Abstract
Spectroscopic studies of extreme-ionization galaxies (EIGs) are critical to our understanding of exotic systems throughout cosmic time. These EIGs exhibit spectral features requiring >54.42 ...eV photons: the energy needed to ionize helium into He
2+
fully and emit He
ii
recombination lines. Spectroscopic studies of EIGs can probe exotic stellar populations or accretion onto intermediate-mass black holes (∼10
2
–10
5
M
⊙
), which are the possibly key contributors to the reionization of the Universe. To facilitate the use of EIGs as probes of high-ionization systems, we focus on ratios constructed from several rest-frame UV/optical emission lines: O
iii
λ
5008, H
β
, Ne
iii
λ
3870, O
ii
λ
λ
3727, 3729, and Ne
v
λ
3427. These lines probe the relative intensity at energies of 35.12, 13.62, 40.96, 13.62, and 97.12 eV, respectively, covering a wider range of ionization than traced by other common rest-frame UV/optical techniques. We use the ratios of these lines (Ne
v
/Ne
iii
≡ Ne53, O
iii
/H
β
, and Ne
iii
/O
ii
), which are nearby in wavelength, mitigating the effects of dust attenuation and uncertainties in flux calibration. We make predictions from photoionization models constructed from
Cloudy
that use a broad range of stellar populations and black hole accretion models to explore the sensitivity of these line ratios to changes in the ionizing spectrum. We compare our models to observations from the Hubble Space Telescope and JWST of galaxies with strong high-ionization emission lines at
z
∼ 0,
z
∼ 2, and 5 <
z
< 8.5. We show that the Ne53 ratio can separate galaxies with ionization from “normal” stellar populations from those with active galactic nuclei and even “exotic” Population III models. We introduce new selection methods to identify galaxies with photoionization driven by Population III stars or intermediate-mass black hole accretion disks that could be identified in upcoming high-redshift spectroscopic surveys.
We have investigated the oxygen and nitrogen chemical abundances in extremely compact star-forming galaxies (SFGs) with redshifts between {approx}0.11 and 0.35, popularly referred to as 'green peas'. ...Direct and strong-line methods sensitive to the N/O ratio applied to their Sloan Digital Sky Survey (SDSS) spectra reveal that these systems are genuine metal-poor galaxies, with mean oxygen abundances {approx}20% solar. At a given metallicity these galaxies display systematically large N/O ratios compared to normal galaxies, which can explain the strong difference between our metallicities measurements and previous ones. While their N/O ratios follow the relation with stellar mass of local SFGs in the SDSS, we find that the mass-metallicity relation of the 'green peas' is offset {approx_gt}0.3 dex to lower metallicities. We argue that recent interaction-induced inflow of gas, possibly coupled with a selective metal-rich gas loss, driven by supernova winds, may explain our findings and the known galaxy properties, namely high specific star formation rates, extreme compactness, and disturbed optical morphologies. The 'green pea' galaxy properties seem to be uncommon in the nearby universe, suggesting a short and extreme stage of their evolution. Therefore, these galaxies may allow us to study in great detail many processes, such as starburst activity and chemical enrichment, under physical conditions approaching those in galaxies at higher redshifts.
Abstract
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 Cosmic Origins Spectrograph (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 interstellar medium (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 out in the warm phase we observe. The outflows’ mass-loading factor increases steeply and inversely with both circular and outflow velocity (log–log slope ∼−1.6), and reaches ∼10 for dwarf galaxies. We find that the outflows typically carry about 10%–100% of the momentum injected by massive stars and about 1%–20% of the kinetic energy. We show that these results place interesting constraints on, and new insights into, models and simulations of galactic winds.
Abstract
Ly
α
line profiles are a powerful probe of interstellar medium (ISM) structure, outflow speed, and Lyman-continuum escape fraction. In this paper, we present the Ly
α
line profiles of the ...Cosmic Origins Spectrograph (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
α
emission profile in the bottom of a damped, Ly
α
absorption trough. Such profiles reveal an inhomogeneous ISM. We successfully fit the damped Ly
α
absorption and the Ly
α
emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Ly
α
exchange between high-
N
H
i
and low-
N
H
i
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
α
peak separation and the O
iii
/O
ii
flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Ly
α
peak separation decreases. We combine measurements of Ly
α
peak separation and Ly
α
red peak asymmetry in a diagnostic diagram, which identifies six Lyman-continuum leakers in the COS Legacy Archive Spectrocopy SurveY (CLASSY) sample. We find a strong correlation between the Ly
α
trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Ly
α
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
α
photons outside the spectroscopic aperture reshapes Ly
α
profiles because the distances to these compact starbursts span a large range.
Abstract
The Mg
ii
λλ
2796, 2803 doublet has been suggested as a useful indirect indicator for the escape of Ly
α
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
to probe 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 to pure absorption. We find that there is a significant trend (with a possibility of spurious correlations of ∼2%) that galaxies that are detected as strong LyC emitters (LCEs) show larger equivalent widths of Mg
ii
emission, while non-LCEs tend to show evidence of more scattering and absorption features in Mg
ii
. We then find that Mg
ii
strongly correlates with Ly
α
in both equivalent width and escape fraction, regardless of whether emission or absorption dominates the Mg
ii
profiles. Furthermore, we show that, for galaxies that are categorized as Mg
ii
emitters, one can use 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 for selecting galaxies as LCEs and thus serve as an indirect indicator for the escape of Ly
α
and LyC.
Abstract 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 ( 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; and shocks) to UV counterparts proposed in the literature—the so-called “UV–BPT diagrams”—using the HST COS Legacy Archive Spectroscopic SurveY (CLASSY), which is 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, and stellar age). We find that the combination of C iii λ λ 1907,9 He ii λ 1640 and O iii λ 1666 can be a powerful tool to separate between SF, shocks, and AGN at subsolar metallicities. We also confirm that alternative diagrams without O iii λ 1666 still allow us to define an SF-locus, with some caveats. Diagrams including C iv λ λ 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) ≲ 8.3) and high ionization parameter (log( U ) ≳ −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.
Abstract
The Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY (CLASSY) provides the first high-resolution spectral catalog of 45 local high-
z
analogs in the ultraviolet (UV; ...1200–2000 Å) to investigate their stellar and gas properties. Here we present a toolkit of UV interstellar medium (ISM) diagnostics, analyzing the main emission lines of CLASSY spectra (N
iv
λ
λ
1483,87, C
iv
λλ
1548,51, He
ii
λ
1640, O
iii
λ
λ
1661,6, Si
iii
λλ
1883,92, C
iii
λ
1907,9). Specifically, our aim is to provide accurate diagnostics for the reddening
E
(
B
−
V
), electron density
n
e
, electron temperature
T
e
, metallicity 12+log(O/H), and ionization parameter log(
U
), taking the different ISM ionization zones into account. We calibrate our UV toolkit using well-known optical diagnostics, analyzing archival optical spectra for all CLASSY targets. We find that UV density diagnostics estimate
n
e
values that are ∼1–2 dex higher (e.g.,
n
e
(C
iii
λ
λ
1907,9) ∼ 10
4
cm
−3
) than those inferred from their optical counterparts (e.g.,
n
e
(S
ii
λ
λ
6717,31) ∼ 10
2
cm
−3
;
n
e
(Ar
iv
λ
λ
4714,41) ∼ 10
3
cm
−3
).
T
e
derived from the hybrid ratio O
iii
λ
1666/
λ
5007 proves to be reliable, implying differences in determining 12+log(O/H) compared to the optical counterpart O
iii
λ
4363/O
iii
λ
5007 within ∼ ±0.3 dex. We also investigate the relation between the stellar and gas
E
(
B
−
V
), finding consistent values at high specific star formation rates (sSFRs;
log
(
sSFR
)
≳
−
8
yr
−1
), while at low sSFRs we confirmed an excess of dust attenuation in the gas. Finally, we investigate UV line ratios and equivalent widths to provide correlations with 12+log(O/H) and log(
U
), but note that there are degeneracies between the two. With this suite of UV-based diagnostics, we illustrate the pivotal role CLASSY plays in understanding the chemical and physical properties of high-z systems that JWST can observe in the rest-frame UV.
Abstract
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 the Mg
ii
λλ
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 eight LyC emitter candidates, selected to have strong Mg
ii
emission lines. We securely detect LyC emission in 50% (4/8) of the galaxies with 2
σ
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 Ly
α
, 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 Ly
α
and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization.
Abstract To study the chemical evolution across cosmic epochs, we investigate Ne, S, Cl, and Ar abundance patterns in the Cosmic Origins Spectrograph Legacy Archive Spectroscopic SurveY (CLASSY). ...CLASSY comprises local star-forming galaxies (SFGs; 0.02 < z < 0.18) with enhanced star formation rates, making them strong analogues to high- z SFGs. With direct measurements of electron temperature, we derive accurate ionic abundances for all elements and assess ionization correction factors (ICFs) to account for unseen ions and derive total abundances. We find Ne/O, S/O, Cl/O, and Ar/O exhibit constant trends with gas-phase metallicity for 12+log(O/H) < 8.5 but significant correlation for Ne/O and Ar/O with metallicity for 12+log(O/H) > 8.5, likely due to ICFs. Thus, the applicability of the ICFs to integrated spectra of galaxies could bias results, underestimating true abundance ratios. Using CLASSY as a local reference, we assess the evolution of Ne/O, S/O, and Ar/O in galaxies at z > 3, finding no cosmic evolution of Ne/O, while the lack of direct abundance determinations for S/O and Ar/O can bias the interpretation of the evolution of these elements. We determine the fundamental metallicity relationship (FMR) for CLASSY and compare to the high-redshift FMR, finding no evolution. Finally, we perform the first mass–neon relationship analysis across cosmic epochs, finding a slight evolution to high Ne at later epochs. The robust abundance patterns of CLASSY galaxies and their broad range of physical properties provide essential benchmarks for interpreting the chemical enrichment of the early galaxies observed with the JWST.
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