readable’ or ‘understandable’ documents is often included in courses on requirements engineering. However, few researchers, particularly within the software engineering domain, have attempted either to define or to understand the nature of comprehension and it’s implications for guidance on the production of quality requirements.
Therefore, this paper examines thoroughly the nature of textual comprehension, drawing heavily from research in discourse process, and suggests some implications for requirements (and other) software documentation. In essence, we find that the guidance on writing requirements, often prevalent within software engineering, may be based upon assumptions which are an oversimplification of the nature of comprehension. Hence, the paper examines guidelines which have been proposed, in this case for use case descriptions, and the extent to which they agree with discourse process theory; before suggesting refinements to the guidelines which attempt to utilise lessons learned from our richer understanding of the underlying discourse process theory. For example, we suggest subtly different sets of writing guidelines for the different tasks of requirements, specification and design
Photospheric flux cancellation and associated flux rope formation and eruption
We study an evolving bipolar active region that exhibits flux cancellation at
the internal polarity inversion line, the formation of a soft X-ray sigmoid
along the inversion line and a coronal mass ejection. The evolution of the
photospheric magnetic field is described and used to estimate how much flux is
reconnected into the flux rope. About one third of the active region flux
cancels at the internal polarity inversion line in the 2.5~days leading up to
the eruption. In this period, the coronal structure evolves from a weakly to a
highly sheared arcade and then to a sigmoid that crosses the inversion line in
the inverse direction. These properties suggest that a flux rope has formed
prior to the eruption. The amount of cancellation implies that up to 60% of the
active region flux could be in the body of the flux rope. We point out that
only part of the cancellation contributes to the flux in the rope if the arcade
is only weakly sheared, as in the first part of the evolution. This reduces the
estimated flux in the rope to or less of the active region flux. We
suggest that the remaining discrepancy between our estimate and the limiting
value of of the active region flux, obtained previously by the flux
rope insertion method, results from the incomplete coherence of the flux rope,
due to nonuniform cancellation along the polarity inversion line. A hot linear
feature is observed in the active region which rises as part of the eruption
and then likely traces out field lines close to the axis of the flux rope. The
flux cancellation and changing magnetic connections at one end of this feature
suggest that the flux rope reaches coherence by reconnection shortly before and
early in the impulsive phase of the associated flare. The sigmoid is destroyed
in the eruption but reforms within a few hours after a moderate amount of
further cancellation has occurred.Comment: Astron. Astrophys., in pres
Compact High-Velocity Clouds at High Resolution
Six examples of the compact, isolated high-velocity clouds catalogued by
Braun & Burton (1999) and identified with a dynamically cold ensemble of
primitive objects falling towards the barycenter of the Local Group have been
imaged with the Westerbork Synthesis Radio Telescope; an additional ten have
been imaged with the Arecibo telescope. The imaging reveals a characteristic
core/halo morphology: one or several cores of cool, relatively
high-column-density material, are embedded in an extended halo of warmer,
lower-density material. Several of the cores show kinematic gradients
consistent with rotation; these CHVCs are evidently rotationally supported and
dark-matter dominated. The imaging data allows several independent estimates of
the distances to these objects, which lie in the range 0.3 to 1.0 Mpc. The CHVC
properties resemble what might be expected from very dark dwarf irregular
galaxies.Comment: 12 pages, 7 figures, to appear in "The Chemical Evolution of the
Milky Way: Stars versus Clusters", eds. F. Matteuchi and F. Giovannelli,
Kluwer Academic Publisher
On the kinematics of the Local cosmic void
We collected the existing data on the distances and radial velocities of
galaxies around the Local Void in the Aquila/Hercules to examine the peculiar
velocity field induced by its underdensity. A sample of 1056 galaxies with
distances measured from the Tip of the Red Giant Branch, the Cepheid
luminosity, the SNIa luminosity, the surface brightness fluctuation method, and
the Tully-Fisher relation has been used for this purpose. The amplitude of
outflow is found to be ~300 km/s. The galaxies located within the void produce
the mean intra-void number density about 1/5 of the mean external number
density of galaxies. The void's population has a lower luminosity and a later
morphological type with the medians: M_B = -15.7^m and T = 8 (Sdm),
respectively.Comment: Version 1. 14 pages, 8 figures, 2 tables. Accepted to Astrophysics,
Volume 54, Issue
Cold gas accretion in galaxies
Evidence for the accretion of cold gas in galaxies has been rapidly
accumulating in the past years. HI observations of galaxies and their
environment have brought to light new facts and phenomena which are evidence of
ongoing or recent accretion:
1) A large number of galaxies are accompanied by gas-rich dwarfs or are
surrounded by HI cloud complexes, tails and filaments. It may be regarded as
direct evidence of cold gas accretion in the local universe. It is probably the
same kind of phenomenon of material infall as the stellar streams observed in
the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI
have been found in nearby spiral galaxies. While a large fraction of this gas
is produced by galactic fountains, it is likely that a part of it is of
extragalactic origin. 3) Spirals are known to have extended and warped outer
layers of HI. It is not clear how these have formed, and how and for how long
the warps can be sustained. Gas infall has been proposed as the origin. 4) The
majority of galactic disks are lopsided in their morphology as well as in their
kinematics. Also here recent accretion has been advocated as a possible cause.
In our view, accretion takes place both through the arrival and merging of
gas-rich satellites and through gas infall from the intergalactic medium (IGM).
The infall may have observable effects on the disk such as bursts of star
formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold
gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates
needed to sustain the observed star formation (~1 Msol/yr), additional infall
of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages.
Full-resolution version available at
http://www.astron.nl/~oosterlo/accretionRevie
On staying grounded and avoiding Quixotic dead ends
The 15 articles in this special issue on The Representation of Concepts illustrate the rich variety of theoretical positions and supporting research that characterize the area. Although much agreement exists among contributors, much disagreement exists as well, especially about the roles of grounding and abstraction in conceptual processing. I first review theoretical approaches raised in these articles that I believe are Quixotic dead ends, namely, approaches that are principled and inspired but likely to fail. In the process, I review various theories of amodal symbols, their distortions of grounded theories, and fallacies in the evidence used to support them. Incorporating further contributions across articles, I then sketch a theoretical approach that I believe is likely to be successful, which includes grounding, abstraction, flexibility, explaining classic conceptual phenomena, and making contact with real-world situations. This account further proposes that (1) a key element of grounding is neural reuse, (2) abstraction takes the forms of multimodal compression, distilled abstraction, and distributed linguistic representation (but not amodal symbols), and (3) flexible context-dependent representations are a hallmark of conceptual processing
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