Ultraviolet (UV; rest-frame $\sim1200-2000$ A) spectra provide a wealth of
diagnostics to characterize fundamental galaxy properties, such as their
chemical enrichment, the nature of their stellar ...populations, and their amount
of Lyman-continuum (LyC) radiation. In this work, we leverage publicly released
JWST data to construct the rest-frame UV-to-optical composite spectrum of a
sample of 63 galaxies at $5.6<z<9$, spanning the wavelength range from 1500 to
5200 A. Based on the composite spectrum, we derive an average dust attenuation
$E(B-V)_\mathrm{gas}=0.16^{+0.10}_{-0.11}$ from \hb/\hg, electron density $n_e
= 570^{+510}_{-290}$ cm$^{-3}$ from the O II doublet ratio, electron
temperature $T_e = 17000^{+1500}_{-1500}$ K from the O III $\lambda4363$/ O
III $\lambda5007$ ratio, and an ionization parameter
$\log(U)=-2.18^{+0.03}_{-0.03}$ from the O III/O II ratio. Using a direct
$T_e$ method, we calculate an oxygen abundance
$12+\log\mathrm{(O/H)}=7.67\pm0.08$ and the carbon-to-oxygen (C/O) abundance
ratio $\log\mathrm{(C/O)}=-0.87^{+0.13}_{-0.10}$. This C/O ratio is smaller
than compared to $z=0$ and $z=2$ - 4 star-forming galaxies, albeit with
moderate significance. This indicates the reionization-era galaxies might be
undergoing a rapid build-up of stellar mass with high specific star-formation
rates. A UV diagnostic based on the ratios of C III
$\lambda\lambda1907,1909$/He II $\lambda1640$ versus O III $\lambda1666$/He II
$\lambda1640$ suggests that the star formation is the dominant source of
ionization, similar to the local extreme dwarf galaxies and $z\sim2$ - 4 He
II-detected galaxies. The O III/O II and C IV/C III ratios of the
composite spectrum are marginally larger than the criteria used to select
galaxies as LyC leakers, suggesting that some of the galaxies in our sample are
strong contributors to the reionizing radiation.
The Mg II 2796, 2803 doublet has been suggested to be a useful indirect indicator for the escape of Ly-alpha and Lyman continuum (LyC) photons in local star-forming galaxies. However, studies to date ...have focused on small samples of galaxies with strong Mg II or strong LyC emission. Here we present the first study of Mg II probing a large dynamic range of galaxy properties, using newly obtained high signal-to-noise, moderate-resolution spectra of Mg II for a sample of 34 galaxies selected from the Low-redshift Lyman Continuum Survey. We show that the galaxies in our sample have Mg II profiles ranging from strong emission to P-Cygni profiles, and to pure absorption. We find there is a significant trend (with a possibility of spurious correlations of ~ 2%) that galaxies detected as strong LyC Emitters (LCEs) also show larger equivalent widths of Mg II emission, and non-LCEs tend to show evidence of more scattering and absorption features in Mg II We then find Mg II strongly correlates with Ly-alpha in both equivalent width and escape fraction, regardless of whether the emission or absorption dominates the Mg II profiles. Furthermore, we present that, for galaxies categorized as Mg II emitters (MgE), one can adopt the information of Mg II, metallicity, and dust to estimate the escape fraction of LyC within a factor of 3. These findings confirm that Mg II lines can be used as a tool to select galaxies as LCEs and to serve as an indirect indicator for the escape of Ly-alpha and LyC.
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, $z>6$).
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 $\lambda\lambda$1907,9 He II $\lambda1640$ and O III
$\lambda$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 $\lambda$1666 still allow us to define a SF-locus with some caveats.
Diagrams including C IV $\lambda\lambda$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) $\lesssim8.3$) and high ionization
parameter (log($U$) $\gtrsim-2.5$) environments. Overall, CLASSY and our UV
toolkit will be crucial in interpreting the spectra of the earliest galaxies
that JWST is currently revealing.
We present a sample of 1165 extreme emission-line galaxies (EELGs) at 4<z<9 selected using James Webb Space Telescope (JWST) NIRCam photometry in the Cosmic Evolution Early Release Science (CEERS) ...program. We use a simple method to photometrically identify EELGs with Hb + OIII (combined) or Ha emission of observed-frame equivalent width EW >5000 AA. JWST/NIRSpec spectroscopic observations of a subset (34) of the photometrically selected EELGs validate our selection method: all spectroscopically observed EELGs confirm our photometric identification of extreme emission, including some cases where the SED-derived photometric redshifts are incorrect. We find that the medium-band F410M filter in CEERS is particularly efficient at identifying EELGs, both in terms of including emission lines in the filter and in correctly identifying the continuum between Hb + OIII and Ha in the neighboring broad-band filters. We present examples of EELGs that could be incorrectly classified at ultra-high redshift (z>12) as a result of extreme Hb + OIII emission blended across the reddest photometric filters. We compare the EELGs to the broader (sub-extreme) galaxy population in the same redshift range and find that they are consistent with being the bluer, high equivalent width tail of a broader population of emission-line galaxies. The highest-EW EELGs tend to have more compact emission-line sizes than continuum sizes, suggesting that active galactic nuclei are responsible for at least some of the most extreme EELGs. Photometrically inferred emission-line ratios are consistent with ISM conditions with high ionization and moderately low metallicity, consistent with previous spectroscopic studies.
During the first 500 million years of cosmic history, the first stars and galaxies formed, seeding the Universe with heavy elements and eventually reionizing the intergalactic medium. Observations ...with JWST have uncovered a surprisingly high abundance of candidates for early star-forming galaxies, with distances (redshifts, \(z\)), estimated from multi-band photometry, as large as \(z\approx 16\), far beyond pre-JWST limits. While generally robust, such photometric redshifts can suffer from degeneracies and occasionally catastrophic errors. Spectroscopic measurement is required to validate these sources and to reliably quantify physical properties that can constrain galaxy formation models and cosmology. Here we present JWST spectroscopy that confirms redshifts for two very luminous galaxies with \(z > 11\), but also demonstrates that another candidate with suggested \(z\approx 16\) instead has \(z = 4.9\), with an unusual combination of nebular line emission and dust reddening that mimics the colors expected for much more distant objects. These results reinforce evidence for the early, rapid formation of remarkably luminous galaxies, while also highlighting the necessity of spectroscopic verification. The large abundance of bright, early galaxies may indicate shortcomings in current galaxy formation models, or deviation from physical properties (such as the stellar initial mass function) that are generally believed to hold at later times.
We report the results of analyses of galactic outflows in a sample of 45 low-redshift starburst galaxies in the COS Legacy Archive Spectroscopic SurveY (CLASSY), augmented by five additional similar ...starbursts with COS data. The outflows are traced by blueshifted absorption-lines of metals spanning a wide range of ionization potential. The high quality and broad spectral coverage of CLASSY data enable us to disentangle the absorption due to the static ISM from that due to outflows. We further use different line multiplets and doublets to determine the covering fraction, column density, and ionization state as a function of velocity for each outflow. We measure the outflow's mean velocity and velocity width, and find that both correlate in a highly significant way with the star-formation rate, galaxy mass, and circular velocity over ranges of four orders-of-magnitude for the first two properties. We also estimate outflow rates of metals, mass, momentum, and kinetic energy. We find that, at most, only about 20% of silicon created and ejected by supernovae in the starburst is carried in the warm phase we observe. The outflows' mass-loading factor increases steeply and inversely with both circular and outflow velocity (log-log slope \(\sim\) -1.6), and reaches \(\sim 10\) for dwarf galaxies. We find that the outflows typically carry about 10 to 100% of the momentum injected by massive stars and about 1 to 20% of the kinetic energy. We show that these results place interesting constraints on, and new insights into, models and simulations of galactic winds.
We report the discovery of an accreting supermassive black hole at z=8.679, in CEERS_1019, a galaxy previously discovered via a Ly\(\alpha\)-break by Hubble and with a Ly\(\alpha\) redshift from ...Keck. As part of the Cosmic Evolution Early Release Science (CEERS) survey, we observed this source with JWST/NIRSpec spectroscopy, MIRI and NIRCam imaging, and NIRCam/WFSS slitless spectroscopy. The NIRSpec spectra uncover many emission lines, and the strong O III emission line confirms the ground-based Ly\(\alpha\) redshift. We detect a significant broad (FWHM~1200 km/s) component in the H\(\beta\) emission line, which we conclude originates in the broad-line region of an active galactic nucleus (AGN), as the lack of a broad component in the forbidden lines rejects an outflow origin. This hypothesis is supported by the presence of high-ionization lines, as well as a spatial point-source component embedded within a smoother surface brightness profile. The mass of the black hole is log(\(M_{BH}/M_{\odot})=6.95{\pm}0.37\), and we estimate that it is accreting at 1.2 (\(\pm\)0.5) x the Eddington limit. The 1-8 \(\mu\)m photometric spectral energy distribution (SED) from NIRCam and MIRI shows a continuum dominated by starlight and constrains the host galaxy to be massive (log M/M\(_{\odot}\)~9.5) and highly star-forming (SFR~30 M\(_{\odot}\) yr\(^{-1}\)). Ratios of the strong emission lines show that the gas in this galaxy is metal-poor (Z/Z\(_{\odot}\)~0.1), dense (n\(_{e}\)~10\(^{3}\) cm\(^{-3}\)), and highly ionized (log U~-2.1), consistent with the general galaxy population observed with JWST at high redshifts. We use this presently highest-redshift AGN discovery to place constraints on black hole seeding models and find that a combination of either super-Eddington accretion from stellar seeds or Eddington accretion from massive black hole seeds is required to form this object by the observed epoch.
A&A 663, A59 (2022) Combining 66 ultraviolet (UV) spectra and ancillary data from the
Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier
studies, we form a statistical ...sample of star-forming galaxies at $z \sim 0.3$
to study the role of the cold interstellar medium (ISM) gas in the leakage of
ionizing radiation. We first constrain the massive star content (ages and
metallicities) and UV attenuation, by fitting the stellar continuum with a
combination of simple stellar population models. The models, together with
accurate LyC flux measurements, allow to determine the absolute LyC photon
escape fraction for each galaxy ($f_{\rm esc}^{\rm abs}$). We measure the
equivalent widths and residual fluxes of multiple HI and low-ionization state
(LIS) lines, and the geometrical covering fraction adopting the picket-fence
model. The $f_{\rm esc}^{\rm abs}$ spans a wide range, with a median (0.16,
0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in
the LyC. The HI and LIS line equivalent widths scale with the UV luminosity and
attenuation, and inversely with the residual flux of the lines. The HI and LIS
residual fluxes are correlated, indicating that the neutral gas is spatially
traced by the LIS transitions. We find the observed trends of the absorption
lines and the UV attenuation are primarily driven by the covering fraction. The
non-uniform gas coverage demonstrates that LyC photons escape through
low-column density channels in the ISM. The equivalent widths and residual
fluxes of the UV lines strongly correlate with $f_{\rm esc}^{\rm abs}$: strong
LyC leakers show weak absorption lines, low UV attenuation, and large
Ly$\alpha$ equivalent widths. We finally show that simultaneous UV absorption
line and dust attenuation measurements can predict, on average, the escape
fraction of galaxies and the method can be applied to galaxies across a wide
redshift range.
Strong nebular emission lines are an important diagnostic tool for tracing the evolution of star-forming galaxies across cosmic time. However, different observational setups can affect these lines, ...and the derivation of the physical nebular properties. We analyze 12 local star-forming galaxies from the COS Legacy Spectroscopy SurveY (CLASSY) to assess the impact of using different aperture combinations on the determination of the physical conditions and gas-phase metallicity. We compare optical spectra observed with the SDSS aperture, which has a 3" of diameter similar to COS, to IFU and longslit spectra, including new LBT/MODS observations of five CLASSY galaxies. We calculate the reddening, electron densities and temperatures, metallicities, star formation rates, and equivalent widths (EWs). We find that measurements of the electron densities and temperatures, and metallicity remained roughly constant with aperture size, indicating that the gas conditions are relatively uniform for this sample. However, using the IFU observations of 3 galaxies, we find that the E(B-V) values derived from the Balmer ratios decrease ( by up to 53%) with increasing aperture size. The values change most significantly in the center of the galaxies, and level out near the COS aperture diameter of 2.5". We examine the relative contributions from the gas and stars using the H\(\alpha\) and OIII \(\lambda\)5007 EWs as a function of aperture light fraction, but find little to no variations within a given galaxy. These results imply that the optical spectra provide nebular properties appropriate for the FUV CLASSY spectra, even when narrow 1.0" long-slit observations are used.
Star-forming galaxies are considered the likeliest source of the H I ionizing Lyman Continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z >~ 6, the ...neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the Epoch of Reionization. Emission from Mg II \ly\ly 2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for 8 LyC emitter candidates, selected to have strong Mg II emission lines. We securely detect LyC emission in 50% (4/8) galaxies with 2\(\sigma\) significance. This high detection rate suggests that strong Mg II emitters might be more likely to leak LyC than similar galaxies without strong Mg II. Using photoionization models, we constrain the escape fraction of Mg II as ~ 15 -- 60%. We confirm that the escape fraction of Mg II correlates tightly with that of Lyman-alpha (LyA), which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg II emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg II emission can be adopted to estimate the escape fraction of LyA and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the Epoch of Reionization.
