196 research outputs found
Is the interstellar gas of starburst galaxies well mixed?
The extent to which the ISM in galaxies is well mixed is not yet settled.
Measured metal abundances in the diffuse neutral gas of star--forming gas--rich
dwarf galaxies are deficient with respect to that of the ionized gas. The
reasons, if real, are not clear and need to be based on firm grounds. Far-UV
spectroscopy of giant HII regions such as NGC604 in the spiral galaxy M33 using
FUSE allows us to investigate possible systematic errors in the metallicity
derivation. We still find underabundances of nitrogen, oxygen, argon, and iron
in the neutral phase by a factor of~6. This could either be explained by the
presence of less chemically evolved gas pockets in the sightlines or by dense
clouds out of which HIIregions form. Those could be more metallic than the
diffuse medium.Comment: 4 pages, 2 figures;contribution to Starbursts: from 30 Dor to Lyman
Break Galaxies, 6 -10 September 2004, Institute of Astronomy, University of
Cambridge, U
Spatially resolved integral field spectroscopy of the ionized gas in IZw18
We present a detailed 2D study of the ionized ISM of IZw18 using new PMAS-IFU
optical observations. IZw18 is a high-ionization galaxy which is among the most
metal-poor starbursts in the local Universe. This makes IZw18 a local benchmark
for understanding the properties most closely resembling those prevailing at
distant starbursts. Our IFU-aperture (~ 1.4 kpc x 1.4 kpc) samples the entire
IZw18 main body and an extended region of its ionized gas. Maps of relevant
emission lines and emission line ratios show that higher-excitation gas is
preferentially located close to the NW knot and thereabouts. We detect a
Wolf-Rayet feature near the NW knot. We derive spatially resolved and
integrated physical-chemical properties for the ionized gas in IZw18. We find
no dependence between the metallicity-indicator R23 and the ionization
parameter (as traced by [OIII]/[OII]) across IZw18. Over ~ 0.30 kpc^2, using
the [OIII]4363 line, we compute Te[OIII] values (~ 15000 - 25000 K), and oxygen
abundances are derived from the direct determinations of Te[OIII]. More than
70% of the higher-Te[OIII] (> 22000 K) spaxels are HeII4686-emitting spaxels
too. From a statistical analysis, we study the presence of variations in the
ISM physical-chemical properties. A galaxy-wide homogeneity, across hundreds of
parsecs, is seen in O/H. Based on spaxel-by-spaxel measurements, the
error-weighted mean of 12 + log(O/H) = 7.11 +/- 0.01 is taken as the
representative O/H for IZw18. Aperture effects on the derivation of O/H are
discussed. Using our IFU data we obtain, for the first time, the IZw18
integrated spectrum.Comment: Accepted for publication in MNRAS, 13 pages, 10 figures, 4 table
A Uniform Analysis of the Ly-alpha forest at z = 0 - 5: I. The sample and distribution of clouds at z > 1.7
We present moderate resolution data for 39 QSOs at z 2 obtained at
the Multiple Mirror Telescope. These data are combined with spectra of
comparable resolution of 60 QSOs with redshifts greater than 1.7 found in the
literature to investigate the distribution of Ly-alpha forest lines in redshift
and equivalent width. We find a value for , the parameter describing
the number distribution of Ly-alpha forest lines in redshift, of
for lines stronger than a rest equivalent width of 0.32 , in good
agreement with some previous studies. The Kolmogorov-Smirnov test was applied
to the data and it is found that this single power law is a good fit over the
relevant redshift ranges. Simulations of the Lyman alpha forest were performed
to determine the completeness of the line lists and to test how well the
analysis the underlying line statistics, given this level of completeness.Comment: minor corrections to text, 37 Latex pages, 11 encapsulated Postscript
figures, uses emulateapj.sty, To appear in the Sept. 2000 ApJS, line lists
and spectra available at http://qso.as.arizona.edu/~jscott/Spectra/index.htm
Kinematics of Interstellar Gas in Nearby UV-Selected Galaxies Measured with HST/STIS Spectroscopy
We measure Doppler shifts of interstellar absorption lines in HST/STIS
spectra of individual star clusters in nearby UV-selected galaxies. Values for
systemic velocities, which are needed to quantify outflow speeds, are taken
from the literature, and verified with stellar lines. We detect outflowing gas
in eight of 17 galaxies via low-ionization lines (e.g., CII, SiII, AlII), which
trace cold and/or warm gas. The starbursts in our sample are intermediate in
luminosity (and mass) to dwarf galaxies and luminous infrared galaxies (LIRGs),
and we confirm that their outflow speeds (ranging from -100 km/s to nearly -520
km/s with an accuracy of ~80 km/s) are intermediate to those previously
measured in dwarf starbursts and LIRGs. We do not detect the outflow in
high-ionization lines (such as CIV or SiIV); higher quality data will be needed
to empirically establish how velocities vary with the ionization state of the
outflow. We do verify that the low-ionization UV lines and optical NaI doublet
give roughly consistent outflow velocities solidifying an important link
between studies of galactic winds at low and high redshift. To obtain higher
signal-to-noise, we create a local average composite spectrum, and compare it
to the high-z Lyman Break composite spectrum. Surprisingly, the low-ionization
lines show similar outflow velocities in the two samples. We attribute this to
a combination of weighting towards higher luminosities in the local composite,
as well as both samples being on average brighter than the ``turnover''
luminosity in the v-SFR relation.Comment: 41 pages, 14 figures, accepted for publication in The Astrophysical
Journa
The nature of the Lyman-alpha emission region of FDF-4691
In order to study the origin of the strong Lyman-alpha emission of
high-redshift starburst galaxies we observed and modeled the emission of the z
= 3.304 galaxy FDF-4691 (rest-frame EW = 103 Angstroem). The observations show
that FDF-4691 is a young starburst galaxy with a (for this redshift) typical
metallicity. The broad, double-peaked profile of the Lyman-alpha emission line
can be explained assuming a highly turbulent emission region in the inner part
of the starburst galaxy, and a surrounding extended shell of low-density
neutral gas with a normal dust/gas ratio and with Galactic dust properties. The
detection of the Lyman-alpha emission line is explained by the intrinsic broad
Lyman-alpha emission and a low HI column density of the neutral shell. A low
dust/gas ratio in the neutral shell is not needed to explain the strong
Lyman-alpha line.Comment: Accepted for publication in A&A Letter
Theoretical Modeling of Starburst Galaxies
We have modeled a large sample of infrared starburst galaxies using both the
PEGASE v2.0 and STARBURST99 codes to generate the spectral energy distribution
of the young star clusters. PEGASE utilizes the Padova group tracks while
STARBURST99 uses the Geneva group tracks, allowing comparison between the two.
We used our MAPPINGS III code to compute photoionization models which include a
self-consistent treatment of dust physics and chemical depletion. We use the
standard optical diagnostic diagrams as indicators of the hardness of the EUV
radiation field in these galaxies. These diagnostic diagrams are most sensitive
to the spectral index of the ionizing radiation field in the 1-4 Rydberg
region. We find that warm infrared starburst galaxies contain a relatively hard
EUV field in this region. The PEGASE ionizing stellar continuum is harder in
the 1-4 Rydberg range than that of STARBURST99. As the spectrum in this regime
is dominated by emission from Wolf-Rayet (W-R) stars, this difference is most
likely due to the differences in stellar atmosphere models used for the W-R
stars. We believe that the stellar atmospheres in STARBURST99 are more
applicable to the starburst galaxies in our sample, however they do not produce
the hard EUV field in the 1-4 Rydberg region required by our observations. The
inclusion of continuum metal blanketing in the models may be one solution.
Supernova remnant (SNR) shock modeling shows that the contribution by
mechanical energy from SNRs to the photoionization models is << 20%. The models
presented here are used to derive a new theoretical classification scheme for
starbursts and AGN galaxies based on the optical diagnostic diagrams.Comment: 36 pages, 16 figures, to be published in ApJ, July 20, 200
The interplay between ionized gas and massive stars in the HII galaxy IIZw70: integral field spectroscopy with PMAS
We performed an integral field spectroscopic study for the HII galaxy IIZw70
in order to investigate the interplay between its ionized interstellar medium
(ISM) and the massive star formation (SF). Observations were taken in the
optical spectral range (3700-6800 A) with the Potsdam Multi-Aperture
Spectrophotometer (PMAS) attached to the 3.5 m telescope at CAHA. We created
and analysed maps of spatially distributed emission-lines, continuum emission
and properties of the ionized ISM (e.g. physical-chemical conditions, dust
extinction, kinematics). We investigated the relation of these properties to
the spatial distribution and evolutionary stage of the massive stars. For the
first time we have detected the presence of Wolf-Rayet (WR) stars in this
galaxy. The peak of the ionized gas emission coincides with the location of the
WR bump. The region of the galaxy with lower dust extinction corresponds to the
region that shows the lowest values of velocity dispersion and radial velocity.
The overall picture suggests that the ISM of this region is being disrupted via
photoionization and stellar winds, leading to a spatial decoupling between
gas+stars and dust clouds. The bulk of dust appears to be located at the
boundaries of the region occupied by the probable ionizing cluster. We also
found that this region is associated to the nebular emission in HeII4686 and to
the intensity maximum of most emission lines. This indicates that the hard
ionizing radiation responsible for the HeII4686 nebular emission can be related
to the youngest stars. Within 0.4 x 0.3 kpc^2 in the central burst, we
derived O/H using direct determinations of Te[OIII]. We found abundances in the
range 12+log(O/H)=7.65-8.05, yielding an error-weighted mean of
12+log(O/H)=7.86 0.05.Comment: 10 pages, 10 figures, accepted for publication in A&A, minor changes
adde
A Stellar Population Gradient in VII Zw 403 - Implications for the Formation of Blue Compact Dwarf Galaxies
We present evidence for the existence of an old stellar halo in the Blue
Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf
galaxy for which a clear spatial segregation of the resolved stellar content
into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations
indicate that active star formation occurs in the central region, but is
strikingly absent at large radii. Instead, a globular-cluster-like red giant
branch suggests the presence of an old (> 10 Gyr) and metal poor
(=-1.92) stellar population in the halo. While the vast majority of
Blue Compact Dwarf galaxies has been recognized to possess halos of red color
in ground-based surface photometry, our observations of VII Zw 403 establish
for the first time a direct correspondence between a red halo color and the
presence of old, red giant stars. If the halos of Blue Compact Dwarf galaxies
are all home to such ancient stellar populations, then the fossil record
conflicts with delayed-formation scenarios for dwarfs.Comment: Accepted for publication in the Ap
Molecular hydrogen, deuterium and metal abundances in the damped Ly-alpha system at z = 3.025 toward QSO 0347-3819
We have detected in high resolution spectra of the quasar Q0347--3819
obtained with the UVES spectrograph at the VLT/Kueyen telescope over 80
absorption features in the Lyman and Werner H2 bands at the redshift of a
damped Ly-alpha system at z = 3.025. The z = 3.025 system spans over 80 km/s
and exhibits a multicomponent velocity structure in the metal lines. The main
component at z = 3.024855 shows a total H2 column density N(H2) =
(4.10\pm0.21)*10^{14} cm^{-2} and a fractional molecular abundance f(H2) =
(1.94\pm0.10)*10^{-6} derived from the H2 lines arising from J=0 to 5
rotational levels of the ground electronic-vibrational state. For the first
time we unambiguously reveal a pronounced [alpha-element/iron-peak] enhancement
of [O,Si/Zn] = 0.6\pm0.1 (6 sigma c.l.) at high redshift. The simultaneous
analysis of metal and hydrogen lines leads to D/H = (3.75\pm0.25)*10^{-5}. This
value is consistent with standard big bang nucleosynthesis if the
baryon-to-photon ratio, eta, lies within the range 4.37*10^{-10} <= eta <=
5.32*10^{-10}, implying 0.016 <= Omega_b h^2_100 <= 0.020.Comment: 32 pages, 16 ps figures, accepted to Ap
I ZW 18 -- a New Wolf-Rayet Galaxy
We report the discovery of broad Wolf-Rayet emission lines in the Multiple
Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the
lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet
(W-R) bumps at the wavelengths 4650 and 5800 are detected
indicating the presence of WN and WC stars. The total numbers of WN and WC
stars inferred from the luminosities of the broad He II 4686 and C IV
5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to
O stars number ratio is equal to about 0.02, in satisfactory agreement with the
value predicted by massive stellar evolution models with enhanced mass loss
rates. The WC stars in the northwest component of I Zw 18 can be responsible
for the presence of the nebular He II 4686 emission line, however the
observed intensity of this line is several times larger than model predictions,
and other sources of ionizing radiation at wavelengths shorter than 228\AA are
necessary.Comment: Ap.J.Lett., in pres
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