643 research outputs found
A Far-Ultraviolet View of Starburst Galaxies
Recent observational and theoretical results on starburst galaxies related to
the wavelength regime below 1200 A are discussed. The review covers stars,
dust, as well as hot and cold gas. This wavelength region follows trends
similar to those seen at longer wavelengths, with several notable exceptions.
Even the youngest stellar populations show a turn-over in their spectral energy
distributions, and line-blanketing is much more pronounced. Furthermore, the O
VI line allows one to probe gas at higher temperatures than possible with lines
at longer wavelengths. Molecular hydrogen lines (if detected) provide a glimpse
of the cold phase. I cover the crucial wavelength regime below 912 A and the
implications of recent attempts to detect the escaping ionizing radiation.Comment: 8 pages, 3 figures, Invited Talk, Starbursts--From 30 Doradus to
Lyman-Break Galaxies, ed. R. de Grijs & R. M. Gonzalez Delgado (Dordrecht:
Kluwer
Ly profile, dust, and prediction of Ly escape fraction in Green Pea Galaxies
We studied Lyman- (Ly) escape in a statistical sample of 43
Green Peas with HST/COS Ly spectra. Green Peas are nearby star-forming
galaxies with strong [OIII]5007 emission lines. Our sample is four
times larger than the previous sample and covers a much more complete range of
Green Pea properties. We found that about 2/3 of Green Peas are strong
Ly line emitters with rest-frame Ly equivalent width \AA.
The Ly profiles of Green Peas are diverse. The Ly escape
fraction, defined as the ratio of observed Ly flux to intrinsic
Ly flux, shows anti-correlations with a few Ly kinematic
features -- both the blue peak and red peak velocities, the peak separations,
and FWHM of the red portion of the Ly profile. Using properties
measured from SDSS optical spectra, we found many correlations -- Ly
escape fraction generally increases at lower dust reddening, lower metallicity,
lower stellar mass, and higher [OIII]/[OII] ratio. We fit their Ly
profiles with the HI shell radiative transfer model and found Ly escape
fraction anti-correlates with the best-fit . Finally, we fit an
empirical linear relation to predict Ly escape fraction from the dust
extinction and Ly red peak velocity. The standard deviation of this
relation is about 0.3 dex. This relation can be used to isolate the effect of
IGM scatterings from Ly escape and to probe the IGM optical depth along
the line of sight of each Ly emission line galaxy in the JWST
era.Comment: 15 pages, 11 figures, 3 tables, machine-readable tables included. ApJ
in-pres
The Star Formation History of NGC 1705: a Post-Starburst Galaxy on the Verge of Activity
We infer the star formation history in different regions of the blue compact
dwarf NGC 1705 by comparing synthetic color-magnitude diagrams with HST optical
and near-infrared photometry. We find that NGC 1705 is not a young galaxy
because its star formation commenced at least 5 Gyr ago. On the other hand, we
confirm the existence of a recent burst of star formation between 15 and 10 Myr
ago. We also find evidence for new strong activity, which started 3 Myr ago and
is still continuing. The old population is spread across the entire galaxy,
while the young and intermediate stars are more concentrated in the central
regions. We derive an almost continuous star formation with variable rate, and
exclude the presence of long quiescent phases between the episodes during the
last ~1 Gyr. The central regions experienced an episode of star formation of
\~0.07 Msun/yr (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago.
This coincides with the strong activity in the central super star cluster. We
find a rate of ~0.3 Msun/yr for the youngest ongoing burst which started ~3 Myr
ago. This is higher than in other dwarfs and comparable to the rate of NGC
1569. The star formation rate of earlier episodes is not especially high and
falls in the range 10^{-3}-10^{-1} Msun/yr. The IMF is close to the Salpeter
value or slightly steeper.Comment: 34 pages, including 6 tables and 14 .ps figures (9 in colour), AJ in
pres
On the Escape of Ionizing Radiation from Starbursts
Far-ultraviolet spectra obtained with show that the strong
1036 interstellar absorption-line is essentially black in five of
the UV-brightest local starburst galaxies. Since the opacity of the neutral ISM
below the Lyman-edge will be significantly larger than in the line, these
data provide strong constraints on the escape of ionizing radiation from these
starbursts. Interpreted as a a uniform absorbing slab, the implied optical
depth at the Lyman edge is huge (). Alternatively, the areal
covering factor of opaque material is typically 94%. Thus, the fraction
of ionizing stellar photons that escape the ISM of each galaxy is small: our
conservative estimates typically yield . Inclusion of
extinction due to dust will further decrease . An analogous analysis
of the rest-UV spectrum of the star-forming galaxy at =2.7
leads to similar constraints on . These new results agree with the
constraints provided by direct observations below the Lyman edge in a few other
local starbursts. However, they differ from the recently reported properties of
star-forming galaxies at 3. We assess the idea that the strong
galactic winds seen in many powerful starbursts clear channels through their
neutral ISM. We show empirically that such outflows may be a necessary - but
not sufficient - part of the process for creating a relatively porous ISM. We
note that observations will soon document the cosmic evolution in the
contribution of star-forming galaxies to the metagalactic ionizing background,
with important implications for the evolution of the IGM.Comment: 17 pages; ApJ, in pres
The Nuclear and Circum-nuclear Stellar Population in Seyfert 2 Galaxies: Implications for the Starburst-AGN Connection
We report the results of a spectroscopic investigation of a sample of 20 of
the brightest type 2 Seyfert nuclei. Our goal is to search for the direct
spectroscopic signature of massive stars, and thereby probe the role of
circumnuclear starbursts in the Seyfert phenomenon. The method used is based on
the detection of the higher order Balmer lines and HeI lines in absorption and
the Wolf-Rayet feature at 4680 \AA in emission. These lines are strong
indicators of the presence of young (a few Myrs) and intermediate-age (a few
100 Myrs) stellar populations. In over half the sample, we have detected HeI
and/or strong stellar absorption features in the high-order (near-UV) Balmer
series together with relatively weak lines from an old stellar population. In
three others we detect a broad emission feature near 4680 \AA that is most
plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants
of the most massive stars). We therefore conclude that the blue and near-UV
light of over half of the sample is dominated by young and/or intermediate age
stars. The ``young'' Seyfert 2's have have larger far-IR luminosities, cooler
mid/far-IR colors, and smaller [OIII]/H flux ratios than the ``old''
ones. These differences are consistent with a starburst playing a significant
energetic role in the former class. We consider the possibility that there may
be two distinct sub-classes of Seyfert 2 nuclei (``starbursts'' and ``hidden
BLR''). However, the fact that hidden BLRs have been found in three of the
``young'' nuclei argues against this, and suggests that nuclear starbursts may
be a more general part of the Seyfert phenomenon.Comment: To be published in ApJ, 546, Jan 10, 200
Probing Star Formation at Low Metallicity: The Radio Emission of Super Star Clusters in SBS0335-052
We present high-resolution radio continuum observations of the nascent
starburst in the metal-poor galaxy SBS 0335-052. These radio data were taken
with the Very Large Array and include observations at 0.7cm, 1.3cm, 2cm, 3.6cm,
and 6cm. These observations enable us to probe the thermal radio nebulae
associated with the extremely young star-forming regions in this galaxy. Two
discrete and luminous star-forming regions are detected in the south of the
galaxy that appear to be associated with massive star clusters previously
identified at optical wavelengths. However, the remaining optically-identified
massive star clusters are not clearly associated with radio emission (either
thermal or non-thermal) down to the sensitivity limits of these radio data. The
spectral energy distributions of the two radio-detected clusters are consistent
with being purely thermal, and the entire region has an inferred ionizing flux
of ~1.2 x 10^ 53 s^-1, which is equivalent to ~12,000 "typical" O-type stars
(type O7.5 V). The observations presented here have resolved out a significant
contribution from diffuse non-thermal emission detected previously, implying a
previous episode of significant star formation. The current star formation rate
(SFR) for this southern region alone is ~1.3 M_sun yr^-1, or ~ 23M_sun yr^-1
kpc^-2. This SFR derived from thermal radio emission also suggests that
previous optical recombination line studies are not detecting a significant
fraction of the current star formation in SBS 0335-052. From model fits to the
radio spectral energy distribution, we infer a global mean density in the two
youngest clusters of n_e > 10^3-10^4 cm^-3. In addition, a comparison between
the compact and diffuse radio emission indicates that up to ~50% of the
ionizing flux could be leaking out of the compact HII regions.Comment: accepted AJ, 14 pages, 5 figure
The BHK Color Diagram: a New Tool to Study Young Stellar Populations
A new method to derive age differences between the various super star
clusters observed in starburst galaxies using the two color diagram (B-H) vs
(H-K) is presented. This method offers a quick and easy way to differentiate
very young and intermediate age stellar populations even if data on extinction
are unavailable. In this case, discrimination of regions younger and older than
4 Myr is feasible. With the availability of data on extinction, the time
resolution can be improved significantly. The application of the method to the
starbursting system Arp 299 is presented. The validity of the method is
confirmed by comparing the equivalent width of the H-alpha line with the
chronological map of the northern part of NGC 3690.Comment: 32 pages, 7 figures, 1 table, AJ accepte
The Complex Star Formation History of NGC 1569
We present new results on the star formation history of the dwarf irregular
galaxy NGC 1569. The data were obtained with Hubble Space Telescope's
NICMOS/NIC2 in the F110W (J) and F160W (H) near-infrared (NIR) filters and
interpreted with the synthetic color-magnitude diagram method. The galaxy
experienced a complex star formation (SF) activity. The best fit to the data is
found by assuming three episodes of activity in the last 1-2 Gyr. The most
recent and strong episode constrained by these NIR data started ~37 Myr ago and
ended ~13 Myr ago, although we cannot exclude the possibility that up to three
SF episodes occurred in this time interval. The average star-formation rate
(SFR) of the episode is 3.2 Msun yr-1 kpc-2, in agreement with literature data.
A previous episode produced stars between 150 Myr and 40 Myr ago, with a mean
SFR about 2/3 lower than the mean SFR of the youngest episode. An older SF
episode occurred about 1 Gyr ago. All these SFRs are 2-3 orders of magnitude
higher than those derived for late-type dwarfs of the Local Group. In all cases
an initial mass function similar to Salpeter's allows for a good reproduction
of the data, but we cannot exclude flatter mass functions. These results have
been obtained adopting a distance of 2.2 Mpc and a reddening E(B-V)=0.56. A
larger distance would require younger episodes and higher SFRs. We have
explored some possible scenarios using the astrated mass in the best fit model,
in order to constrain the past star formation history. We cannot rule out a low
rate in the past SF but we can safely conclude that the last 1-2 Gyr have been
peculiar.Comment: Accepted by AJ, 2005, vol 129, 14 pages, 9 figure
Polarimetric Evidence of Non-Spherical Winds
Polarization observations yield otherwise unobtainable information about the
geometrical structure of unresolved objects. In this talk we review the
evidences for non-spherically symmetric structures around Luminous Hot Stars
from polarimetry and what we can learn with this technique. Polarimetry has
added a new dimension to the study of the envelopes of Luminous Blue Variables,
Wolf-Rayet stars and B[e] stars, all of which are discussed in some detail.Comment: 8 pages, 2 encapsulated Postscript figures, uses lamuphys.sty.
Invited review to appear in IAU Coll. 169, Variable and Non-Spherical Stellar
Winds in Luminous Hot Stars, eds. B. Wolf, A.Fullerton and O. Stahl
(Springer
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