arXiv subjects

N. G. Guseva

Publications and source records attributed to N. G. Guseva.

At least 19 recordsLinked to original sources

arXiv2026

Chemical composition and kinematics of ionised gas in low-mass star-forming galaxies with extremely high [OIII]/[OII] ratios

We present Very Large Telescope/Xshooter spectrophotometric observations of eleven low-redshift (z<0.085) compact star-forming galaxies (`high O32 sample'). These galaxies are characterised by extremely high emission-line ratios [OIII]$λ$5007/[OII]3727, ranging from 11 to 42. Galaxies with such high ratios are thought to be promising candidates for leaking large amounts of Lyman continuum radiation. They are characterized by low oxygen abundances 12+log(O/H)\,=7.5-8.0 and low stellar masses M*~10^6-10^8 Msun. Strong emission lines of various ions in all spectra are used to derive helium and oxygen abundances, and N/O, Ne/O, S/O, Cl/O, Ar/O and Fe/O abundance ratios. We also derived macroscopic velocity dispersions sigma(lambda) from various emission lines of different ions. We find that sigma(4861) of the Hbeta emission line is increased with increasing stellar mass and decreasing O32 ratio. On the other hand, sigma(lambda)/sigma(4861) ratios for various lines are close to 1. Exceptions are sigma(lambda)/sigma(4861) of two lines, HeII 4686 and HeI 10830, which are considerably higher than unity and of four lines, [OII] 3726,3729, [SII] 6717,6731, with sigma(lambda)/sigma(4861) lower than unity. The two former lines are likely produced in the inner parts of HII regions and are broadened by dynamical processes generated by massive stars, and by radiative scattering in the case of the HeI 10830 emission line. Emission in the four latter lines is produced mainly in the outer and likely more quiet parts of HII regions.

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arXiv2025

Feedback and dynamical masses in high-$z$ galaxies: the advent of high-resolution NIRSpec spectroscopy

Stellar feedback is an essential step in the baryon cycle of galaxies, but it remains unconstrained beyond Cosmic Noon. We study the ionized gas kinematics, dynamical mass and gas-flow properties of a sample of 16 sub-$L^{\star}$ star-forming galaxies at $4\leq z\leq7.6$, using high-resolution JWST/NIRSpec observations. The emission lines are resolved, with velocity dispersions ($σ_{\rm gas}{\rm~(km~s^{-1})}\simeq38-96$) comparable to more massive galaxies at Cosmic Noon. From $σ_{\rm gas}$ and the galaxy size ($r_e=400-960~$pc), we estimate the dynamical mass to be $\log M_{\rm dyn}/M_{\odot}=9.25-10.25$. Stellar-to-dynamical mass ratios are low ($\log M_{\star}/M_{\rm dyn}\in[-0.5,-2]$) and decrease with increasing SFR surface density ($Σ_{\rm SFR}$). We estimate the gas surface densities assuming a star-formation law, but the gas masses do not balance the baryon-to-dynamical mass ratios, which would require a decrease in the star-formation efficiency. We find evidence of ionized outflows in five out of the sixteen galaxies, based on the need of broad components to reproduce the emission-line wings. We only observe outflows from galaxies undergoing recent bursts of star formation ${\rm SFR_{10}/SFR_{100}\geq1}$, with elevated $Σ_{\rm SFR}$ and low $M_{\star}/M_{\rm dyn}$. This links high gas surface densities to increased outflow incidence and lower $M_{\star}/M_{\rm dyn}$. With moderate outflow velocities ($v_{\rm flow}{\rm~(km~s^{-1})}=150-250$) and mass outflow rates ($\dot{M}_{\rm flow}/{\rm M_{\odot} yr^{-1}}=0.2-5$), these high-redshift galaxies appear more efficient at removing baryons than low-redshift galaxies with similar $M_{\star}$, showing mass loading-factors of $\dot{M}_{\rm flow}/{\rm SFR}=0.04-0.4$. For their given dynamical mass, the outflow velocities exceed the escape velocities, meaning that they may eventually enrich the Circumgalactic Medium.

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arXiv2025

A great diversity of spectral shapes in the ionising spectra of z ~ 0.6-1 galaxies revealed by HST/COS and possible detection of nebular LyC emission

We present observations of eleven compact star-forming galaxies in the redshift range z = 0.6145 - 1.0053, with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). We aim to spectroscopically measure for the first time the Lyman continuum (LyC) over a wider rest-frame wavelength range of ~ 600 - 900A compared to ~ 850 - 900A in previous studies of galaxies at z ~ 0.3 - 0.4. The HST data are supplemented by SDSS spectra of all galaxies and by a VLT/Xshooter spectrum of one galaxy, J0232+0025. These data are used to derive the spectral energy distribution in the entire UV and optical range, the stellar mass, and the chemical composition from the nebular emission lines. We detect stellar LyC emission in seven out of eleven galaxies with escape fractions, f_esc(LyC), in the range of ~ 2 - 60%, and establish upper limits for f_esc(LyC) in the remaining galaxies. We discover for the first time nebular LyC emission as a bump just bluewards of the LyC limit at 912A in two galaxies, J0232+0025 and J1021+0436. We find a similar bump among our earlier studies in a less distant galaxy J1243+4646 with z = 0.4317. We conclude that the use of the LyC continuum in the wavelength range close to the LyC limit, which contains both the stellar and nebular continua, requires special consideration to not overestimate the observed f_esc(LyC).

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arXiv2025

A negligible contribution of two luminous $z$ ~ 7.5 galaxies to the ionizing photon budget of reionization

We present indirect constraints on the absolute escape fraction of ionizing photons ($f_{\rm esc}^{\rm LyC}$) of the system GN 42912 which comprises two luminous galaxies ($M_{\rm UV}$ magnitudes of -20.89 and -20.37) at $z\sim7.5$, GN 42912-NE and GN 42912-SW, to determine their contribution to the ionizing photon budget of the Epoch of Reionization (EoR). The high-resolution James Webb Space Telescope NIRSpec and NIRCam observations reveal the two galaxies are separated by only ~0.1$"$ (0.5 kpc) on the sky and have a 358 km s$^{-1}$ velocity separation. GN 42912-NE and GN 42912-SW are relatively massive for this redshift (log($M_\ast/M_\odot$) $\sim$ 8.4 and 8.9, respectively), with gas-phase metallicities of 18 per cent and 23 per cent solar, O$_{32}$ ratios of 5.3 and $>5.8$, and $β$ slopes of $-1.92$ and $-1.51$, respectively. We use the Mg II$λλ$2796,2803 doublet to constrain $f_{\rm esc}^{\rm LyC}$. Mg II has an ionization potential close to that of neutral hydrogen and, in the optically thin regime, can be used as an indirect tracer of the LyC leakage. We establish realistic conservative upper limits on $f_{\rm esc}^{\rm LyC}$ of 8.5 per cent for GN 42912-NE and 14 per cent for GN 42912-SW. These estimates align with $f_{\rm esc}^{\rm LyC}$ trends observed with $β$, O$_{32}$, and the H$β$ equivalent width at $z<4$. The small inferred ionized region sizes ($<0.3$ pMpc) around both galaxies indicate they have not ionized a significant fraction of the surrounding neutral gas. While these $z>7$ $f_{\rm esc}^{\rm LyC}$ constraints do not decisively determine a specific reionization model, they support a minor contribution from these two relatively luminous galaxies to the EoR.

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arXiv2024

[Ne v] emission from a faint epoch of reionization-era galaxy: evidence for a narrow-line intermediate mass black hole

Here we present high spectral resolution $\textit{JWST}$ NIRSpec observations of GN42437, a low-mass (log(M$_\ast/M_\odot)=7.9$), compact ($r_e < 500$pc), extreme starburst galaxy at $z=5.59$ with 13 emission line detections. GN42437 has a low-metallicity (5-10% Z$_\odot$) and its rest-frame H$α$ equivalent width suggests nearly all of the observed stellar mass formed within the last 3 Myr. GN42437 has an extraordinary 7$σ$ significant [Ne V] 3427 $\mathring{\rm A}$ detection. The [Ne V] line has a rest-frame equivalent width of $11\pm2\mathring{\rm A}$, [Ne V]/H$α=0.04\pm0.007$, [Ne V]/[Ne III] 3870$\mathring{\rm A} = 0.26\pm0.04$, and [Ne V]/He II 4687 $\mathring{\rm A} = 1.2\pm0.5$. Ionization from massive stars, shocks, or high-mass X-ray binaries cannot simultaneously produce these [Ne V] and low-ionization line ratios. Reproducing the complete nebular structure requires both massive stars and accretion onto a black hole. We do not detect broad lines nor do the traditional diagnostics indicate that GN42437 has an accreting black hole. Thus, the very-high-ionization emission lines powerfully diagnose faint narrow-line black holes at high-redshift. We approximate the black hole mass in a variety of ways as log(M$_{\rm BH}/M_\odot) \sim 5-7$. This black hole mass is consistent with local relations between the black hole mass and the observed velocity dispersion, but significantly more massive than the stellar mass would predict. Very-high-ionization emission lines may reveal samples to probe the formation and growth of the first black holes in the universe.

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arXiv2023

Monitoring broad emission-line components in spectra of the two low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840

We report the discovery of broad components with P-Cygni profiles of the hydrogen and helium emission lines in the two low-redshift low-metallicity dwarf compact star-forming galaxies (SFG), SBS 1420+540 and J1444+4840. We found small stellar masses of 10^{6.24} and 10^{6.59} M$_\odot$, low oxygen abundances 12+log O/H of 7.75 and 7.45, high velocity dispersions reaching $σ$ ~700 and ~1200km/s, high terminal velocities of the stellar wind of ~1000 and ~1000-1700km/s, respectively, and large EW(H$β$) of ~300A for both. For SBS 1420+540, we succeeded in capturing an eruption phase by monitoring the variations of the broad-to-narrow component flux ratio. We observe a sharp increase of that ratio by a factor 4 in 2017 and a decrease by about an order of magnitude in 2023. The peak luminosity of ~10^{40}ergs/s of the broad component in $L$(H$α$) lasted for about 6 years out of a three-decades monitoring. This leads us to conclude that there is probably a LBV candidate (LBVc) in this galaxy. As for J1444+4840, its very high $L$(H$α$) of about 10^{41}ergs/s, close to values observed in active galactic nuclei (AGNs) and Type IIn Supernovae (SNe), and the variability of no more than 20 per cent of the broad-to-narrow flux ratio of the hydrogen and helium emission lines over a 8-year monitoring do not allow us to definitively conclude that it contains a LBVc. On the other hand, the possibility that the line variations are due to a long-lived stellar transient of type LBV/SNIIn cannot be ruled out.

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arXiv2023

Extremely strong CIV 1550 nebular emission in the extremely low-metallicity star-forming galaxy J2229+2725

Using Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) observations of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J2229+2725, we have discovered an extremely strong nebular CIV 1549, 1551 emission-line doublet, with an equivalent width of 43A, several times higher than the value observed so far in low-redshift SFGs. Together with other extreme characteristics obtained from optical spectroscopy (oxygen abundance 12+log(O/H)=7.085+/-0.031, ratio O32 = I([OIII]5007)/I([OII]3727) ~ 53, and equivalent width of the Hbeta emission line EW(Hbeta) = 577A), this galaxy greatly increases the range of physical properties for dwarf SFGs at low redshift and is a likely analogue of the high-redshift dwarf SFGs responsible for the reionization of the Universe. We find the ionizing radiation in J2229+2725 to be stellar in origin and the high EW(CIV 1549,1551) to be due to both extreme ionization conditions and a high carbon abundance, with a corresponding log C/O = -0.38, that is ~ 0.4 dex higher than the average value for nearby low-metallicity SFGs.

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arXiv2023

J1046+4047: an extremely low-metallicity dwarf star-forming galaxy with O32 = 57

Using the optical spectrum obtained with the Kitt Peak Ohio State Multi-Object Spectrograph (KOSMOS) mounted on the Apache Point Observatory (APO) 3.5m Telescope and the Sloan Digital Sky Survey (SDSS) spectrum, we study the properties of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J1046+4047. The galaxy, with a redshift z=0.0487, was selected from the Data Release 16 (DR16) of the SDSS. Its properties are among the most extreme for SFGs in several ways. Its oxygen abundance 12+log(O/H) = 7.091+/-0.016 is among the lowest ever observed. With an absolute magnitude Mg = -16.51 mag, a low stellar mass Mstar = 1.8x10^6 Msun and a very low mass-to-light ratio Mstar/Lg~0.0029 (in solar units), J1046+4047 has a very high specific star-formation rate sSFR~430 Gyr^-1, indicating very active ongoing star formation. Another striking feature of J1046+4047 is that it possesses a ratio O32 = I([OIII]5007)/I([OII]3727) ~57. Using this extremely high O32, we have confirmed and improved the strong-line calibration for the determination of oxygen abundances in the most metal-deficient galaxies, in the range 12+log(O/H) < 7.65. This improved method is appropriate for all galaxies with O32<60 and extends an applicability to highest observed O32 ratios.We find the Halpha emission line in J1046+4047 to be enhanced by some non-recombination processes and thus can not be used for the determination of interstellar extinction.

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arXiv2023

Ly-alpha emission in low-redshift most metal-deficient compact star-forming galaxies

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of nine most metal-deficient compact star-forming galaxies with oxygen abundances 12+log(O/H)=6.97-7.23, redshifts z=0.02811-0.13320, and stellar masses M*<10^7Msun. We aim to study the properties of Ly-alpha emission in these extremely metal-deficient objects. We find that all nine galaxies are Ly-alpha emitters (LAEs). We examine various relations between the Ly-alpha escape fraction fesc(Ly-alpha) and other characteristics - such as absolute UV magnitude, oxygen abundance, O32 ratio, stellar mass, Lyman-alpha luminosity and equivalent width EW(Ly-alpha), ionizing photon production efficiency and velocity separation Vsep between the two peaks of the Ly-alpha profile - of a large sample of LAEs, including our lowest-metallicity galaxies and other objects from the literature. We find a relatively tight correlation between fesc(Ly-alpha) and two characteristics, EW(Ly-alpha) and Vsep, whereas no correlation is found between fesc(Ly-alpha) and the oxygen abundance. We also find a relatively tight relation between the Ly-alpha and LyC escape fractions. We propose to use the latter relation to estimate indirectly the escaping ionizing radiation in LAEs, when direct measurements of LyC emission are not possible. We show that the global properties of low-z LAEs are very similar to those of z>6 galaxies. They are thus ideal local proxies for studying physical processes during the epoch of reionization of the Universe.

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arXiv2022

First look with JWST spectroscopy: $z \sim 8$ galaxies resemble local analogues

Deep images and near-IR spectra of galaxies in the field of the lensing cluster SMACS J0723.3-7327 were recently taken in the Early Release Observations program of JWST. Among these, two NIRSpec spectra of galaxies at $z=7.7$ and one at $z=8.5$ were obtained, revealing for the first time rest-frame optical emission line spectra of galaxies in the epoch of reionization, including the detection of the important[OIII]4363 auroral line (see JWST PR 2022-035). We present an analysis of the emission line properties of these galaxies, finding that these galaxies have a high excitation (as indicated by high ratios of [OIII]/[OII], [NeIII]/[OII]), strong [OIII]4363/H$γ$, high equivalent widths, and other properties which are typical of low-metallicity star-forming galaxies. Using the direct method we determine oxygen abundances of $12+\log(O/H)=7.9$ in two $z=7.7$ galaxies, and a lower metallicity of $12+\log(O/H)\approx 7.4-7.5$ in the $z=8.5$ galaxy using different strong line methods. More accurate metallicity determinations will require better data. With stellar masses estimated from SED fits, we find that the three galaxies lie close to or below the $z \sim 2$ mass-metallicity relation. Overall, these first galaxy spectra at $z \sim 8$ show a strong resemblance of the emission lines properties of galaxies in the epoch of reionization with those of relatively rare local analogues previously studied from the SDSS. Clearly, the first JWST observations demonstrate already the incredible power of spectroscopy to reveal properties of galaxies in the early Universe.

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arXiv2022

Optical spectroscopy of the extremely metal-deficient star-forming galaxy HSC J1631+4426: a test of the strong-line method

Recently, Kojima and co-authors have reported a record low oxygen abundance, 12+logO/H=6.90+/-0.03 in the low-mass star-forming galaxy HSC J1631+4426. This exceptionally low oxygen abundance was obtained by the direct method, using the [OIII]4363 emission line. However, using the strong-line method by Izotov et al. (2019b), these authors have derived a significantly higher metallicity 12+logO/H=7.175+/-0.005. To clarify the situation, we have obtained new observations of HSC J1631+4426 with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). We have derived a higher oxygen abundance, 12+logO/H=7.14+/-0.03, using the direct method, a value similar to the oxygen abundance obtained by the strong-line method. Thus, HSC J1631+4426 has a metallicity close to that of the well known blue compact dwarf galaxy IZw18.

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arXiv2022

Lyman alpha and Lyman continuum emission of MgII-selected star-forming galaxies

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of seven compact low-mass star-forming galaxies at redshifts, z,in the range 0.3161-0.4276, with various O3Mg2=[OIII]5007/MgII 2796+2803 and Mg2=MgII 2796/MgII 2803 emission-line ratios. We aim to study the dependence of leaking Lyman continuum (LyC) emission on the characteristics of MgII emission together with the dependences on other indirect indicators of escaping ionizing radiation. LyC emission with escape fractions fesc(LyC)=3.1-4.6 per cent is detected in four galaxies, whereas only 1sigma upper limits of fesc(LyC) in the remaining three galaxies were derived. A strong narrow Ly-alpha emission line with two peaks separated by Vsep~298-592 km/s was observed in four galaxies with detected LyC emission and very weak Ly-alpha emission is observed in galaxies with LyC non-detections. Our new data confirm the tight anti-correlation between fesc(LyC) and Vsep found for previous low-redshift galaxy samples. Vsep remains the best indirect indicator of LyC leakage among all considered indicators. It is found that escaping LyC emission is detected predominantly in galaxies with Mg2>1.3. A tendency of an increase of fesc(LyC) with increasing of both the O3Mg2 and Mg2 is possibly present. However, there is substantial scatter in these relations not allowing their use for reliable prediction of fesc(LyC).

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arXiv2022

Decade-long time-monitoring of candidate Luminous Blue Variable Stars in the two very metal-deficient compact dwarf galaxies DDO 68 and PHL 293B

We have studied the spectral time variations of candidate luminous blue variable stars (cLBV) in two low-metallicity blue compact dwarf galaxies, DDO 68 and PHL 293B. The LBV in DDO 68, located in HII region #3, shows an outburst, with an increase of more than 1000 times in Halpha luminosity during the period 2008-2010. The broad emission of the HI and HeI lines display a P Cygni profile, with a relatively constant terminal velocity of ~800 km/s, reaching a maximum luminosity L(Halpha) of ~2x10^38 erg/s, with a FWHM of ~1000-1200 km/s. On the other hand, since the discovery of a cLBV in 2001 in PHL 293B, the fluxes of the broad components and the broad-to-narrow flux ratios of the HI and HeI emission lines in this galaxy have remained nearly constant over 16 years, with small variations. The luminosity of the broad Halpha component varies between ~2x10^38 erg/s and ~10^39 erg/s, with the FWHM varying in the range ~500-1500 km/s. Unusually persistent P Cygni features are clearly visible until the end of 2020 despite a decrease of the broad-to-narrow flux ratio in the most recent years. A terminal velocity of ~800 km/s is measured from the P Cygni profile, similar to the one in DDO 68, although the latter is 3.7 more metal-deficient than PHL 293B. The relative constancy of the broad Halpha luminosity in PHL 293B suggests that it is due to a long-lived stellar transient of type LBV/SN IIn.

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arXiv2021

Large Binocular Telescope observations of new six compact star-forming galaxies with [NeV] 3426A emission

We report the discovery of [NeV]3426 emission, in addition to HeII4686 emission, in six compact star-forming galaxies. These observations considerably increase the sample of eight such galaxies discovered earlier by our group. For four of the new galaxies, the optical observations are supplemented by near-infrared spectra. All galaxies, but one, have HII regions that are dense, with electron number densities of ~300-700 cm-3. They are all characterised by high Hbeta equivalent widths EW(Hbeta)~190-520A and high O32=[OIII]5007/[OII]3727 ratios of 10-30, indicating young starburst ages and the presence of high ionization radiation. All are low-metallicity objects with 12+logO/H=7.46-7.88. The spectra of all galaxies show a low-intensity broad component of the Halpha line and five out of six objects show Wolf-Rayet features. Comparison with photoionization models shows that pure stellar ionization radiation from massive stars is not hard enough to produce such strong [NeV] and HeII emission in our galaxies. The [NeV]3426/HeII4686 flux ratio of ~1.2 in J1222+3602 is consistent with some contribution of active galactic nucleus ionizing radiation. However, in the remaining five galaxies, this ratio is considerably lower, <0.4. The most plausible models are likely to be non-uniform in density, where HeII and [NeV] lines are emitted in low-density channels made by outflows and illuminated by harder ionizing radiation from radiative shocks propagating through these channels, whereas [OIII] emission originates in denser regions exposed to softer stellar ionizing sources.

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arXiv2021

J2229+2725: an extremely low-metallicity dwarf compact star-forming galaxy with an exceptionally high [OIII]5007/[OII]3727 flux ratio of 53

Using the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS), we have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J2229+2725. This galaxy with a redshift z=0.0762 was selected from the Data Release 16 (DR16) of the Sloan Digital Sky Survey (SDSS). Its properties derived from the LBT observations are most extreme among SFGs in several ways. Its oxygen abundance 12+logO/H=7.085+/-0.031 is among the lowest ever observed for a SFG. With its very low metallicity, an absolute magnitude Mg=-16.39 mag, a low stellar mass Mstar=9.1x10^6 Msun and a very low mass-to-light ratio Mstar/Lg~0.0166 (in solar units), J2229+2725 deviates strongly from the luminosity-metallicity relation defined by the bulk of the SFGs in the SDSS. J2229$+$2725 has a very high specific star-formation rate sSFR~75 Gyr^-1, indicating very active ongoing star formation. Three other features of J2229+2725 are most striking, being the most extreme among lowest-metallicity SFGs: 1) a ratio O32=I([OIII]5007)/I([OII]3727)~53, 2) an equivalent width of the Hbeta emission line EW(Hbeta) of 577A, and 3) an electron number density of ~1000 cm^-3. These properties imply that the starburst in J2229+2725 is very young. Using the extremely high O32 in J2229+2725, we have improved the strong-line calibration for the determination of oxygen abundances in the most metal-deficient galaxies, in the range 12 + logO/H<7.3.

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arXiv2021

Lyman continuum leakage from low-mass galaxies with Mstar < 1.E8 Msun

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of nine low-mass star-forming galaxies at redshifts, z, in the range 0.3179-0.4524, with stellar masses Mstar < 10^8 M and veruny high specific star-formation rates sSFR~150-630 Gyr^{-1}, aiming to study the dependence of leaking Lyman continuum (LyC) emission on stellar mass and some other characteristics of the galaxy. We detect LyC emission in four out of nine galaxies with escape fractions, fesc(LyC), in the range of 11-35 per cent, and establish upper limits for fesc(LyC) in the remaining five galaxies. We observe a narrow Ly-alpha emission line with two peaks in seven galaxies and likely more complex Ly-alpha profiles in the two remaining galaxies. The velocity separation between the peaks Vsep varies in the range from ~229 km/s to ~512 km/s. Our additional data on low-mass galaxies confirm and strengthen the tight anti-correlation between fesc(LyC) and Vsep found for previous low-redshift galaxy samples with higher stellar masses. Vsep remains the best indirect indicator of LyC leakage. It is better than O32 on which fesc(LyC) depends weakly, with a large scatter. Finally, contrary to expectations, we find no increase of fesc(LyC) with decreasing galaxy stellar mass Mstar.

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arXiv2021

Low-redshift compact star-forming galaxies as analogues of high-redshift star-forming galaxies

We compare the relations among various integrated characteristics of ~25,000 low-redshift (z<1.0) compact star-forming galaxies (CSFGs) from Data Release 16 (DR16) of the Sloan Digital Sky Survey (SDSS) and of high-redshift (z>1.5) star-forming galaxies (SFGs) with respect to oxygen abundances, stellar masses M*, far-UV absolute magnitudes M(FUV), star-formation rates SFR and specific star-formation rates sSFR, Lyman-continuum photon production efficiencies (xi_ion), UV continuum slopes β, [OIII]5007/[OII]3727 and [NeIII]3868/[OII]3727 ratios, and emission-line equivalent widths EW([OII]3727), EW([OIII]5007), and EW(Hα). We find that the relations for low-z CSFGs with high equivalent widths of the Hβemission line, EW(Hβ)>100A, and high-z SFGs are very similar, implying close physical properties in these two categories of galaxies. Thus, CSFGs are likely excellent proxies for the SFGs in the high-z Universe. They also extend to galaxies with lower stellar masses, down to ~10^6 Msun, and to absolute FUV magnitudes as faint as -14 mag. Thanks to their proximity, CSFGs can be studied in much greater detail than distant SFGs. Therefore, the relations between the integrated characteristics of the large sample of CSFGs studied here can prove very useful for our understanding of high-z dwarf galaxies in future observations with large ground-based and space telescopes.

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arXiv2020

Reconciling escape fractions and observed line emission in Lyman-continuum-leaking galaxies

Finding and elucidating the properties of Lyman-continuum(LyC)-emitting galaxies is an important step for our understanding of cosmic reionization. Although the z - 0.3-0.4 LyC emitters found recently show strong optical emission lines, no consistent quantitative photoionization model taking into account the escape of ionizing photons and inhomogenous interstellar medium (ISM) geometry of these galaxies has yet been constructed. We hence construct one- and two-zone photoionization models accounting for the observed LyC escape, which we compare to the observed emission line measurements. We find that one-zone density-bounded photoionization models cannot reproduce the emission lines of the LyC leakers because they systematically underpredict the lines of species of low ionization potential, as [OI] and [SII]. Introducing a two-zone model, with differing ionization parameter and a variable covering fraction and where one of the zones is density-bounded, we show that the observed emission line ratios and escape fractions of the LyC emitters are well reproduced. The [OI] excess, which is observed in some LyC leakers, can be naturally explained in this model, e.g., by emission from low-ionization and low-filling-factor gas. LyC emitters with a high escape fraction (fesc > 38%) are deficient both in [OI]6300A and in [SII]6716,6731A. We also confirm that a [SII] deficiency can be used to select LyC emitter candidates. Finally, we find indications for a possible dichotomy in terms of escape mechanisms for LyC photons between galaxies with relatively low (fesc < 10%) and higher escape fractions. We conclude that two-zone photoionization models are sufficient and required to explain the observed emission line properties of z - 0.3-0.4 LyC emitters. These models provide a first step towards the use of optical emission lines and their ratios as quantitative diagnostics of LyC escape from galaxies.

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