We present a spectroscopic study of metal-deficient dwarf galaxy candidates, selected from the SDSS DR12. The oxygen abundances were derived using the direct method in galaxies with the electron ...temperature-sensitive emission line O iiiλ4363 Å measured with an accuracy better than 30%. The oxygen abundances for the remaining galaxies with larger uncertainties of the O iiiλ4363 Å line fluxes were calculated using a strong-line semi-empirical method by Izotov and Thuan. The resulting sample consists of 287 low-metallicity candidates with oxygen abundances below 12 + log O/H = 7.65 including 23 extremely metal-deficient (XMD) candidates with 12 + log O/H ≤ 7.35. Ten out of sixteen XMDs known so far (or ~60%) have been discovered by our team using the direct method. Three XMDs were found in the present study. We study relations between global parameters of low-metallicity galaxies, including absolute optical magnitudes, Hβ luminosities (or equivalently star formation rates), stellar masses, mid-infrared colours, and oxygen abundances. Low-metallicity and XMD galaxies strongly deviate to lower metallicities in L–Z, L(Hβ)–Z and M∗–Z diagrams than in relations obtained for large samples of low-redshift, star-forming galaxies with non-restricted metallicities. These less chemically evolved galaxies with stellar masses ≈106–108M⊙, Hβ luminosities ≈1038–1041 erg s-1, SFR ≈ 0.01–1.0 M⊙ yr-1, and sSFR ~ 50 Gyr-1 have physical conditions which may be characteristic of high-redshift low-mass star-forming galaxies which are still awaiting discovery.
ABSTRACT
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 $M_\star \, \lt $ 108 M⊙ and very 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 α emission line with two peaks in seven galaxies and likely more complex Ly α profiles in the two remaining galaxies. The velocity separation between the peaks Vsep varies in the range from ∼229 to ∼512 km s−1. Our additional data on low-mass galaxies confirm and strengthen the tight anticorrelation 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 M⋆.
Abstract
We report the detection of the Lyman continuum (LyC) radiation of the compact star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble ...Space Telescope. This galaxy, at a redshift of z = 0.3690, is characterized by a high emission-line flux ratio O32 = O iii λ5007/O ii λ3727 = 11.5. The escape fraction of the LyC radiation fesc(LyC) in this galaxy is 46 per cent, the highest value found so far in low-redshift SFGs and one of the highest values found in galaxies at any redshift. The narrow double-peaked Ly α emission line is detected in the spectrum of J1154+2443 with a separation between the peaks Vsep of 199 km s−1, one of the lowest known for Ly α-emitting galaxies, implying a high fesc(Ly α). Comparing the extinction-corrected Ly α/H β flux ratio with the case B value, we find fesc(Ly α) = 98 per cent. Our observations, combined with previous detections in the literature, reveal an increase of O32 with increasing fesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The surface brightness profile derived from the COS acquisition image reveals a bright star-forming region in the centre and an exponential disc in the outskirts with a disc scale length α = 1.09 kpc. J1154+2443, compared to other known low-redshift LyC leakers, is characterized by the lowest metallicity, 12+log O/H = 7.65 ± 0.01, the lowest stellar mass M⋆ = 108.20 M⊙, a similar star formation rate SFR = 18.9 M⊙ yr−1, and a high specific SFR of 1.2 × 10−7 yr−1.
We study relations between global characteristics of low-redshift (0 < z < 1) compact star-forming galaxies, including absolute optical magnitudes, H β emission-line luminosities (or equivalently ...star formation rates), stellar masses, and oxygen abundances. The sample consists of 5182 galaxies with high-excitation H ii regions selected from the SDSS DR7 and SDSS/BOSS DR10 surveys adopting a criterion O iii λ4959/H β ≥ 1. These data were combined with the corresponding data for high-redshift (2 ≲ z ≲ 3) star-forming galaxies. We find that in all diagrams low-z and high-z star-forming galaxies are closely related indicating a very weak dependence of metallicity on stellar mass, redshift, and star formation rate. This finding argues in favour of the universal character of the global relations for compact star-forming galaxies with high-excitation H ii regions over redshifts 0 < z < 3.
ABSTRACT
We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of eight compact star-forming galaxies at redshifts z = 0.02811–0.06540, with low oxygen ...abundances 12 + log(O/H) = 7.43–7.82 and extremely high emission-line flux ratios O32 = O iii λ5007/O ii λ3727 ∼ 22–39, aiming to study the properties of Ly α emission in such conditions. We find a diversity in Ly α properties. In five galaxies Ly α emission line is strong, with equivalent width (EW) in the range 45–190 Å. In the remaining galaxies, weak Ly α emission with EW(Ly α) ∼ 2–7 Å is superposed on a broad Ly α absorption line, indicating a high neutral hydrogen column density N(H i) ∼ (1 – 3)× 1021 cm−2. We examine the relation between the Ly α escape fraction fesc(Ly α) and the Lyman continuum escape fraction fesc(LyC), using direct measures of the latter in eleven low-redshift LyC leakers, to verify whether fesc(Ly α) can be an indirect measure of escaping LyC radiation. The usefulness of O32, of the Ly α equivalent width EW(Ly α), and of the Ly α peak separation Vsep as indirect indicators of Ly α leakage is also discussed. It is shown that there is no correlation between O32 and fesc(Ly α). We find an increase of fesc(Ly α) with increasing EW(Ly α) for EW(Ly α) <100 Å, but for higher EW(Ly$\alpha)\gtrsim$150 Å the fesc(Ly α) is nearly constant attaining the value of ∼0.25. We find an anticorrelation between fesc(Ly α) and Vsep, though not as tight as the one found earlier between fesc(LyC) and Vsep. This finding makes Vsep a promising indirect indicator of both the Ly α and ionizing radiation leakage.
We present a sample of low-redshift (z < 0.133) candidates for extremely low-metallicity star-forming galaxies with oxygen abundances 12 + log O/H < 7.4 selected from the Data Release 14 (DR14) of ...the Sloan Digital Sky Survey (SDSS). Three methods are used to derive their oxygen abundances. Among these methods two are based on strong O IIλ3727 Å, O IIIλ4959 Å, and O IIIλ5007 Å emission lines, which we call strong-line and semi-empirical methods. These were applied for all galaxies. We have developed one of these methods, the strong-line method, in this paper. This method is specifically focused on the accurate determination of metallicity in extremely low-metallicity galaxies and may not be used at higher metallicities with 12 + log O/H ≳ 7.5. The third, the direct Te method, was applied for galaxies with detected O IIIλ4363 emission lines. All three methods give consistent abundances and can be used in combination or separately for selection of lowest-metallicity candidates. However, the strong-line method is preferable for spectra with a poorly detected or undetected O IIIλ4363 emission line. In total, our list of selected candidates for extremely low-metallicity galaxies includes 66 objects.
We studied a large sample of ~14 000 dwarf star-forming galaxies with strong emission lines. These low-metallicity galaxies with oxygen abundances of 12 +log O/H ~7.4−8.5 are selected from the Sloan ...Digital Sky Survey (SDSS) and distributed in the redshift range of z ~0−0.6. We modelled spectral energy distributions (SED) of all galaxies, which were based on the SDSS spectra in the visible range of 0.38 μm−0.92 μm and included both the stellar and ionised gas emission. These SEDs were extrapolated to the UV and mid-infrared ranges to cover the wavelength range of 0.1 μm−22 μm. The SDSS spectroscopic data were supplemented by photometric data from the GALEX, SDSS, 2MASS, WISE, IRAS, and NVSS all-sky surveys. Using these data, we derived global characteristics of the galaxies, such as their element abundances, luminosities, and stellar masses. The luminosities and stellar masses range within the sample over ~5 orders of magnitude, thereby linking low-mass and low-luminosity blue compact dwarf galaxies to luminous galaxies, which are similar to high-redshift Lyman-break galaxies. It was found that the luminosity L(Hβ) of the Hβ emission line, a characteristic of the youngest stellar population with an age of a few Myr, is correlated with luminosities in other wavelength ranges. This implies that the most recent burst of star formation makes a significant contribution to the emission in the visible range and dominates in other wavelength ranges. It was also found that the contribution of the young population to the galaxy luminosity is higher for galaxies with higher L(Hβ) and higher equivalent widths EW(Hβ). We found 20 galaxies with very red WISE mid-infrared m(3.4 μm)− m(4.6 μm) colour (≥2 mag), which suggests the important contribution of the hot (with a temperature of several hundred degree) dust emission in these galaxies. Our analysis of the balance between the luminosity in the WISE bands that covered a wavelength range of 3.4 μm−22 μm and the luminosity of the emission absorbed at shorter wavelengths showed that the luminosity of the hot dust emission is increased with increasing L(Hβ) and EW(Hβ). We demonstrated that the emission emerging from young star-forming regions is the dominant dust-heating source for temperatures to several hundred degrees in the sample 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 ...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 (
ξ
ion
), UV continuum slopes
β
, O
III
λ
5007/O
II
λ
3727 and Ne
III
λ
3868/O
II
λ
3727 ratios, and emission-line equivalent widths EW(O
II
λ
3727), EW(O
III
λ
5007), and EW(H
α
). We find that the relations for low-
z
CSFGs with high equivalent widths of the H
β
emission line, EW(H
β
) ≥ 100 Å, 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
M
⊙
, 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.
Quantum information-theoretical measure in terms of Shannon and Fisher entropy in conjugate position and momentum spaces provides important information about the localization/delocalization patterns ...of the inter-atomic charge density under arbitrary confining environments. In this article, we have attempted to employ such measures to the ground, excited, and the virtual states arising out of two-photon transitions (
1
s
→
n
l
;
n
=
2
-
4
,
l
=
0
,
2
) of weakly coupled classical plasma embedded H iso-electronic ions (nuclear charge,
Z
=
2
-
5
). The wavefunction for the said states is essentially a linear combination of the Slater-type orbitals, the coefficients of which are generated from a fourth-order time-dependent perturbation theory within the variational framework. A complementary nature has been noted in the Shannon and Fisher measures versus the plasma screening parameter plot in the conjugate spaces. A novel scaling law has been proposed to replicate the variation of the Shannon and Fisher entropy
w.r.t.
Z
for the virtual as well as real 2
p
states of free and plasma confined ions.
Graphic abstract
Abstract
The quantum information theoretic measures in terms of Shannon entropy and Fisher entropy (both in position and momentum spaces) on the ground, excited as well as virtual states arising out ...of the two-photon transitions (1
s
→
nl
;
n
= 2 − 4,
l
= 0, 2) of H atom embedded in classical weakly coupled plasma environment are done for the first time. Fourth order time dependent perturbation theory is adopted within a variational framework for calculating the two photon excitation energies and their respective wavefunctions from an analysis of the pole positions of the non linear response of the system. The representation of virtual state follows from an analysis of the linear response at such poles using a novel method developed by us. Ground and perturbed state wave functions of appropriate symmetries are represented by linear combination of Slater-type orbitals. The analytic form of the momentum space wave functions of ground, excited and virtual states are determined by taking Fourier transformation of the respective position space wave functions. The quantum information measures give interesting insights on the delocalization patterns of the all the real and virtual states under question
w.r.t.
the increase in plasma strength. The estimated data values are found to be in excellent agreement with the few existing in literature for the ground as well as excited states participating in the two-photon transitions. Such data for the virtual states are completely new and can be set as benchmark for future works in related disciplines.
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