1,133 research outputs found
Measuring gravitational lens time delays using low-resolution radio monitoring observations
Obtaining lensing time delay measurements requires long-term monitoring
campaigns with a high enough resolution (< 1 arcsec) to separate the multiple
images. In the radio, a limited number of high-resolution interferometer arrays
make these observations difficult to schedule. To overcome this problem, we
propose a technique for measuring gravitational time delays which relies on
monitoring the total flux density with low-resolution but high-sensitivity
radio telescopes to follow the variation of the brighter image. This is then
used to trigger high-resolution observations in optimal numbers which then
reveal the variation in the fainter image. We present simulations to assess the
efficiency of this method together with a pilot project observing radio lens
systems with the Westerbork Synthesis Radio Telescope (WSRT) to trigger Very
Large Array (VLA) observations. This new method is promising for measuring time
delays because it uses relatively small amounts of time on high-resolution
telescopes. This will be important because instruments that have high
sensitivity but limited resolution, together with an optimum usage of followup
high-resolution observations from appropriate radio telescopes may in the
future be useful for gravitational lensing time delay measurements by means of
this new method.Comment: 10 pages, 7 figures, accepted by MNRA
Stellar collisions in accreting protoclusters: a Monte Carlo dynamical study
We explore the behaviour of accreting protoclusters with a Monte Carlo
dynamical code in order to evaluate the relative roles of accretion, two body
relaxation and stellar collisions in the cluster evolution. We corroborate the
suggestion of Clarke & Bonnell that the number of stellar collisions should
scale as (independent of other cluster parameters, where
N is the number of stars in the cluster and the rate of mass
accretion) and thus strengthen the argument that stellar collisions are more
likely in populous (large N) clusters. We however find that the estimates of
Clarke & Bonnell were pessimistic in the sense that we find that more than 99 %
of the stellar collisions occur within the post-adiabatic regime as the cluster
evolves towards core collapse, driven by a combination of accretion and
two-body relaxation. We discuss how the inclusion of binaries may reduce the
number of collisions through the reversal of core collapse but also note that
it opens up another collisional channel involving the merger of stars within
hard binaries; future Nbody simulations are however required in order to
explore this issue.Comment: 9 pages, 9 figures; accepted for publication in MNRAS. This version
contains minor revisions after referee's comments
Dynamic plantwide modeling, uncertainty and sensitivity analysis of a pharmaceutical upstream synthesis: Ibuprofen case study
Gravothermal oscillations in two-component models of star clusters
In this paper, gravothermal oscillations are investigated in two-component
clusters with a range of different stellar mass ratios and total component mass
ratios. The critical number of stars at which gravothermal oscillations first
appeared is found using a gas code. The nature of the oscillations is
investigated and it is shown that the oscillations can be understood by
focusing on the behaviour of the heavier component, because of mass
segregation. It is argued that, during each oscillation, the re-collapse of the
cluster begins at larger radii while the core is still expanding. This
re-collapse can halt and reverse a gravothermally driven expansion. This
material outside the core contracts because it is losing energy both to the
cool expanding core and to the material at larger radii. The core collapse
times for each model are also found and discussed. For an appropriately chosen
case, direct N -body runs were carried out, in order to check the results
obtained from the gas model, including evidence of the gravothermal nature of
the oscillations and the temperature inversion that drives the expansion.Comment: 13 pages, 18 figures and 8 tables. Accepted for publication in MNRA
Cold gas outflows from the Small Magellanic Cloud traced with ASKAP
Feedback from massive stars plays a critical role in the evolution of the
Universe by driving powerful outflows from galaxies that enrich the
intergalactic medium and regulate star formation. An important source of
outflows may be the most numerous galaxies in the Universe: dwarf galaxies.
With small gravitational potential wells, these galaxies easily lose their
star-forming material in the presence of intense stellar feedback. Here, we
show that the nearby dwarf galaxy, the Small Magellanic Cloud (SMC), has atomic
hydrogen outflows extending at least 2 kiloparsecs (kpc) from the star-forming
bar of the galaxy. The outflows are cold, , and may have formed
during a period of active star formation million years (Myr) ago. The
total mass of atomic gas in the outflow is solar masses, , or % of the total atomic gas of the galaxy. The inferred
mass flux in atomic gas alone, , is up to an order of magnitude greater than the star
formation rate. We suggest that most of the observed outflow will be stripped
from the SMC through its interaction with its companion, the Large Magellanic
Cloud (LMC), and the Milky Way, feeding the Magellanic Stream of hydrogen
encircling the Milky Way.Comment: Published in Nature Astronomy, 29 October 2018,
http://dx.doi.org/10.1038/s41550-018-0608-
On the mass-radius relation of hot stellar systems
Most globular clusters have half-mass radii of a few pc with no apparent
correlation with their masses. This is different from elliptical galaxies, for
which the Faber-Jackson relation suggests a strong positive correlation between
mass and radius. Objects that are somewhat in between globular clusters and
low-mass galaxies, such as ultra-compact dwarf galaxies, have a mass-radius
relation consistent with the extension of the relation for bright ellipticals.
Here we show that at an age of 10 Gyr a break in the mass-radius relation at
~10^6 Msun is established because objects below this mass, i.e. globular
clusters, have undergone expansion driven by stellar evolution and hard
binaries. From numerical simulations we find that the combined energy
production of these two effects in the core comes into balance with the flux of
energy that is conducted across the half-mass radius by relaxation. An
important property of this `balanced' evolution is that the cluster half-mass
radius is independent of its initial value and is a function of the number of
bound stars and the age only. It is therefore not possible to infer the initial
mass-radius relation of globular clusters and we can only conclude that the
present day properties are consistent with the hypothesis that all hot stellar
systems formed with the same mass-radius relation and that globular clusters
have moved away from this relation because of a Hubble time of stellar and
dynamical evolution.Comment: 5 pages, 3 figures, MNRAS Letters (accepted
Gravitational waves from intermediate-mass black holes in young clusters
Massive young clusters (YCs) are expected to host intermediate-mass black
holes (IMBHs) born via runaway collapse. These IMBHs are likely in binaries and
can undergo mergers with other compact objects, such as stellar mass black
holes (BHs) and neutron stars (NSs). We derive the frequency of such mergers
starting from information available in the Local Universe. Mergers of IMBH-NS
and IMBH-BH binaries are sources of gravitational waves (GWs), which might
allow us to reveal the presence of IMBHs. We thus examine their detectability
by current and future GW observatories, both ground- and space-based. In
particular, as representative of different classes of instruments we consider
Initial and Advanced LIGO, the Einstein gravitational-wave Telescope (ET) and
the Laser Interferometer Space Antenna (LISA). We find that IMBH mergers are
unlikely to be detected with instruments operating at the current sensitivity
(Initial LIGO). LISA detections are disfavored by the mass range of IMBH-NS and
IMBH-BH binaries: less than one event per year is expected to be observed by
such instrument. Advanced LIGO is expected to observe a few merger events
involving IMBH binaries in a 1-year long observation. Advanced LIGO is
particularly suited for mergers of relatively light IMBHs (~100 Msun) with
stellar mass BHs. The number of mergers detectable with ET is much larger: tens
(hundreds) of IMBH-NS (IMBH-BH) mergers might be observed per year, according
to the runaway collapse scenario for the formation of IMBHs. We note that our
results are affected by large uncertainties, produced by poor observational
constraints on many of the physical processes involved in this study, such as
the evolution of the YC density with redshift.[abridged]Comment: 29 pages, 4 figures, accepted for publication in Ap
A runaway collision in a young star cluster as the origin of the brightest supernova
Supernova 2006gy in the galaxy NGC 1260 is the most luminous one recorded
\cite{2006CBET..644....1Q, 2006CBET..647....1H, 2006CBET..648....1P,
2006CBET..695....1F}. Its progenitor might have been a very massive (
\msun) star \cite{2006astro.ph.12617S}, but that is incompatible with hydrogen
in the spectrum of the supernova, because stars \msun are believed to
have shed their hydrogen envelopes several hundred thousand years before the
explosion \cite{2005A&A...429..581M}. Alternatively, the progenitor might have
arisen from the merger of two massive stars \cite{2007ApJ...659L..13O}. Here we
show that the collision frequency of massive stars in a dense and young cluster
(of the kind to be expected near the center of a galaxy) is sufficient to
provide a reasonable chance that SN 2006gy resulted from such a bombardment. If
this is the correct explanation, then we predict that when the supernova fades
(in a year or so) a dense cluster of massive stars becomes visible at the site
of the explosion
NGC 326: X-shaped no more
We present new 144-MHz LOFAR observations of the prototypical ‘X-shaped’ radio galaxy NGC 326, which show that the formerly known wings of the radio lobes extend smoothly into a large-scale, complex radio structure. We argue that this structure is most likely the result of hydrodynamical effects in an ongoing group or cluster merger, for which pre-existing X-ray and optical data provide independent evidence. The large-scale radio structure is hard to explain purely in terms of jet reorientation due to the merger of binary black holes, a previously proposed explanation for the inner structure of NGC 326. For this reason, we suggest that the simplest model is one in which the merger-related hydrodynamical processes account for all the source structure, though we do not rule out the possibility that a black hole merger has occurred. Inference of the black hole-black hole merger rate from observations of X-shaped sources should be carried out with caution in the absence of deep, sensitive low-frequency observations. Some X-shaped sources may be signposts of cluster merger activity, and it would be useful to investigate the environments of these objects more generally
LOFAR MSSS: The Scaling Relation between AGN Cavity Power and Radio Luminosity at Low Radio Frequencies
This article has been accepted for publication in a forthcoming issue of Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.We present a new analysis of the widely used relation between cavity power and radio luminosity in clusters of galaxies with evidence for strong AGN feedback. We study the correlation at low radio frequencies using two new surveys - the First Alternative Data Release of the TIFR GMRT Sky Survey (TGSS ADR1) at 148 MHz and LOFAR's first all-sky survey, the Multifrequency Snapshot Sky Survey (MSSS) at 140 MHz. We find a scaling relation , with a logarithmic slope of , which is in good agreement with previous results based on data at 327 MHz. The large scatter present in this correlation confirms the conclusion reached at higher frequencies that the total radio luminosity at a single frequency is a poor predictor of the total jet power. We show that including measurements at 148 MHz alone is insufficient to reliably compute the bolometric radio luminosity and reduce the scatter in the correlation. For a subset of four well-resolved sources, we examine the detected extended structures at low frequencies and compare with the morphology known from higher frequency images and Chandra X-ray maps. In Perseus we discuss details in the structures of the radio mini-halo, while in the 2A 0335+096 cluster we observe new diffuse emission associated with multiple X-ray cavities and likely originating from past activity. For A2199 and MS 0735.6+7421, we confirm that the observed low-frequency radio lobes are confined to the extents known from higher frequencies. This new low-frequency analysis highlights the fact that existing cavity power to radio luminosity relations are based on a relatively narrow range of AGN outburst ages. We discuss how the correlation could be extended using low frequency data from the LOFAR Two-metre Sky Survey (LoTSS) in combination with future, complementary deeper X-ray observations.Peer reviewe
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