J/A+A/712/A19 Chemical composition in low-mass galaxies (Izotov+, 2026)
Chemical composition and kinematics of ionised gas in low-mass star-forming
galaxies with extremely high [O III]/[O II] ratios.
Izotov Y.I., Guseva N.G., Schaerer D., Amorin R.O
<Astron. Astrophys. 712, A19 (2026)>
=2026A&A...712A..19I 2026A&A...712A..19I (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies ; Ultraviolet ; Optical ; Infrared ; Spectroscopy ;
Photometry ; Abundances ; Line Profiles ; Equivalent widths
Keywords: galaxies: abundances - galaxies: evolution - galaxies: irregular -
galaxies: ISM
Abstract:
We present Very Large Telescope/Xshooter spectrophotometric
observations of eleven low-redshift (z<0.085) compact star-forming
galaxies (the high O 32 sample). These galaxies are characterised by
extremely high emission-line ratios, [O III]5007/[O II]3727, ranging
from 11 to 42. Galaxies with such high ratios are promising candidates
for leaking large amounts of Lyman continuum radiation. They are
characterised by low oxygen abundances, 12+log(O/H)=7.5-8.0, and low
stellar masses, M*∼106-108M_☉. We used strong emission lines
of various ions in all spectra 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 increases with increasing stellar mass and
decreasing O 32 ratio. In contrast, sigma(lambda)/sigma(4861) ratios
for various lines are close to unity. Exceptions are the
sigma(lambda)/sigma(4861) of two lines, HeII 4686 and HeI 10830, which
are considerably higher than unity, and those of four lines, [OII]
3726, 3729, [SII] 6717, and 6731, for which sigma(lambda)/sigma(4861)
is lower than unity. The HeII 4686 and He I 10830 lines are likely
produced in the inner parts of HII regions and are broadened by
dynamical processes generated by massive stars, and, in the case of
the HeI 10830 emission line, by radiative scattering. Emission in the
[OII] and [SII] lines is produced mainly in the outer and likely more
quieter parts of HII regions.
Description:
The studies of low-redshift compact star-forming galaxies (CSFGs)
selected from the Sloan Digital Sky Survey (SDSS) have shown that they
are low-mass and low-metallicity galaxies with star formation occuring
in short bursts of a few million years duration. Their properties,
together with the fact that CSFGs and high-redshift star-forming
galaxies follow similar mass-metallicity and luminosity-metallicity
relations, suggest that CSFGs are good local counterparts of the
high-redshift dwarf star-forming galaxies The strong emission lines in
the optical spectra of H II regions in CSFGs are powered by numerous
O-stars, which produce plenty of ionising radiation. This makes CSFGs
promising candidates for considerable leaking ionising radiation into
the intergalactic medium (IGM). The objects were selected from the
SDSS by their compactness, strong emission lines, extremely high
emission-line ratios O32=[OIII]λ5007/[OII]λ3727 and
absence of Active Galactic Nuclei (AGNs) spectral features.
Additionally, the objects must have low declinations ≲ 20 degree to
be accessible for observations with the XShooter VLT. The sample
includes 11 relatively bright galaxies at z < 0.085. More,
characteristics of the CSFGs are derived from their SDSS spectra and
GALEX photometry, the observed fluxes transformed to luminosities and
absolute magnitudes with adopting luminosity NED distances (see more
in section selection of CSFGs).
As discussed observation section, we have obtained Xshooter
spectrophotometric observations of the galaxies in the wavelength
range 3200-24000Å with the three-arm spectrograph splitting
emission from the object into three beams, UVB, VIS, and NIR..
Emission-line fluxes were measured using the IRAF splot routine.
Intensities of emission lines in spectra in the wavelength range from
310 to 2200nm of 11 galaxies with extremely high [OIII]/[OII]
ratios are presented in tableb1a.dat. Extinction coefficients,
rest-frame equivalent widths and observed fluxes of Hbeta emission
line are given in tableb1b.dat. Abundances of helium, nitrogen,
oxygen, neon, sulfur, chlorine, argon, and iron are presented in
tableb2.dat.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 72 11 General characteristics of CSFGs galaxies
derived from SDSS spectra and GALEX photometry
tableb1a.dat 149 113 Emission-line fluxes in 11 galaxies
tableb1b.dat 52 11 Extinction coefficients, rest-frame equivalent
widths and the observed fluxes of the Hbeta
emission line in 11 galaxies
tableb2.dat 526 11 Chemical abundances in 11 galaxies
--------------------------------------------------------------------------------
See also:
J/A+A/688/A159 : Blueberry galaxies up to 200Mpc (Kouroumpatzakis+, 2024)
J/A+A/448/955 : Abundances of emission galaxies in SDSS-DR3 (Izotov+, 2006)
J/MNRAS/442/3565 : L-σ relation for massive star formation (Chavez+,2014)
J/ApJ/893/96 : CHAOS IV: NGC3184 LBT obs. & 3 other gal. abundances
(Berg+, 2020)
J/ApJ/847/38 : Extreme emission line galaxies at z≲0.05 (Yang+, 2017)
J/ApJ/803/34 : z∼4-10 galaxies from HST legacy fields (Bouwens+, 2015)
J/ApJ/602/200 : HII regions abundances in blue compact galaxies
(Izotov+, 2004)
J/ApJS/261/31 : COS Legacy Archive Sp. Survey (CLASSY) treasury (Berg+,2022)
J/ApJS/161/240 : High-ionization emission in metal-deficient BCDs
(Thuan+, 2005)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Galaxy CSFGs name (Name)
12- 13 I2 h RAh Right ascension (J2000) (RA)
15- 16 I2 min RAm Right ascension (J2000) (RA)
18- 22 F5.2 s RAs Right ascension (J2000) (RA)
24 A1 --- DE- Declination sign (J2000) (Dec)
25- 26 I2 deg DEd Declination (J2000) (Dec)
28- 29 I2 arcmin DEm Declination (J2000) (Dec)
31- 35 F5.2 arcsec DEs Declination (J2000) (Dec)
37- 43 F7.5 --- z Redshift (Redshift)
45- 49 F5.2 mag gmag SDSS g-band magnitude (g)
51- 54 F4.2 [-] A(O) Oxygen abundance derived by the direct
Te-method from the Xshooter spectrum
(12+log(O/H))
56- 58 I3 0.1nm EWHb Rest-frame equivalent width of the
Hβ emission line in the Xshooter
spectrum (EW(Hβ)
60- 61 I2 --- O32 The oxygen line ratio O32 as
[OIII]λ5007/[OII]λ3727
(O32)
63- 66 F4.2 [Msun] logM* The stellar mass (logM*)
68- 72 F5.2 [10+7Hz/J] logXi The logarithm of the production
efficiency of ionizing photons
(logΞion)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tableb1a.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Line Name of the emission line (Line)
9- 17 F9.3 nm Lambda Rest-frame wavelength of the emission line
(Lambda)
18- 23 F6.3 --- F01 Flux of the emission line relative to Hbeta
emission line in the galaxy J0007+0226 (F01)
24- 29 F6.3 --- e_F01 Flux error of the emission line relative to
Hbeta emission line in the galaxy J0007+0226
(EF01)
30- 35 F6.3 --- F02 Flux of the emission line relative to Hbeta
emission line in the galaxy J0009+0234 (F02)
36- 41 F6.3 --- e_F02 Flux error of the emission line relative to
Hbeta emission line in the galaxy J0009+0234
(EF02)
42- 47 F6.3 --- F03 Flux of the emission line relative to Hbeta
emission line in the galaxy J0159+0751 (F03)
48- 53 F6.3 --- e_F03 Flux error of the emission line relative to
Hbeta emission line in the galaxy J0159+0751
(EF03)
54- 59 F6.3 --- F04 Flux of the emission line relative to Hbeta
emission line in the galaxy J0240-0828 (F04)
60- 65 F6.3 --- e_F04 Flux error of the emission line relative to
Hbeta emission line in the galaxy J0240-0828
(EF04)
66- 71 F6.3 --- F05 Flux of the emission line relative to Hbeta
emission line in the galaxy J0825+1846 (F05)
72- 77 F6.3 --- e_F05 Flux error of the emission line relative to
Hbeta emission line in the galaxy J0825+1846
(EF05)
78- 83 F6.3 --- F06 Flux of the emission line relative to Hbeta
emission line in the galaxy J1255-0213 (F06)
84- 89 F6.3 --- e_F06 Flux error of the emission line relative to
Hbeta emission line in the galaxy J1255-0213
(EF06)
90- 95 F6.3 --- F07 Flux of the emission line relative to Hbeta
emission line in the galaxy J1310+0852 (F07)
96-101 F6.3 --- e_F07 Flux error of the emission line relative to
Hbeta emission line in the galaxy J1310+0852
(EF07)
102-107 F6.3 --- F08 Flux of the emission line relative to Hbeta
emission line in the galaxy J1323-0132 (F08)
108-113 F6.3 --- e_F08 Flux error of the emission line relative to
Hbeta emission line in the galaxy J1323-0132
(EF08)
114-119 F6.3 --- F09 Flux of the emission line relative to Hbeta
emission line in the galaxy J1325+0744 (F09)
120-125 F6.3 --- e_F09 Flux error of the emission line relative to
Hbeta emission line in the galaxy J1325+0744
(EF09)
126-131 F6.3 --- F10 Flux of the emission line relative to Hbeta
emission line in the galaxy J1411+0550 (F10)
132-137 F6.3 --- e_F10 Flux error of the emission line relative to
Hbeta emission line in the galaxy J1411+0550
(EF10)
138-143 F6.3 --- F11 Flux of the emission line relative to Hbeta
emission line in the galaxy J1526+2113 (F11)
144-149 F6.3 --- e_F11 Flux error of the emission line relative to
Hbeta emission line in the galaxy J1526+2113
(EF11)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tableb1b.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Name of the galaxy (Name)
11- 16 F6.3 --- CHb Extinction coefficient derived from the decrement
of hydrogen Balmer emission lines (CHB)
17- 22 F6.3 --- e_CHb Error of extinction coefficient derived from the
decrement of hydrogen Balmer emission lines (ECHB)
23- 28 F6.2 nm EWHb Rest-frame equivalent width of the Hbeta emission
line (EWHB)
29- 33 F5.2 nm e_EWHb Error of the rest-frame equivalent width of the
Hbeta emission line (EEWHB)
34- 44 E11.3 W/m2 FHb Observed flux of the Hbeta emission line (FHB)
45- 52 E8.1 W/m2 e_FHb Error of observed flux of the Hbeta emission line
(EFHB)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tableb2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Name of the galaxy (Name)
11- 16 I6 K TOIII Electron temperature for OIII ion (TOIII)
17- 20 I4 K e_TOIII Error of the electron temperature for
OIII ion (ETOIII)
21- 26 I6 K TOII Electron temperature for OII ion (TOII)
27- 30 I4 K e_TOII Error of the electron temperature for
OII ion (ETOII)
31- 36 I6 K TSIII Electron temperature for SIII ion (TSIII)
37- 40 I4 K e_TSIII Error of the electron temperature for
SIII ion (ETSIII)
41- 44 I4 cm-3 nSII Electron number density derived from the
[SII] 671.7/673.1 emission-line ratio
(NSII)
45- 48 I4 cm-3 e_nSII Error of the electron number density
derived from the [SII] 671.7/673.1
emission-line ratio (ENSII)
49- 52 I4 cm-3 nOII Electron number density derived from the
[OII] 327.6/327.9 emission-line ratio
(NOII)
53- 56 I4 cm-3 e_nOII Error of the electron number density
derived from the [OII] 327.6/327.9
emission-line ratio (ENOII)
57- 62 I6 K THE5 Electron temperature for HeII ion
derived from five HeI emission lines
(THE5)
63- 66 I4 cm-3 neHE5 Electron number density for HeII ion
derived from five HeI emission lines
(NEHE5)
67- 71 F5.2 --- Tau5 Optical depth in He I 3889 emission line
derived from five HeI emission lines
(TAU5)
72- 78 F7.4 --- Y5 Weighted mean He mass fraction derived
from derived from five HeI emission
lines (Y5)
79- 84 I6 K THE6 Electron temperature for HeII ion
derived from six HeI emission lines
(THE6)
85- 88 I4 cm-3 neHE6 Electron number density for HeII ion
derived from six HeI emission lines
(NEHE6)
89- 93 F5.2 --- Tau6 Optical depth in in He I 3889 emission
line derived from six HeI emission lines
(TAU6)
94-100 F7.4 --- Y6 Weighted mean He mass fraction derived
from derived from six HeI emission lines
(Y6)
101-109 E9.3 --- O+/H+ The O+/H+ abundance ratio (OP)
110-118 E9.3 --- e_O+/H+ Error of O+/H+ abundance ratio (EOP)
119-127 E9.3 --- O2+/H+ The O2+/H+ abundance ratio (O2P)
128-136 E9.3 --- e_O2+/H+ Error of O2+/H+ abundance ratio (EO2P)
137-145 E9.3 --- O3+/H+ The O3+/H+ abundance ratio (O3P)
146-154 E9.3 --- e_O3+/H+ Error of O3+/H+ abundance ratio (EO3P)
155-160 F6.3 [-] A(O) The total oxygen abundance 12+log(O/H)
(O)
161-166 F6.3 [-] e_A(O) Error of total oxygen abundance
12+log(O/H) (EO)
167-175 E9.3 --- N+/H+ The N+/H+ abundance ratio (NP)
176-184 E9.3 --- e_N+/H+ Error of N+/H+ abundance ratio (ENP)
185-191 F7.3 --- ICF(N) Ionisation correction factor ICF(N) of
nitrogen (ICFN)
192-198 F7.3 [-] log(N/O) The logarithm of N/O abundance ratio (NO)
199-204 F6.3 [-] e_log(N/O) Error of log(N/O) abundance ratio (ENO)
205-213 E9.3 --- Ne2+/H+ The Ne2+/H+ abundance ratio (NE2P)
214-222 E9.3 --- e_Ne2+/H+ Error of Ne2+/H+ abundance ratio (ENE2P)
223-228 F6.3 --- ICF(NE) Ionisation correction factor ICF(Ne) of
neon (ICFNE)
229-235 F7.3 [-] log(NE/O) The logaritm of Ne/O abundance ratio
(NEO)
236-241 F6.3 [-] e_log(NE/O) Error of log of Ne/O abundance ratio
(ENEO)
242-250 E9.3 --- S+/H+ The S+/H+ abundance ratio (SP)
251-259 E9.3 --- e_S+/H+ Error of S+/H+ abundance ratio (ESP)
260-268 E9.3 --- S2+/H+6312 The S2+/H+ abundance ratio derived from
[SIII]631.2 emission line (S2P6312)
269-277 E9.3 --- e_S2+/H+6312 Error of S2+/H+ abundance ratio derived
from [SIII]631.2 emission line (ES2P6312)
278-283 F6.3 --- ICF(S) Ionisation correction factor ICF(S) of
sulfur (ICFS)
284-290 F7.3 [-] log(S/O6312) The logarithm of S/O abundance ratio
using [SIII]631.2 emission line (SO6312)
291-296 F6.3 [-] e_log(S/O6312) Error of log of S/O abundance ratio
using [SIII]631.2 emission line (ESO6312)
297-305 E9.3 --- S2+/H+9069 The S2+/H+ abundance ratio derived from
[SIII]906.9 emission line (S2P9069)
306-314 E9.3 --- e_S2+/H+9069 Error of S2+/H+ abundance ratio using
[SIII]906.9 emission line (ES2P9069)
315-321 F7.3 [-] log(S/O9069) The logarithm of S/O abundance ratio
using [SIII]906.9 emission line (SO9069)
322-327 F6.3 [-] e_log(S/O9069) Error of log of S/O abundance ratio
using [SIII]906.9 emission line (ESO9069)
328-336 E9.3 --- S2+/H+9531 The S2+/H+ abundance ratio derived from
[SIII]953.1 emission line (S2P9531)
337-345 E9.3 --- e_S2+/H+9531 Error of S2+/H+ abundance ratio using
[SIII]953.1 emission line (ES2P9531)
346-352 F7.3 [-] log(S/O9531) The logarithm of S/O abundance ratio
using [SIII]953.1 emission line (SO9531)
353-358 F6.3 [-] e_log(S/O9531) Error of log(S/O9531) abundance ratio
using [SIII]953.1 emission line (ESO9531)
359-367 E9.3 --- Cl2+/H+ The Cl2+/H+ abundance ratio (CL2P)
368-376 E9.3 --- e_Cl2+/H+ Error of Cl2+/H+ abundance ratio (ECL2P)
377-382 F6.3 --- ICF(Cl) Ionisation correction factor ICF(Cl) of
chlorine (ICFCL)
383-389 F7.3 [-] log(Cl/O) The logarithm of Cl/O abundance ratio
(CLO)
390-395 F6.3 [-] e_log(Cl/O) Error of log(Cl/O) abundance ratio (ECLO)
396-404 E9.3 --- Ar2+/H+ The Ar2+/H+ abundance ratio (AR2P)
405-413 E9.3 --- e_Ar2+/H+ Error of Ar2+/H+ abundance ratio (EAR2P)
414-422 E9.3 --- Ar3+/H+ The Ar3+/H+ abundance ratio (AR3P)
423-431 E9.3 --- e_Ar3+/H+ Error of Ar3+/H+ abundance ratio (EAR3P)
432-437 F6.3 --- ICF(Ar) Ionisation correction factor ICF(Ar) of
argon (ICFAR)
438-444 F7.3 [-] log(Ar/O) The logarithm of Ar/O abundance ratio
(ARO)
445-450 F6.3 [-] e_log(Ar/O) Error of log(Ar/O) abundance ratio (EARO)
451-459 E9.3 --- Fe2+/H+4658 The Fe2+/H+ abundance ratio derived from
[FeIII]465.8 emission line (FE2P466)
460-468 E9.3 --- e_Fe2+/H+4658 Error of Fe2+/H+ abundance ratio derived
from [FeIII]465.8 emission line
(EFE2P466)
469-475 F7.3 --- ICF(Fe4658) Ionisation correction factor ICF(Fe)
using [FeIII]465.8 emission line
(ICFFE466)
476-482 F7.3 [-] log(Fe/O)4658 The logarithm of Fe/O abundance ratio
using [FeIII]465.8 emission line (FEO466)
483-488 F6.3 [-] e_log(Fe/O)4658 Error of log of Fe/O abundance ratio
using [FeIII]465.8 emission line
(EFEO466)
489-497 E9.3 --- Fe2+/H+ The Fe2+/H+ abundance ratio derived from
[FeIII]498.8 emission line (FE2P499)
498-506 E9.3 --- e_Fe2+/H+ Error of Fe2+/H+ abundance ratio derived
from [FeIII]498.8 emission line
(EFE2P499)
507-513 F7.3 --- ICF(Fe4998) Ionisation correction factor ICF(Fe) of
iron (ICFFE499)
514-520 F7.3 [-] log(Fe/O)4998 The logarithm of Fe/O abundance ratio
using [FeIII]499.8 emission line (FEO499)
521-526 F6.3 [-] e_log(Fe/O)4998 Error of log(Fe/O) abundance ratio using
[FeIII]499.8 emission line (EFEO499)
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Acknowledgements:
Yuri Izotov, yizotov(at)bitp.kyiv.ua
License: CC-BY-4.0
(End) Yuri Izotov [BITP, Ukraine], Luc Trabelsi [CDS] 15-Jun-2026