88 research outputs found
A Rare Encounter with Very Massive Stars in NGC 3125-A1
Super star cluster A1 in the nearby starburst galaxy NGC 3125 is
characterized by broad He\ii \lam1640 emission (full width at half maximum,
km s) of unprecedented strength (equivalent width,
\AA). Previous attempts to characterize the massive star content
in NGC 3125-A1 were hampered by the low resolution of the UV spectrum and the
lack of co-spatial panchromatic data. We obtained far-UV to near-IR
spectroscopy of the two principal emitting regions in the galaxy with the Space
Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS)
onboard the Hubble Space Telescope (\hst). We use these data to study three
clusters in the galaxy, A1, B1, and B2. We derive cluster ages of 3-4 Myr,
intrinsic reddenings of , 0.15, and 0.13, and cluster masses of
, , and M, respectively.
A1 and B2 show O\vb \lam1371 absorption from massive stars, which is rarely
seen in star-forming galaxies, and have Wolf-Rayet (WR) to O star ratios of
and 0.10, respectively. The high ratio of
A1 cannot be reproduced by models that use a normal IMF and generic WR star
line luminosities. We rule out that the extraordinary He\ii \lam1640 emission
and O\vb \lam1371 absorption of A1 are due to an extremely flat upper IMF
exponent, and suggest that they originate in the winds of very massive
() stars. In order to reproduce the properties of peculiar
clusters such as A1, the present grid of stellar evolution tracks implemented
in Starburst99 needs to be extended to masses .Comment: Accepted for publication in ApJ. 34 pages, 12 figure
Shining A Light On Galactic Outflows: Photo-Ionized Outflows
We study the ionization structure of galactic outflows in 37 nearby, star
forming galaxies with the Cosmic Origins Spectrograph on the Hubble Space
Telescope. We use the O I, Si II, Si III, and Si IV ultraviolet absorption
lines to characterize the different ionization states of outflowing gas. We
measure the equivalent widths, line widths, and outflow velocities of the four
transitions, and find shallow scaling relations between them and galactic
stellar mass and star formation rate. Regardless of the ionization potential,
lines of similar strength have similar velocities and line widths, indicating
that the four transitions can be modeled as a co-moving phase. The Si
equivalent width ratios (e.g. Si IV/Si II) have low dispersion, and little
variation with stellar mass; while ratios with O I and Si vary by a factor of 2
for a given stellar mass. Photo-ionization models reproduce these equivalent
width ratios, while shock models under predict the relative amount of high
ionization gas. The photo-ionization models constrain the ionization parameter
(U) between -2.25 < log(U) < -1.5, and require that the outflow metallicities
are greater than 0.5 Z. We derive ionization fractions for the
transitions, and show that the range of ionization parameters and stellar
metallicities leads to a factor of 1.15-10 variation in the ionization
fractions. Historically, mass outflow rates are calculated by converting a
column density measurement from a single metal ion into a total Hydrogen column
density using an ionization fraction, thus mass outflow rates are sensitive to
the assumed ionization structure of the outflow.Comment: 30 pages, 17 tables, 14 figures. Accepted for publication in MNRA
Carbon Abundances in Starburst Galaxies of the Local Universe
The cosmological origin of carbon, the fourth most abundant element in the
Universe, is not well known and matter of heavy debate. We investigate the
behavior of C/O to O/H in order to constrain the production mechanism of
carbon. We measured emission-line intensities in a spectral range from 1600 to
10000 \AA\ on Space Telescope Imaging Spectrograph (STIS) long-slit spectra of
18 starburst galaxies in the local Universe. We determined chemical abundances
through traditional nebular analysis and we used a Markov Chain Monte Carlo
(MCMC) method to determine where our carbon and oxygen abundances lie in the
parameter space. We conclude that our C and O abundance measurements are
sensible. We analyzed the behavior of our sample in the [C/O] vs. [O/H] diagram
with respect to other objects such as DLAs, neutral ISM measurements, and disk
and halo stars, finding that each type of object seems to be located in a
specific region of the diagram. Our sample shows a steeper C/O vs. O/H slope
with respect to other samples, suggesting that massive stars contribute more to
the production of C than N at higher metallicities, only for objects where
massive stars are numerous; otherwise intermediate-mass stars dominate the C
and N production.Comment: Accepted for publication in Ap
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
Spectroscopic detections of CIII]1909 at z~6-7: A new probe of early star forming galaxies and cosmic reionisation
Deep spectroscopic observations of z~6.5 galaxies have revealed a marked
decline with increasing redshift in the detectability of Lyman-alpha emission.
While this may offer valuable insight into the end of the reionisation process,
it presents a fundamental challenge to the detailed spectroscopic study of the
many hundreds of photometrically-selected distant sources now being found via
deep HST imaging, and particularly those bright sources viewed through
foreground lensing clusters. In this paper we demonstrate the validity of a new
way forward via the convincing detection of an alternative diagnostic line,
CIII]1909, seen in spectroscopic exposures of two star forming galaxies at
z=6.029 and 7.213. The former detection is based on a 3.5 hour X-shooter
spectrum of a bright (J=25.2) gravitationally-lensed galaxy behind the cluster
Abell 383. The latter detection is based on a 4.2 hour MOSFIRE spectra of one
of the most distant spectroscopically confirmed galaxies, GN-108036, with
J=25.2. Both targets were chosen for their continuum brightness and
previously-known redshift (based on Lyman-alpha), ensuring that any CIII]
emission would be located in a favorable portion of the near-infrared sky
spectrum. We compare our CIII] and Lyman-alpha equivalent widths in the context
of those found at z~2 from earlier work and discuss the motivation for using
lines other than Lyman-alpha to study galaxies in the reionisation era.Comment: 10 pages, 6 figures, submitted to MNRA
Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST
Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below . Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
() and present uniformly large specific star
formation rates (sSFR ). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III] equivalent widths comparable to those
seen in systems at . Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
() presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the -ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.Comment: 27 pages, 13 figures, 11 tables, accepted for publication in MNRA
Scaling Relations Between Warm Galactic Outflows and Their Host Galaxies
We report on a sample of 51 nearby, star-forming galaxies observed with the
Cosmic Origin Spectrograph on the Hubble Space Telescope. We calculate Si II
kinematics and densities arising from warm gas entrained in galactic outflows.
We use multi-wavelength ancillary data to estimate stellar masses (M),
star-formation rates (SFR), and morphologies. We derive significant
correlations between outflow velocity and SFR, M
and v. Some mergers drive outflows faster than these
relations prescribe, launching the outflow faster than the escape velocity.
Calculations of the mass outflow rate reveal strong scaling with SFR and M. Additionally, mass-loading efficiency factors
(mass outflow rate divided by SFR) scale approximately as M. Both
the outflow velocity and mass-loading scaling suggest that these outflows are
powered by supernovae, with only 0.7% of the total supernovae energy converted
into the kinetic energy of the warm outflow. Galaxies lose some gas if
log(M/M) < , while more massive galaxies retain all of
their gas, unless they undergo a merger. This threshold for gas loss can
explain the observed shape of the mass-metallicity relation.Comment: 28 pages, 15 figures, submitted to Ap
Ultraviolet Emission Lines in Young Low Mass Galaxies at z~2: Physical Properties and Implications for Studies at z>7
We present deep spectroscopy of 17 very low mass (M* ~ 2.0x10^6 Msun to
1.4x10^9 Msun) and low luminosity (M_UV ~ -13.7 to -19.9) gravitationally
lensed galaxies in the redshift range z~1.5-3.0. Deep rest-frame ultraviolet
spectra reveal large equivalent width emission from numerous lines (NIV],
OIII], CIV, Si III], CIII]) which are rarely seen in individual spectra of more
massive star forming galaxies. CIII] is detected in 16 of 17 low mass star
forming systems with rest-frame equivalent widths as large as 13.5 Angstroms.
Nebular CIV emission is present in the most extreme CIII] emitters, requiring
an ionizing source capable of producing a substantial component of photons with
energies in excess of 47.9 eV. Photoionization models support a picture whereby
the large equivalent widths are driven by the increased electron temperature
and enhanced ionizing output arising from metal poor gas and stars, young
stellar populations, and large ionization parameters. The young ages implied by
the emission lines and continuum SEDs indicate that the extreme line emitters
in our sample are in the midst of a significant upturn in their star formation
activity. The low stellar masses, blue UV colors, and large sSFRs of our sample
are similar to those of typical z>6 galaxies. Given the strong attenuation of
Ly-alpha in z>6 galaxies we suggest that CIII] is likely to provide our best
probe of early star forming galaxies with ground-based spectrographs and one of
the most efficient means of confirming z>10 galaxies with the James Webb Space
Telescope.Comment: 22 pages, 8 figures, accepted for publication in MNRA
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