33 research outputs found
Photometric identification and MMT spectroscopy of new extremely metal-poor galaxies: towards a better understanding of young stellar populations at low metallicity
Extremely metal-poor star-forming galaxies (XMPs) represent one of our only
laboratories for study of the low-metallicity stars we expect to encounter at
early epochs. But as our understanding of the universe has improved, it
has become clear that the majority of known XMPs within 100 Mpc host
significantly less prominent massive star populations than their
reionization-era counterparts, severely limiting their utility as testbeds for
interpreting spectral features found at the highest redshifts. Here we present
a new photometric selection technique designed to identify nearby XMPs
dominated by young stellar populations comparable to those expected in the
reionization era. We apply our technique to uncover candidate XMPs in SDSS
imaging at magnitudes , extending significantly below the
completeness limits of the SDSS spectroscopic survey. Spectroscopic
observations with the MMT confirm that 32 of the 53 uniformly metal-poor and
high specific star formation rate targets we observed have gas-phase oxygen
abundances (), including two in the
range of the lowest-metallicity galaxies known, . Our
observations shed new light onto the longstanding mystery of He II emission in
star-forming galaxies: we find that the equivalent width of the He II high-ionization emission line does not scale with that of H in our
sample, suggesting that binary evolution or other processes on Myr
timescales contribute substantially to the -ionizing photon
budget in this metallicity regime. Applying such selection techniques coupled
with deep spectroscopy to next-generation photometric surveys like LSST may
eventually provide a basis for an empirical understanding of metal-poor massive
stars.Comment: 16 pages, 10 figures, accepted for publication in 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
A Panchromatic Study of Massive Stars in the Extremely Metal-poor Local Group Dwarf Galaxy Leo A*
We characterize massive stars (M > 8 Mâ) in the nearby (D ⌠0.8 Mpc) extremely metal-poor (Z ⌠5% Zâ) galaxy Leo A using Hubble Space Telescope ultraviolet (UV), optical, and near-infrared (NIR) imaging along with Keck/Low-Resolution Imaging Spectrograph and MMT/Binospec optical spectroscopy for 18 main-sequence OB stars. We find that: (a) 12 of our 18 stars show emission lines, despite not being associated with an H ii region, suggestive of stellar activity (e.g., mass loss, accretion, binary star interaction), which is consistent with previous predictions of enhanced activity at low metallicity; (b) six are Be stars, which are the first to be spectroscopically studied at such low metallicityâthese Be stars have unusual panchromatic SEDs; (c) for stars well fit by the TLUSTY nonlocal thermodynamic equilibrium models, the photometric and spectroscopic values of and agree to within âŒ0.01 dex and âŒ0.18 dex, respectively, indicating that near-UV/optical/NIR imaging can be used to reliably characterize massive (M ⌠8â30 Mâ) main-sequence star properties relative to optical spectroscopy; (d) the properties of the most-massive stars in H II regions are consistent with constraints from previous nebular emission line studies; and (e) 13 stars with M > 8Mâ are >40 pc from a known star cluster or H II region. Our sample comprises âŒ50% of all known massive stars at Z âČ 10% Zâwith derived stellar parameters, high-quality optical spectra, and panchromatic photometry
CLASSY VII Ly\alpha\ Profiles: The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-Era Analogs
Lyman-alpha line profiles are a powerful probe of ISM structure, outflow
speed, and Lyman continuum escape fraction. In this paper, we present the
Ly line profiles of the COS Legacy Archive Spectroscopic SurveY, a
sample rich in spectroscopic analogs of reionization-era galaxies. A large
fraction of the spectra show a complex profile, consisting of a double-peaked
Ly emission profile in the bottom of a damped, Ly absorption
trough. Such profiles reveal an inhomogeneous interstellar medium (ISM). We
successfully fit the damped Ly absorption (DLA) and the Ly
emission profiles separately, but with complementary covering factors, a
surprising result because this approach requires no Ly exchange between
high- and low- paths. The combined distribution
of column densities is qualitatively similar to the bimodal distributions
observed in numerical simulations. We find an inverse relation between
Ly peak separation and the [O III]/[O II] flux ratio, confirming that
the covering fraction of Lyman-continuum-thin sightlines increases as the
Ly peak separation decreases. We combine measurements of Ly
peak separation and Ly red peak asymmetry in a diagnostic diagram which
identifies six Lyman continuum leakers in the CLASSY sample. We find a strong
correlation between the Ly trough velocity and the outflow velocity
measured from interstellar absorption lines. We argue that greater vignetting
of the blueshifted Ly peak, relative to the redshifted peak, is the
source of the well-known discrepancy between shell-model parameters and
directly measured outflow properties. The CLASSY sample illustrates how
scattering of Ly photons outside the spectroscopic aperture reshapes
Ly profiles as the distances to these compact starbursts span a large
range.Comment: 40 pages, 19 figures, 5 tables, submitted to ApJ, comments welcom
CLASSY VIII: Exploring the Source of Ionization with UV ISM diagnostics in local High- Analogs
In the current JWST era, rest-frame UV spectra play a crucial role in
enhancing our understanding of the interstellar medium (ISM) and stellar
properties of the first galaxies in the epoch of reionization (EoR, ).
Here, we compare well-known and reliable optical diagrams sensitive to the main
ionization source (i.e., star formation, SF; active galactic nuclei, AGN;
shocks) to UV counterparts proposed in the literature - the so-called ``UV-BPT
diagrams'' - using the HST COS Legacy Archive Spectroscopic SurveY (CLASSY),
the largest high-quality, high-resolution and broad-wavelength range atlas of
far-UV spectra for 45 local star-forming galaxies. In particular, we explore
where CLASSY UV line ratios are located in the different UV diagnostic plots,
taking into account state-of-the-art photoionization and shock models and, for
the first time, the measured ISM and stellar properties (e.g., gas-phase
metallicity, ionization parameter, carbon abundance, stellar age). We find that
the combination of C III] 1907,9 He II and O III]
1666 can be a powerful tool to separate between SF, shocks and AGN at
sub-solar metallicities. We also confirm that alternative diagrams without O
III] 1666 still allow us to define a SF-locus with some caveats.
Diagrams including C IV 1548,51 should be taken with caution
given the complexity of this doublet profile. Finally, we present a discussion
detailing the ISM conditions required to detect UV emission lines, visible only
in low gas-phase metallicity (12+log(O/H) ) and high ionization
parameter (log() ) environments. Overall, CLASSY and our UV
toolkit will be crucial in interpreting the spectra of the earliest galaxies
that JWST is currently revealing.Comment: 31 pages, submitted to ApJ, comments welcom
The COS Legacy Archive Spectroscopy SurveY (CLASSY) Treasury Atlas
Far-ultraviolet (FUV; ~1200-2000 angstroms) spectra are fundamental to our
understanding of star-forming galaxies, providing a unique window on massive
stellar populations, chemical evolution, feedback processes, and reionization.
The launch of JWST will soon usher in a new era, pushing the UV spectroscopic
frontier to higher redshifts than ever before, however, its success hinges on a
comprehensive understanding of the massive star populations and gas conditions
that power the observed UV spectral features. This requires a level of detail
that is only possible with a combination of ample wavelength coverage,
signal-to-noise, spectral-resolution, and sample diversity that has not yet
been achieved by any FUV spectral database.
We present the COS Legacy Spectroscopic SurveY (CLASSY) treasury and its
first high level science product, the CLASSY atlas. CLASSY builds on the HST
archive to construct the first high-quality (S/N_1500 >~ 5/resel),
high-resolution (R~15,000) FUV spectral database of 45 nearby (0.002 < z <
0.182) star-forming galaxies. The CLASSY atlas, available to the public via the
CLASSY website, is the result of optimally extracting and coadding 170
archival+new spectra from 312 orbits of HST observations.
The CLASSY sample covers a broad range of properties including stellar mass
(6.2 < logM_star(M_sol) < 10.1), star formation rate (-2.0 < log SFR (M_sol/yr)
< +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization
(0.5 < O_32 < 38.0), reddening (0.02 < E(B-V < 0.67), and nebular density (10 <
n_e (cm^-3) < 1120). CLASSY is biased to UV-bright star-forming galaxies,
resulting in a sample that is consistent with z~0 mass-metallicity
relationship, but is offset to higher SFRs by roughly 2 dex, similar to z >~2
galaxies. This unique set of properties makes the CLASSY atlas the benchmark
training set for star-forming galaxies across cosmic time.Comment: Accepted for publication in Ap
JWST reveals a possible galaxy merger in triply-lensed MACS0647JD
MACS0647JD is a triply-lensed galaxy originally discovered with
the Hubble Space Telescope. Here we report new JWST imaging, which clearly
resolves MACS0647JD as having two components that are either merging
galaxies or stellar complexes within a single galaxy. Both are very small, with
stellar masses and radii . The brighter
larger component "A" is intrinsically very blue (), likely due
to very recent star formation and no dust, and is spatially extended with an
effective radius . The smaller component "B" appears redder
(), likely because it is older () with mild dust
extinction (), and a smaller radius . We
identify galaxies with similar colors in a high-redshift simulation, finding
their star formation histories to be out of phase. With an estimated stellar
mass ratio of roughly 2:1 and physical projected separation ,
we may be witnessing a galaxy merger 400 million years after the Big Bang. We
also identify a candidate companion galaxy C away, likely
destined to merge with galaxies A and B. The combined light from galaxies A+B
is magnified by factors of 8, 5, and 2 in three lensed images JD1, 2, and
3 with F356W fluxes , , (AB mag 25.1, 25.6, 26.6).
MACS0647JD is significantly brighter than other galaxies recently discovered
at similar redshifts with JWST. Without magnification, it would have AB mag
27.3 (). With a high confidence level, we obtain a photometric
redshift of based on photometry measured in 6 NIRCam filters
spanning , out to rest-frame. JWST NIRSpec
observations planned for January 2023 will deliver a spectroscopic redshift and
a more detailed study of the physical properties of MACS0647JD.Comment: 27 pages, 14 figures, submitted to Natur
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Low-Metallicity Stars and High-Redshift Galaxies Through the Lens of Local Metal-Poor Star-Forming Regions
Over the last decade, unprecedented imaging campaigns and deep spectroscopy have delivered our first glimpse of galaxies in the reionization era, and the results have stymied interpretation. The prominent nebular emission in CIII], CIV, and other high-ionization lines detected in the first rest-UV spectra at these redshifts suggests that hard ionizing radiation fields are common at z>6, in striking contrast to typical star-forming systems at lower redshift. The difficulty in interpreting this emission is fundamentally tied to the lack of empirical constraints on massive stars at the very low metallicities we expect to encounter in the first billion years of the Universe. Nearby star-forming regions hosting stars and gas extending to metallicities below that of the SMC (<20% solar) represent a critical laboratory for study of such young stellar populations. In this dissertation, I first present HST/COS UV spectroscopy of HeII-emitters which reveal a clear transition from UV spectra dominated by stellar features above 20% solar metallicity to high-ionization nebular line emission at lower metallicities, tracked by a significant hardening in the inferred ionizing spectrum. I demonstrate that nebular HeII and CIV are ubiquitous among local extremely metal-poor galaxies (XMPs, <10% solar), and that CIV in particular may be an effective signpost of rapidly-assembling systems at these low metallicities in the distant Universe. I then present a new technique for locating XMPs dominated by very young stars in broadband photometry, and apply it to SDSS imaging to uncover 32 such systems at typical effective ages of tens of megayears. I demonstrate that the nebular HeII commonly encountered in such systems is inconsistent with an origin in high-mass X-ray binaries or the most massive short-lived stars, and suggest instead that this line may provide insight onto stripped stars and other uncertain products of binary evolution at very low metallicity. I also present evidence from stellar population synthesis modeling that an overabundance of massive stars or spun-up binary products are necessary to explain the strongest stellar wind features in very young systems at higher metallicities where these winds are prominent. Finally, I conclude by outlining a path forward from testing to direct calibration of models for massive stars below the metallicity of the SMC