2,215 research outputs found
The Role of Starbursts in the Formation of Galaxies & Active Galactic Nuclei
Starbursts are episodes of intense star-formation in the central regions of
galaxies, and are the sites of roughly 25% of the high-mass star-formation in
the local universe. In this contribution I review the role starbursts play in
the formation and evolution of galaxies, the intergalactic medium, and active
galactic nuclei. Four major conclusions are drawn. 1) Starburst galaxies are
good analogues (in fact, the only plausible local analogues) to the known
population of star-forming galaxies at high-redshift. 2) Integrated over cosmic
time, supernova-driven galactic-winds (`superwinds') play an essential role in
the evolution of galaxies and the inter-galactic medium. 3) Circumnuclear
starbursts are an energetically-significant component of the Seyfert
phenomenon. 4) The evolution of the population of the host galaxies of
radio-quiet quasars is significantly different than that of powerful radio
galaxies, and is at least qualitatively consistent with the standard picture of
the hierarchical assembly of massive galaxies at relatively late times.Comment: 16 pages, 4 figures, Royal Society discussion meeting `The formation
of galaxies
The detection of tethered and rising bubbles using multiple acoustic techniques
There exists a range of acoustic techniques for characterizing bubble populations within liquids. Each technique has limitations, and complete characterization of a population requires the sequential or simultaneous use of several, so that the limitations of each find compensation in the others. Here, nine techniques are deployed using one experimental rig, and compared to determine how accurately and rapidly they can characterize given bubble populations. These are, specifically (i) two stationary bubbles attached to a wire; and (ii) injected, rising bubble
Circumnuclear stellar population, morphology and environment of Seyfert 2 galaxies: an evolutionary scenario
We investigate the relation between the characteristics of the circumnuclear
stellar population and both the galaxy morphology and the presence of close
companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present
unambiguous signatures of recent episodes of star formation within 300
pc from the nucleus. When we relate this property with the Hubble type of the
host galaxy, we find that the incidence of recent circumnuclear star formation
increases along the Hubble sequence, and seems to be larger than in non-Seyfert
galaxies for the early Hubble types S0 and Sa, but similar to that in
non-Seyfert galaxies for later Hubble types. Both in early-type and late-type
Seyferts, the presence of recent star-formation is related to the galaxy
morphology in the inner few kiloparsecs, as observed in HST images through the
filter F606W by Malkan et al., who has assigned a late ``inner Hubble type'' to
most Seyfert 2s with recent nuclear star-formation. This new classification is
due to the presence of dust lanes and spiral structures in the inner region.
The presence of recent star formation in Seyfert 2 nuclei is also related to
interactions: among the 13 galaxies of the sample with close companions or in
mergers, 9 have recent star formation in the nucleus. These correlations
between the presence of companions, inner morphology and the incidence of
recent star formation suggest an evolutionary scenario in which the interaction
is responsible for sending gas inwards which both feeds the AGN and triggers
star-formation. The starburst then fades with time and the composite Seyfert 2
+ Starburst nucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellar
population.Comment: 12 pages, 6 figures, 2 table
Five Planets Transiting a Ninth Magnitude Star
The Kepler mission has revealed a great diversity of planetary systems and
architectures, but most of the planets discovered by Kepler orbit faint stars.
Using new data from the K2 mission, we present the discovery of a five planet
system transiting a bright (V = 8.9, K = 7.7) star called HIP 41378. HIP 41378
is a slightly metal-poor late F-type star with moderate rotation (v sin(i) = 7
km/s) and lies at a distance of 116 +/- 18 from Earth. We find that HIP 41378
hosts two sub-Neptune sized planets orbiting 3.5% outside a 2:1 period
commensurability in 15.6 and 31.7 day orbits. In addition, we detect three
planets which each transit once during the 75 days spanned by K2 observations.
One planet is Neptune sized in a likely ~160 day orbit, one is sub-Saturn sized
likely in a ~130 day orbit, and one is a Jupiter sized planet in a likely ~1
year orbit. We show that these estimates for the orbital periods can be made
more precise by taking into account dynamical stability considerations. We also
calculate the distribution of stellar reflex velocities expected for this
system, and show that it provides a good target for future radial velocity
observations. If a precise orbital period can be determined for the outer
Jovian planet through future observations, it will be an excellent candidate
for follow-up transit observations to study its atmosphere and measure its
oblateness.Comment: Accepted by ApJL. 12 pages, 6 figures, 2 table
Stellar Population in LLAGN.I: Ground-based observations
(Abridge): We present a spectroscopic study of the stellar populations of Low
Luminosity AGN (LLAGN). Our main goal is to determine whether the stars who
live in the innermost (100 pc-scale) regions of these galaxies are in some way
related to the emission line properties, which would imply a link between the
stellar population and the ionization mechanism. High signal to noise, ground
based long-slit spectra in the 3500--5500 A interval were collected for 60
galaxies.Our main findings are: (1) Few LLAGN have a detectable young (< 10
Myr) starburst component, indicating that very massive stars do not contribute
significantly to the optical continuum. In particular, no features due to
Wolf-Rayet stars were convincingly detected. (2) High Order Balmer absorption
lines of HI (HOBLs), on the other hand, are detected in 40% of LLAGN.
These features, which are strongest in 100 Myr--1 Gyr intermediate age
stellar populations, are accompanied by diluted metal absorption lines and
bluer colors than other objects in the sample. (3) These intermediate age
populations are very common (50 %) in LLAGN with relatively weak [OI] emission,
but rare (10 %) in LLAGN with stronger [OI]. This is intriguing since LLAGN
with weak [OI] have been previously hypothesized to be ``transition objects''
in which both an AGN and young stars contribute to the emission-line
excitation. Massive stars, if present, are completely outshone by intermediate
age and old stars in the optical. This happens in at least a couple of objects
where independent UV spectroscopy detects young starbursts not seen in the
optical. (4) Objects with predominantly old stars span the whole range of
[OI]/Halpha values, but (5) sources with significant young and/or intermediate
age populations are nearly all (90%) weak [OI] emitters.Comment: To be published in ApJ (April issue). Paper with full resolution
images is at: http://www.iaa.csic.es/~rosa/preprints/preprints.html
http://www.journals.uchicago.edu/ApJ/journa
Late-onset renal vein thrombosis: A case report and review of the literature
AbstractINTRODUCTIONRenal vein thrombosis, a rare complication of renal transplantation, often causes graft loss. Diagnosis includes ultrasound with Doppler, and it is often treated with anticoagulation or mechanical thrombectomy. Success is improved with early diagnosis and institution of treatment.PRESENTATION OF CASEWe report here the case of a 29 year-old female with sudden development of very late-onset renal vein thrombosis after simultaneous kidney pancreas transplant. This resolved initially with thrombectomy, stenting and anticoagulation, but thrombosis recurred, necessitating operative intervention. Intraoperatively the renal vein was discovered to be compressed by a large ovarian cyst.DISCUSSIONCompression of the renal vein by a lymphocele or hematoma is a known cause of thrombosis, but this is the first documented case of compression and thrombosis due to an ovarian cyst.CONCLUSIONEarly detection and treatment of renal vein thrombosis is paramount to restoring renal allograft function. Any woman of childbearing age may have thrombosis due to compression by an ovarian cyst, and screening for this possibility may improve long-term graft function in this population
XMM-Newton Observations of a Complete Sample of Optically Selected Type 2 Seyfert Galaxies
(abridged)The majority of Active Galactic Nuclei (AGN) suffer from
significant obscuration by surrounding dust and gas. X-ray surveys in the 2-10
keV band will miss the most heavily-obscured AGN in which the absorbing column
density exceeds cm (the Compton-thick AGN). It is therefore
vital to know the fraction of AGN that are missed in such X-rays surveys and to
determine if these AGN represent some distinct population in terms of the
fundamental properties of AGN and/or their host galaxies. In this paper we
present the analysis of \textit{XMM-Newton} X-ray data for a complete sample of
17 low-redshift Type 2 Seyfert galaxies chosen from the Sloan Digital Sky
Survey based solely on the high observed flux of the [OIII]5007
emission-line. This line is formed in the Narrow Line Region hundreds of
parsecs away from the central engine. Thus, unlike the X-ray emission, it is
not affected by obscuration due to the torus surrounding the black hole. It
therefore provides a useful isotropic indicator of the AGN luminosity. As
additional indicators of the intrinsic AGN luminosity, we use the Spitzer Space
Telescope to measure the luminosities of the mid-infrared continuum and the
[OIV]25.89m narrow emission-line. We then use the ratio of the 2-10 keV
X-ray luminosity to the [OIII], [OIV], and mid-infrared luminosities to assess
the amount of X-ray obscuration and to distinguish between Compton-thick and
Compton-thin objects. We find that the majority of the sources suffer
significant amounts of obscuration: the observed 2-10 keV emission is depressed
by more than an order-of-magnitude in 11 of the 17 cases (as expected for
Compton-thick sources).Comment: accepted for publication to ApJ; 48 pages, 15 figure
Chronic Arsenic Exposure and Oxidative Stress: OGG1 Expression and Arsenic Exposure, Nail Selenium, and Skin Hyperkeratosis in Inner Mongolia
Arsenic, a human carcinogen, is known to induce oxidative damage to DNA. In this study we investigated oxidative stress and As exposure by determining gene expression of OGG1, which codes for an enzyme, 8-oxoguanine DNA glycosylase, involved in removing 8-oxoguanine in As-exposed individuals. Bayingnormen (Ba Men) residents in Inner Mongolia are chronically exposed to As via drinking water. Water, toenail, and blood samples were collected from 299 Ba Men residents exposed to 0.34â826 ÎŒg/L As. RNA was isolated from blood, and mRNA levels of OGG1 were determined using real-time polymerase chain reaction. OGG1 expression levels were linked to As concentrations in drinking water and nails, selenium concentrations in nails, and skin hyperkeratosis. OGG1 expression was strongly associated with water As concentrations (p < 0.0001). Addition of the quadratic term significantly improved the fit compared with the linear model (p = 0.05). The maximal OGG1 response was at the water As concentration of 149 ÎŒg/L. OGG1 expression was also significantly associated with toenail As concentrations (p = 0.015) but inversely associated with nail Se concentrations (p = 0.0095). We found no significant differences in the As-induced OGG1 expression due to sex, smoking, or age even though the oldest group showed the strongest OGG1 response (p = 0.0001). OGG1 expression showed a dose-dependent increased risk of skin hyperkeratosis in males (trend analysis, p = 0.02), but the trend was not statistically significant in females. The results from this study provide a linkage between oxidative stress and As exposure in humans. OGG1 expression may be useful as a biomarker for assessing oxidative stress from As exposure
The Magnetic Field of the Irregular Galaxy NGC 4214
We examine the magnetic field in NGC 4214, a nearby irregular galaxy, using
multi-wavelength radio continuum polarization data from the Very Large Array.
We find that the global radio continuum spectrum shows signs that free-free
absorption and/or synchrotron losses may be important. The 3cm radio continuum
morphology is similar to that of the Halpha, while the 20cm emission is more
diffuse. We estimate that 50% of the radio continuum emission in the center of
the galaxy is thermal. Our estimate of the magnetic field strength is \uG\ in the center and \uG\ at the edges. We find that the hot
gas, magnetic, and the gravitational pressures are all the same order of
magnitude. Inside the central star forming regions, we find that the thermal
and turbulent pressures of the HII regions dominate the pressure balance. We do
not detect any significant polarization on size scales greater than 200 pc. We
place an upper limit of 8 \uG\ on the uniform field strength in this galaxy. We
suggest that the diffuse synchrotron region, seen to the north of the main body
of emission at 20cm, is elongated due to a uniform magnetic field with a
maximum field strength of 7.6 \uG. We find that, while the shear in NGC 4214 is
comparable to that of the Milky Way, the supernova rate is half that of the
Milky Way and suggest that the star formation episode in NGC 4214 needs
additional time to build up enough turbulence to drive an
dynamo.Comment: Accepted by ApJ. Version with high resolution figures at
http://www.astro.virginia.edu/~aak8t/data/n4214/ms.pd
Stellar Population in LLAGN.II: STIS observations
We present a study of the stellar population in Low Luminosity AGN (LLAGN).
Our goal is to search for spectroscopic signatures of young and intermediate
age stars, and to investigate their relationship with the ionization mechanism
in LLAGN. The method used is based on the stellar population synthesis of the
optical continuum of the innermost (20-100 pc) regions in these galaxies. For
this purpose, we have collected high spatial resolution optical (2900-5700 A)
STIS spectra of 28 nearby LLAGN that are available in the Hubble Space
Telescope archive. The analysis of these data is compared with a similar
analysis also presented here for 51 ground-based spectra of LLAGN. Our main
findings are: (1) No features due to Wolf-Rayet stars were convincingly
detected in the STIS spectra. (2) Young stars contribute very little to the
optical continuum in the ground-based aperture. However, the fraction of light
provided by these stars is higher than 10% in most of the weak-[OI] LLAGN STIS
spectra. (3) Intermediate age stars contribute significantly to the optical
continuum of these nuclei. This population is more frequent in objects with
weak than with strong [OI]. Weak-[OI] LLAGN that have young stars stand out for
their intermediate age population. (4) Most of the strong-[OI] LLAGN have
predominantly old stellar population. A few of these objects also show a
feature-less continuum that contributes significantly to the optical continuum.
These results suggest that young and intermediate age stars do not play a
significant role in the ionization of LLAGN with strong [OI]. However, the
ionization in weak-[OI] LLAGN with young and/or intermediate age population
could be due to stellar processes.Comment: To be published in ApJ (April issue). Paper with full resolution
images is at: http://www.iaa.csic.es/~rosa/preprints/preprints.html
http://www.journals.uchicago.edu/ApJ/journa
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