70 research outputs found
Gas accretion as the origin of chemical abundance gradients in distant galaxies
It has recently been suggested that galaxies in the early Universe can grow
through the accretion of cold gas, and that this may have been the main driver
of star formation and stellar mass growth. Because the cold gas is essentially
primordial, it has a very low abundance of elements heavier than helium
(metallicity). As it is funneled to the centre of a galaxy, it will lead the
central gas having an overall lower metallicity than gas further from the
centre, because the gas further out has been enriched by supernovae and stellar
winds, and not diluted by the primordial gas. Here we report chemical
abundances across three rotationally-supported star-forming galaxies at z~3,
only 2 Gyr after the Big Bang. We find an 'inverse' gradient, with the central,
star forming regions having a lower metallicity than less active ones, opposite
to what is seen in local galaxies. We conclude that the central gas has been
diluted by the accretion of primordial gas, as predicted by 'cold flow' models.Comment: To Appear in Nature Oct 14, 2010; Supplementary Information included
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A single low-energy, iron-poor supernova as the source of metals in the star SMSS J 031300.36-670839.3
The element abundance ratios of four low-mass stars with extremely low
metallicities indicate that the gas out of which the stars formed was enriched
in each case by at most a few, and potentially only one low-energy, supernova.
Such supernovae yield large quantities of light elements such as carbon but
very little iron. The dominance of low-energy supernovae is surprising, because
it has been expected that the first stars were extremely massive, and that they
disintegrated in pair-instability explosions that would rapidly enrich galaxies
in iron. What has remained unclear is the yield of iron from the first
supernovae, because hitherto no star is unambiguously interpreted as
encapsulating the yield of a single supernova. Here we report the optical
spectrum of SMSS J031300.36- 670839.3, which shows no evidence of iron (with an
upper limit of 10^-7.1 times solar abundance). Based on a comparison of its
abundance pattern with those of models, we conclude that the star was seeded
with material from a single supernova with an original mass of ~60 Mo (and that
the supernova left behind a black hole). Taken together with the previously
mentioned low-metallicity stars, we conclude that low-energy supernovae were
common in the early Universe, and that such supernovae yield light element
enrichment with insignificant iron. Reduced stellar feedback both chemically
and mechanically from low-energy supernovae would have enabled first-generation
stars to form over an extended period. We speculate that such stars may perhaps
have had an important role in the epoch of cosmic reionization and the chemical
evolution of early galaxies.Comment: 28 pages, 6 figures, Natur
Galactic and Extragalactic Samples of Supernova Remnants: How They Are Identified and What They Tell Us
Supernova remnants (SNRs) arise from the interaction between the ejecta of a
supernova (SN) explosion and the surrounding circumstellar and interstellar
medium. Some SNRs, mostly nearby SNRs, can be studied in great detail. However,
to understand SNRs as a whole, large samples of SNRs must be assembled and
studied. Here, we describe the radio, optical, and X-ray techniques which have
been used to identify and characterize almost 300 Galactic SNRs and more than
1200 extragalactic SNRs. We then discuss which types of SNRs are being found
and which are not. We examine the degree to which the luminosity functions,
surface-brightness distributions and multi-wavelength comparisons of the
samples can be interpreted to determine the class properties of SNRs and
describe efforts to establish the type of SN explosion associated with a SNR.
We conclude that in order to better understand the class properties of SNRs, it
is more important to study (and obtain additional data on) the SNRs in galaxies
with extant samples at multiple wavelength bands than it is to obtain samples
of SNRs in other galaxiesComment: Final 2016 draft of a chapter in "Handbook of Supernovae" edited by
Athem W. Alsabti and Paul Murdin. Final version available at
https://doi.org/10.1007/978-3-319-20794-0_90-
Direct Evidence for Termination of Obscured Star Formation by Radiatively Driven Outflows in Reddened QSOs
We present optical to far-infrared photometry of 31 reddened QSOs that show
evidence for radiatively driven outflows originating from AGN in their
rest-frame UV spectra. We use these data to study the relationships between the
AGN-driven outflows, and the AGN and starburst infrared luminosities. We find
that FeLoBAL QSOs are invariably IR-luminous, with IR luminosities exceeding
10^{12} Solar luminosities in all cases. The AGN supplies 76% of the total IR
emission, on average, but with a range from 20% to 100%. We find no evidence
that the absolute luminosity of obscured star formation is affected by the
AGN-driven outflows. Conversely, we find an anticorrelation between the
strength of AGN-driven outflows, as measured from the range of outflow
velocities over which absorption exceeds a minimal threshold, and the
contribution from star formation to the total IR luminosity, with a much higher
chance of seeing a starburst contribution in excess of 25% in systems with weak
outflows than in systems with strong outflows. Moreover, we find no convincing
evidence that this effect is driven by the IR luminosity of the AGN. We
conclude that radiatively driven outflows from AGN can have a dramatic,
negative impact on luminous star formation in their host galaxies. We find that
such outflows act to curtail star formation such that star formation
contributes less than ~25% of the total IR luminosity. We also propose that the
degree to which termination of star formation takes place is not deducible from
the IR luminosity of the AGN.Comment: Accepted for publication in Ap
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SUPERNOVA REMNANTS AND THE INTERSTELLAR MEDIUM OF M83: IMAGING AND PHOTOMETRY WITH THE WIDE FIELD CAMERA 3 ON THE HUBBLE SPACE TELESCOPE
We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology, and photometry in continuum-subtracted Hα, [S II], Hβ, [O III], and [O II] filters, we have identified 60 supernova remnant (SNR) candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible, their Hα fluxes are given. Radiative ages and pre-shock densities are derived from those SNRs that have good photometry. The ages lie in the range 2.62 \u3c log (τrad/yr) \u3c 5.0, and the pre-shock densities at the blast wave range over 0.56 \u3c n 0/cm-3 \u3c 1680. Two populations of SNRs have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium (ISM) is pressurized by the hot X-ray plasma. We also find that the ISM in the nuclear region of M83 is characterized by a very high porosity and pressure, and infer an SNR rate of 1 per 70-150 yr for the nuclear (R \u3c 300 pc) region. On the basis of the number of SNRs detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is M min = 16+7 –5 M . Finally, we give evidence for the likely detection of the remnant of the historical supernova, SN1968L
Near-identical star formation rate densities from Hα and FUV at redshift zero
For the first time both H and far-ultraviolet (FUV) observations from an HI-selected sample are used to determine the dust-corrected star formation rate density (SFRD: ) in the local Universe. Applying the two star formation rate indicators on 294 local galaxies we determine log( [M yr Mpc and log() [M yr Mpc. These values are derived from scaling H and FUV observations to the HI mass function. Galaxies were selected to uniformly sample the full HI mass (M) range of the HI Parkes All-Sky Survey (M to M). The approach leads to relatively larger sampling of dwarf galaxies compared to optically-selected surveys. The low HI mass, low luminosity and low surface brightness galaxy populations have, on average, lower H/FUV flux ratios than the remaining galaxy populations, consistent with the earlier results of Meurer. The near-identical H- and FUV-derived SFRD values arise with the low H/FUV flux ratios of some galaxies being offset by enhanced H from the brightest and high mass galaxy populations. Our findings confirm the necessity to fully sample the HI mass range for a complete census of local star formation to include lower stellar mass galaxies which dominate the local Universe.Partial funding for the SINGG and SUNGG surveys came from NASA grants NAG5-13083 (LTSA program), GALEX GI04- 0105-0009 (NASA GALEX Guest Investigator grant) and NNX09AF85G (GALEX archival grant) to G.R. Meurer. FAR acknowledges partial funding from the Department of Physics, University of Western Australia. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration
Biases in abundance derivations for metal-rich nebulae
Using ab-initio photoionization models of giant HII regions, we test methods
for abundance determinations based on a direct measurement of the electron
temperature, now possible even for moderate to high-metallicity objects. We
find that, for metallicities larger than solar, the computed abundances deviate
systematically from the real ones, generally by larger amounts for more
metal-rich HII regions. We discuss the reasons for this, and present diagrams
allowing the reader to better understand the various factors coming into play.
We briefly discuss less classical methods to derive abundances in metal-rich
HII regions. In particular, we comment on the interest of the oxygen and carbon
recombination lines. We also show that, contrary to the case of giant HII
regions, the physical conditions in bright extragalactic planetary nebulae are
such that their chemical composition can be accurately derived even at high
metallicities. Thus, extragalactic planetary nebulae are promising potential
probes of the metallicity of the interstellar medium in the internal parts of
spiral galaxies as well as in metal-rich elliptical galaxies.Comment: 15 pages, 13 figures, Accepted for publication in A&
The origin of dust in galaxies revisited: the mechanism determining dust content
The origin of cosmic dust is a fundamental issue in planetary science. This
paper revisits the origin of dust in galaxies, in particular, in the Milky Way,
by using a chemical evolution model of a galaxy composed of stars, interstellar
medium, metals (elements heavier than helium), and dust. We start from a review
of time-evolutionary equations of the four components, and then, we present
simple recipes for the stellar remnant mass and yields of metal and dust based
on models of stellar nucleosynthesis and dust formation. After calibrating some
model parameters with the data from the solar neighborhood, we have confirmed a
shortage of the stellar dust production rate relative to the dust destruction
rate by supernovae if the destruction efficiency suggested by theoretical works
is correct. If the dust mass growth by material accretion in molecular clouds
is active, the observed dust amount in the solar neighborhood is reproduced. We
present a clear analytic explanation of the mechanism for determining dust
content in galaxies after the activation of accretion growth: a balance between
accretion growth and supernova destruction. Thus, the dust content is
independent of the uncertainty of the stellar dust yield after the growth
activation. The timing of the activation is determined by a critical metal mass
fraction which depends on the growth and destruction efficiencies. The solar
system formation seems to have occurred well after the activation and plenty of
dust would have existed in the proto-solar nebula.Comment: 12 pages, 11 figure
The origin of kinematically distinct cores and misaligned gas discs in galaxies from cosmological simulations
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [Philip Taylor, Christoph Federrath, and Chiaki Kobayashi, ‘The origin of kinematically distinct cores and misaligned gas discs in galaxies from cosmological simulations’, Monthly Notices of the Royal Astronomical Society, Vol. 479(1): 141-152, Sept 2018] is available online at: https://doi.org/10.1093/mnras/sty1439. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.Integral field spectroscopy surveys provide spatially resolved gas and stellar kinematics of galaxies. They have unveiled a range of atypical kinematic phenomena, which require detailed modelling to understand. We present results from a cosmological simulation that includes stellar and active galactic nucleus (AGN) feedback. We find that the distribution of angles between the gas and stellar angular momenta of galaxies is not affected by projection effects. We examine five galaxies (≈6 per cent of well resolved galaxies) that display atypical kinematics; two of the galaxies have kinematically distinct cores (KDC), while the other three have counter-rotating gas and stars. All five form the majority of their stars in the field, subsequently falling into cosmological filaments where the relative orientation of the stellar angular momentum and the bulk gas flow leads to the formation of a counter-rotating gas disc. The accreted gas exchanges angular momentum with pre-existing co-rotating gas causing it to fall to the centre of the galaxy. This triggers low-level AGN feedback, which reduces star formation. Later, two of the galaxies experience a minor merger (stellar mass ratio ~1/10) with a galaxy on a retrograde orbit compared to the spin of the stellar component of the primary. This produces the KDCs, and is a different mechanism than suggested by other works. The role of minor mergers in the kinematic evolution of galaxies may have been underappreciated in the past, and large, high-resolution cosmological simulations will be necessary to gain a better understanding in this area.Peer reviewe
Supernova remnants: the X-ray perspective
Supernova remnants are beautiful astronomical objects that are also of high
scientific interest, because they provide insights into supernova explosion
mechanisms, and because they are the likely sources of Galactic cosmic rays.
X-ray observations are an important means to study these objects.And in
particular the advances made in X-ray imaging spectroscopy over the last two
decades has greatly increased our knowledge about supernova remnants. It has
made it possible to map the products of fresh nucleosynthesis, and resulted in
the identification of regions near shock fronts that emit X-ray synchrotron
radiation.
In this text all the relevant aspects of X-ray emission from supernova
remnants are reviewed and put into the context of supernova explosion
properties and the physics and evolution of supernova remnants. The first half
of this review has a more tutorial style and discusses the basics of supernova
remnant physics and thermal and non-thermal X-ray emission. The second half
offers a review of the recent advances.The topics addressed there are core
collapse and thermonuclear supernova remnants, SN 1987A, mature supernova
remnants, mixed-morphology remnants, including a discussion of the recent
finding of overionization in some of them, and finally X-ray synchrotron
radiation and its consequences for particle acceleration and magnetic fields.Comment: Published in Astronomy and Astrophysics Reviews. This version has 2
column-layout. 78 pages, 42 figures. This replaced version has some minor
language edits and several references have been correcte
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