102 research outputs found
Wolf-Rayet Stars in Starburst Galaxies
Wolf-Rayet stars have been detected in a large number of galaxies
experiencing intense bursts of star formation. All stars initially more massive
than a certain, metallicity-dependent, value are believed to experience the
Wolf-Rayet phase at the end of their evolution, just before collapsing in
supernova explosion. The detection of Wolf-Rayet stars puts therefore important
constraints on the evolutionary status of starbursts, the properties of their
Initial Mass Functions and their star formation regime. In this contribution we
review the properties of galaxies hosting Wolf-Rayet stars, with special
emphasis on the factors that determine their presence and evolution, as well as
their impact on the surrounding medium.Comment: Contribution to the Proceedings of the JENAM 99 conference "The
interplay between massive stars and the ISM", held in Toulouse in September
7-11, 1999. 10 pages, 5 figures. Requires elsart.cls latex macr
Detection of a Super Star Cluster as the Ionizing Source in the Low Luminosity AGN NGC 4303
HST UV STIS imaging and spectroscopy of the low luminosity AGN (LLAGN) NGC
4303 have identified the previously detected UV-bright nucleus of this galaxy,
as a compact, massive and luminous stellar cluster. The cluster with a size
(FWHM) of 3.1 pc, and an ultraviolet luminosity log L (1500 A}(erg/s/A)= 38.33
is identified as a nuclear super star cluster (SSC) like those detected in the
circumnuclear regions of spirals and starburst galaxies. The UV spectrum
showing the characteristic broad P Cygni lines produced by the winds of massive
young stars, is best fitted by the spectral energy distribution of a massive
cluster of 1e5 Msol generated in an instantaneous burst 4 Myr ago. No evidence
for an additional non-thermal ionizing source associated with an accreting
black hole is detected in the ultraviolet. We hypothesize that at least some
LLAGNs in spirals could be understood as the result of the combined ionizing
radiation emitted by an evolving SSC and a black hole (BH) accreting with low
radiative efficiency, coexisting in the inner few parsecs region.Comment: 4 figure
On the influence of physical galaxy properties on Lyman-alpha escape in star-forming galaxies
[abridged] Among the different observational techniques used to select
high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Lya) is of
particular interest as it gives access to the measurement of cosmological
quantities such as the star formation rate of distant galaxy populations.
However, the interpretation of this line and the calibration of such
observables is still subject to serious uncertainties. Therefore, it important
to understand under what conditions the Lya line can be used as a reliable star
formation diagnostic tool. We use a sample of 24 Lya emitters at z ~ 0.3 with
an optical spectroscopic follow-up to calculate the Lya escape fraction and its
dependency upon different physical properties. We also examine the reliability
of Lya as a star formation rate indicator. We combine these observations with a
compilation of Lya emitters selected at z = 0 - 0.3 to assemble a larger
sample. The Lya escape fraction depends clearly on the dust extinction
following the relation fesc(Lya) = C(Lya) x 10^(-0.4 E(B-V) k(Lya)), but with a
shallower slope than previously reported, with k(Lya) ~ 6.67 and C(Lya) = 0.22.
However, the correlation does not follow the expected curve for a simple dust
attenuation. We explore the various mechanisms than lead to fesc(Lya) values
above the continuum extinction curve, i.e. to an enhancement of the Lya output.
We also observe that the strength of Lya and the escape fraction appear
unrelated to the galaxy metallicity. Regarding the reliability of Lya as a star
formation rate (SFR) indicator, we show that the deviation of SFR(Lya) from the
true SFR (as traced by the UV continuum) is a function of the observed SFR(UV),
which can be seen as the decrease of fesc(Lya) with increasing UV luminosity.
Moreover, we observe a redshift-dependence of this relationship revealing the
underlying evolution of fesc(Lya) with redshift.Comment: 17 pages. Accepted for publicatio
Empirical Estimate of Lyman-alpha Escape Fraction in a Statistical Sample of Lyman-alpha Emitters
The Lyman-alpha (Lya) recombination line is a fundamental tool for galaxy
evolution studies and modern observational cosmology. However, subsequent
interpretations are still prone to a number of uncertainties. Besides numerical
efforts, empirical data are urgently needed for a better understanding of Lya
escape process. We empirically estimate the Lyman-alpha escape fraction
fesc(Lya) in a statistically significant sample of z ~ 0 - 0.3 galaxies in
order to calibrate high-redshift Lyman-alpha observations. An optical
spectroscopic follow-up of a sub-sample of 24 Lyman-alpha emitters (LAEs)
detected by GALEX at z ~ 0.2-0.3, combined with a UV-optical sample of local
starbursts, both with matched apertures, allow us to quantify the dust
extinction through Balmer lines, and to estimate the Lyman-alpha escape
fraction from the Halpha flux corrected for extinction in the framework of the
recombination theory. The global escape fraction of Lyman-alpha radiation spans
nearly the entire range of values, from 0.5 to 100 %, and fesc(Lya) clearly
decreases with increasing nebular dust extinction E(B-V). Several objects show
fesc(Lya) greater than fesc(continuum) which may be an observational evidence
for clumpy ISM geometry or for an aspherical ISM. Selection biases and aperture
size effects may still prevail between z ~ 0.2-0.3 LAEs and local starbursts,
which may explain the difference observed for fesc(Lya).Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysic
The escape of Lyman photons from a young starburst: the case of Haro 11
(Abridged) Lyman-alpha (Lya) is a dominant probe of the galaxy population at
high-z. However, interpretation of data drawn from Lya alone hinges on the Lya
escape fraction which, due to the complex radiative transport, may vary
greatly. Here we map the Lya emission from local starburst Haro 11, a Lya
emitter and the only known candidate for low-z Lyman continuum emission (LyC).
To aid in the interpretation we perform a detailed multi-wavelength analysis
and model the stellar population, dust distribution, ionising photon budget,
and star-cluster population. We use archival X-ray observations to further
constrain properties of the starburst and estimate the HI column density.
The Lya morphology is found to be strongly decoupled from stellar and nebular
(H-alpha) morphologies. General surface photometry finds only very slight
correlation between Lya and H-halpha, E(B-V), and stellar age. Only around the
central Lya-bright cluster do we find the Lya/Ha ratio at values predicted by
recombination theory. The total Lya escape fraction is found to be just 3%. We
compute that ~90% of the Lya photons that escape do so after undergoing
multiple resonance scattering events, masking their point of origin. This leads
to a largely symmetric distribution and, by increasing the distance that
photons must travel to escape, decreases the escape probability significantly.
While dust must ultimately be responsible for the destruction of Lya, it plays
little role in governing the observed morphology, which is regulated more by
ISM kinematics and geometry. We find tentative evidence for local Lya
equivalent width in the immediate vicinity of star-clusters being a function of
cluster age, consistent with hydrodynamic studies. We estimate the ionising
photon production and further constrain the escape fraction at 900 AA to <~9% .Comment: In press for MNRAS. 18 pages, 9 figures. Version with full resolution
images to be found at
http://www.astro.su.se/~matthew/english/papers/hayes_lya_haro11_hires.pd
On the contribution of ULXs to stellar feedback: an intermediate mass black hole candidate and the population of ULXs in the low-metallicity starburst galaxy ESO 338-4
X-ray radiation from accreting compact objects is an important part of
stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous
starburst during the last 40 Myr and contains some of the most massive super
star clusters in the nearby Universe. Given its starburst age and its
star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures
of neutron stars and black holes, hence providing an excellent laboratory for
feedback studies. We compared X-ray images and spectra obtained by XMM-Newton
and Chandra telescopes with integral field spectroscopic VLT MUSE observations
in the optical to constrain the nature of strong X-ray emitters. X-ray
observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The
brightest among them, ESO 338~X-1, has X-ray luminosity in excess of 10^{40}
erg/s. We speculate that ESO 338-4 is powered by accretion on an
intermediate-mass (~300Msun) black hole. We show that X-ray radiation from ULXs
and hot superbubbles strongly contributes to HeII ionization and general
stellar feedback in this template starburst galaxy.Comment: A&A, in pres
ORIGIN: metal creation and evolution from the cosmic dawn
Herder, Jan-Willem den et al.ORIGIN is a proposal for the M3 mission call of ESA aimed at the study of metal creation from the epoch of cosmic dawn. Using high-spectral resolution in the soft X-ray band, ORIGIN will be able to identify the physical conditions of all abundant elements between C and Ni to red-shifts of z = 10, and beyond. The mission will answer questions such as: When were the first metals created? How does the cosmic metal content evolve? Where do most of the metals reside in the Universe? What is the role of metals in structure formation and evolution? To reach out to the early Universe ORIGIN will use Gamma-Ray Bursts (GRBs) to study their local environments in their host galaxies. This requires the capability to slew the satellite in less than a minute to the GRB location. By studying the chemical composition and properties of clusters of galaxies we can extend the range of exploration to lower redshifts (z-0.2). For this task we need a high-resolution spectral imaging instrument with a large field of view. Using the same instrument, we can also study the so far only partially detected baryons in the Warm-Hot Intergalactic Medium (WHIM). The less dense part of the WHIM will be studied using absorption lines at low redshift in the spectra for GRBs. The ORIGIN mission includes a Transient Event Detector (coded mask with a sensitivity of 0.4 photon/cm2/s in 10 s in the 5–150 keV band) to identify and localize 2000 GRBs over a five year mission, of which -65 GRBs have a redshift > 7. The Cryogenic Imaging Spectrometer, with a spectral resolution of 2.5 eV, a field of view of 30 arcmin and large effective area below 1 keV has the sensitivity to study clusters up to a significant fraction of the virial radius and to map the denser parts of the WHIM (factor 30 higher than achievable with current instruments). The payload is complemented by a Burst InfraRed Telescope to enable onboard red-shift determination of GRBs (hence securing proper follow up of high-z bursts) and also probes the mildly ionized state of the gas. Fast repointing is achieved by a dedicated ControlledMomentum Gyro and a low background is achieved by the selected low Earth orbit.The team likes to express its appreciation for the support of Astrium UK for the present study.Peer Reviewe
Enfermedad inflamatoria intestinal: depresión y estrategias de afrontamiento
Los objetivos de esta investigación fueron evaluar las estrategias de afrontamiento
del estrés que utilizan los enfermos de Crohn y Colitis Ulcerosa una vez diagnosticada
las enfermedad y relacionar estas estrategias con el estado de ánimo (depresivo - no
depresivo), con el fin de determinar qué estrategias de afrontamiento son las que más
están influyendo en el estado de ánimo del enfermo. La muestra estuvo formada por
40 pacientes, a los que se les administró un cuestionario demográfico, el Inventario
de Depresión de Beck y el Cuestionario de Estrategias de Afrontamiento de Lazarus y
Folkman. Los datos de este estudio reflejan que un alto porcentaje de enfermos de Crohn
y Colitis Ulcerosa (45%) se encuentra deprimido, y que las diferencias en cuanto al sexo
y la situación laboral son estadísticamente significativas, siendo las mujeres y aquellos
pacientes que se encuentran desempleados, jubilados o que se dedican a sus labores los
que mayor puntuación en depresión obtienen. Asimismo, la estrategia de afrontamiento
que mejor predice un estado de ánimo depresivo es la de huida.The objectives of this research were to evaluate the stress coping strategies that
Crohn and Ulcerative Colitis patients use once the illness is diagnosed and to relate
these strategies with the mood (depressive - not depressive), with the aim of determining
which coping strategies are influencing more in the patient mood. The sample was formed
by 40 patients. They completed a demographic questionnaire, the Beck Depression
Inventory and the Coping Strategies Questionnaire by Lazarus and Folkman. The data of this study reflect that a high percentage of Crohn and Ulcerative Colitis patients (45%)
are depressed, and differences relating the sex and employed situation are statistically
significant, being women and unemployed, retired or housewife patients who got a
higher score in depression. Likewise, the coping strategy that better predicts the mood
depression state is the avoidance
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