195 research outputs found
The Ultraviolet Attenuation Law in Backlit Spiral Galaxies
(Abridged) The effective extinction law (attenuation behavior) in galaxies in
the emitted ultraviolet is well known only for actively star-forming objects
and combines effects of the grain properties, fine structure in the dust
distribution, and relative distributions of stars and dust. We use GALEX, XMM
Optical Monitor, and HST data to explore the UV attenuation in the outer parts
of spiral disks which are backlit by other UV-bright galaxies, starting with
candidates provided by Galaxy Zoo participants. Our analysis incorporates
galaxy symmetry, using non-overlapping regions of each galaxy to derive error
estimates on the attenuation measurements. The entire sample has an attenuation
law close to the Calzetti et al. (1994) form; the UV slope for the overall
sample is substantially shallower than found by Wild et al. (2011), a
reasonable match to the more distant galaxies in our sample but not to the
weighted combination including NGC 2207. The nearby, bright spiral NGC 2207
alone gives accuracy almost equal to the rest of our sample, and its outer arms
have a very low level of foreground starlight. This "grey" law can be produced
from the distribution of dust alone, without a necessary contribution from
differential escape of stars from dense clouds. The extrapolation needed to
compare attenution between backlit galaxies at moderate redshifts, and local
systems from SDSS data, is mild enough to allow use of galaxy overlaps to trace
the cosmic history of dust. For NGC 2207, the covering factor of clouds with
small optical attenuation becomes a dominant factor farther into the
ultraviolet, which opens the possibility that widespread diffuse dust dominates
over dust in star-forming regions deep into the ultraviolet. Comparison with
published radiative-transfer models indicates that the role of dust clumping
dominates over differences in grain populations, at this spatial resolution.Comment: In press, Astronomical Journa
Galaxy Zoo: an independent look at the evolution of the bar fraction over the last eight billion years from HST-COSMOS
The Galactic Interstellar Object Population: A Framework for Prediction and Inference
The Milky Way is thought to host a huge population of interstellar objects
(ISOs), numbering approximately around the Sun, which
are formed and shaped by a diverse set of processes ranging from planet
formation to galactic dynamics. We define a novel framework: firstly to predict
the properties of this Galactic ISO population by combining models of processes
across planetary and galactic scales, and secondly to make inferences about the
processes modelled, by comparing the predicted population to what is observed.
We predict the spatial and compositional distribution of the Galaxy's
population of ISOs by modelling the Galactic stellar population with data from
the APOGEE survey and combining this with a protoplanetary disk chemistry
model. Selecting ISO water mass fraction as an example observable quantity, we
evaluate its distribution both at the position of the Sun and averaged over the
Galactic disk; our prediction for the Solar neighbourhood is compatible with
the inferred water mass fraction of 2I/Borisov. We show that the well-studied
Galactic stellar metallicity gradient has a corresponding ISO compositional
gradient. We also demonstrate the inference part of the framework by using the
current observed ISO composition distribution to constrain the parent star
metallicity dependence of the ISO production rate. This constraint, and other
inferences made with this framework, will improve dramatically as the Vera C.
Rubin Observatory Legacy Survey of Space and Time (LSST) progresses and more
ISOs are observed. Finally, we explore generalisations of this framework to
other Galactic populations, such as that of exoplanets.Comment: Accepted to A
Extended X-ray emission in the IC 2497 - Hanny's Voorwerp system: energy injection in the gas around a fading AGN
We present deep Chandra X-ray observations of the core of IC 2497, the galaxy
associated with Hanny's Voorwerp and hosting a fading AGN. We find extended
soft X-ray emission from hot gas around the low intrinsic luminosity
(unobscured) AGN ( erg s). The
temperature structure in the hot gas suggests the presence of a bubble or
cavity around the fading AGN (\mbox{E_{\rm bub}} \sim 10^{54} - 10^{55}
erg). A possible scenario is that this bubble is inflated by the fading AGN,
which after changing accretion state is now in a kinetic mode. Other
possibilities are that the bubble has been inflated by the past luminous quasar
( erg s), or that the temperature gradient is
an indication of a shock front from a superwind driven by the AGN. We discuss
the possible scenarios and the implications for the AGN-host galaxy
interaction, as well as an analogy between AGN and X-ray binaries lifecycles.
We conclude that the AGN could inject mechanical energy into the host galaxy at
the end of its lifecycle, and thus provide a source for mechanical feedback, in
a similar way as observed for X-ray binaries.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Unproceedings of the Fourth .Astronomy Conference (.Astronomy 4), Heidelberg, Germany, July 9-11 2012
The goal of the .Astronomy conference series is to bring together
astronomers, educators, developers and others interested in using the Internet
as a medium for astronomy. Attendance at the event is limited to approximately
50 participants, and days are split into mornings of scheduled talks, followed
by 'unconference' afternoons, where sessions are defined by participants during
the course of the event. Participants in unconference sessions are discouraged
from formal presentations, with discussion, workshop-style formats or informal
practical tutorials encouraged. The conference also designates one day as a
'hack day', in which attendees collaborate in groups on day-long projects for
presentation the following morning. These hacks are often a way of
concentrating effort, learning new skills, and exploring ideas in a practical
fashion. The emphasis on informal, focused interaction makes recording
proceedings more difficult than for a normal meeting. While the first
.Astronomy conference is preserved formally in a book, more recent iterations
are not documented. We therefore, in the spirit of .Astronomy, report
'unproceedings' from .Astronomy 4, which was held in Heidelberg in July 2012.Comment: 11 pages, 1 figure, .Astronomy 4, #dotastr
Revealing the cosmic evolution of boxy/peanut-shaped bulges from HST COSMOS and SDSS
Abstract Vertically thickened bars, observed in the form of boxy/peanut (B/P) bulges, are found in the majority of massive barred disc galaxies in the local Universe, including our own. B/P bulges indicate that their host bars have suffered violent bending instabilities driven by anisotropic velocity distributions. We investigate for the first time how the frequency of B/P bulges in barred galaxies evolves from z = 1 to z ≈ 0, using a large sample of non-edge-on galaxies with masses M⋆ > 1010 M⊙, selected from the HST COSMOS survey. We find the observed fraction increases from at z = 1 to % at z = 0.2. We account for problems identifying B/P bulges in galaxies with low inclinations and unfavourable bar orientations, and due to redshift-dependent observational biases with the help of a sample from the Sloan Digital Sky Survey, matched in resolution, rest-frame band, signal-to-noise ratio and stellar mass and analysed in the same fashion. From this, we estimate that the true fraction of barred galaxies with B/P bulges increases from ∼10% at z ≈ 1 to at z = 0. In agreement with previous results for nearby galaxies, we find a strong dependence of the presence of a B/P bulge on galaxy stellar mass. This trend is observed in both local and high-redshift galaxies, indicating that it is an important indicator of vertical instabilities across a large fraction of the age of the Universe. We propose that galaxy formation processes regulate the thickness of galaxy discs, which in turn affect which galaxies experience violent bending instabilities of the bar
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