We present new or improved oxygen abundances (O/H) for the nearby Lynx-Cancer void updated galaxy sample. They are obtained via the SAO 6-m telescope spectroscopy (25 objects), or derived from the ...Sloan Digital Sky Survey spectra (14 galaxies, of which for seven objects O/H values were unknown). For eight galaxies with detected O iii λ4363 line, O/H values are derived via the direct (T
e) method. For the remaining objects, O/H was estimated via semi-empirical and empirical methods. For all accumulated O/H data for 81 galaxies of this void (with 40 of them derived via T
e method), their relation ‘O/H versus M
B’ is compared with that for similar late-type galaxies from denser environments (the Local Volume ‘reference sample’). We confirm our previous conclusion derived for a subsample of 48 objects: void galaxies show systematically reduced O/H for the same luminosity with respect to the reference sample, in average by 0.2 dex, or by a factor of ∼1.6. Moreover, we confirm the fraction of ∼20 per cent of strong outliers, with O/H of two to four times lower than the typical values for the ‘reference’ sample. The new data are consistent with the conclusion on the slower evolution of the main void galaxy population. We obtained Hα velocity for the faint optical counterpart of the most gas-rich (M(H i)/L
B = 25) void object J0723+3624, confirming its connection with the respective H i blob. For similar extremely gas-rich dwarf J0706+3020, we give a tentative O/H ∼(O/H)⊙/45. In Appendix A, we present the results of calibration of semi-empirical method by Izotov & Thuan and of empirical calibrators by Pilyugin & Thuan and Yin et al. on the sample of ∼150 galaxies from the literature with O/H measured by T
e method.
ABSTRACT
In the framework of an ongoing project aimed at searching for and studying eXtremely Metal-Poor (XMP) very gas-rich blue dwarfs in nearby voids, we conducted spectroscopy with the 11-m ...Southern African Large Telescope (SALT) of 26 candidates, preselected in the first paper of this series (PEPK19). For 23 of them, we detected oxygen lines, allowing us to estimate the gas O/H ratio. For 10 of them, the oxygen abundance is found to be very low, in the range of 12 + log (O/H) = 6.95–7.30 dex. Of those, four void dwarfs have 12 + log (O/H) < 7.19, or Z < Z⊙/30. For the majority of observed galaxies, the faint line O iii λ4363 Å used to estimate O/H with the direct Te method appeared either too noisy or was not detected. We therefore use the semi-empirical method of Izotov & Thuan for these spectra, or, when applicable, the new ‘Strong line’ method of Izotov et al. We present and discuss the results for all void dwarfs observed in this work. We also compare their O/H values with O/H values of ∼140 void galaxies available from our recent papers. We address the properties of the newly found unusual void XMP dwarfs and compare them with those for 10 known prototype void XMP objects. The latter small group is outstanding based on their very small mass fraction of stars (only 0.01–0.02 of the baryonic mass), the blue colours of stars in the outer body indicating a non-cosmological age for the main star-forming episode), and the low gas metallicity (several times lower than expected for their luminosity).
ABSTRACT
In the framework of the ongoing project, aimed at the systematical studying galaxies in nearby voids, we conducted spectroscopy with the Southern African Large Telescope (SALT) of 62 objects ...from the Nearby Void Galaxy (NVG) sample. They include 8 remaining objects of the 60 pre-selected candidates to eXtremely Metal-Poor (XMP) dwarfs, two known void XMP dwarfs and 52 void dwarfs residing within the Local Volume. For 47 galaxies residing in the nearby voids, we obtained spectra of the diverse quality. For 42 of them, we detected the Hydrogen and Oxygen lines that allowed us to get estimates of O/H in the observed H ii regions. For 12 of the 42 objects, we detected the faint line O iiiλ4363, that allowed us to directly derive the electron temperature Te and obtain their gas O/H by the direct method. 14 objects with the undetected O iiiλ4363 line fall to the lowest metallicities range (12+log(O/H) ≲ 7.5 dex). For them, we use a carefully checked new empirical ‘Strong line’ method of Izotov et al. For 14 other objects with only strong lines detected and with 12+log(O/H) of ∼7.5–8.0 dex, we used the modified version of ‘semi-empirical’ method of Izotov and Thuan. It accounts for effect of the excitation parameter O32 on Te. 16 new galaxies are found with parameter 12+log(O/H) ≲ 7.39 dex. Of them, four have 12+log(O/H) = 7.07–7.20 dex. Of the 60 observed NVG objects, 15 have mistaken radial velocities in HyperLEDA. They do not reside in the nearby voids.
ABSTRACT
We introduce a project aimed at systematically searching for eXtremely Metal-Poor (XMP) very gas-rich blue dwarfs in voids in the nearby Universe. Several such galaxies were first identified ...in the course of an unbiased study of the galaxy population in the nearby Lynx-Cancer void. These very rare and unusual galaxies appear to be the best proxies for the so-called Very Young Galaxies defined recently in the model simulations by Tweed et al. (2018). We discuss the main properties of 10 prototype objects residing in nearby voids and formulate criteria to search for similar dwarfs in other voids. The recently published sample of 1354 Nearby Void Galaxies is used to identify a subsample of 60 void dwarf XMP candidates. We provide a list of these XMP candidates with their main parameters and finding charts. These candidates are the subjects of subsequent spectral, photometric, and H i studies in the accompanying papers. Looking ahead, with reference to the submitted accompanying papers, we find that this study results in the discovery of many new XMP dwarfs with 12+log (O/H) ∼ 7.0–7.3 dex.
ABSTRACT
We present the second part of results of the ongoing project aimed at searching for and studying eXtremely Metal-Poor (XMP) – adopted as those with Zgas ≲ Z⊙/30, or with 12+log (O/H) ≲ ...7.21 dex − very gas-rich blue dwarfs in voids. They were first identified in the course of the ‘unbiased’ study of the galaxy population in the nearby Lynx–Cancer void. These very rare and unusual galaxies seem to be the best proxies of so-called Very Young Galaxies (VYGs) defined recently in model simulations by Tweed et al. To date, for 16 pre-selected void XMP candidates, using the Big Telescope Alt-azimuth (BTA), the SAO 6-m telescope, we have obtained spectra suitable for the determination of O/H. For majority of the observed galaxies, the principal line O iii λ4363 , used for the direct classical Te method of O/H determination, is undetected. Therefore, to estimate O/H, we use a new ‘strong-lines’ method by Izotov et al. This appears to be the most accurate empirical O/H estimator for the range of 12+log (O/H) ≲ 7.4–7.5. For objects with higher O/H, we use the semi-empirical method by Izotov & Thuan with our modification accounting for variance of the excitation parameter O32. Six of those 16 candidates are found, with confidence, to be XMP dwarfs. In addition, eight studied galaxies are less metal-poor, with 12+log (O/H) = 7.24–7.33, and these can also fall into the category of VYG candidates. Taking into account our recently published work and the previously known (nine prototype galaxies) XMP gas-rich void objects, the new findings increase the number of this type of galaxy known to date to a total of 19.
ABSTRACT
KK242 is a LV dwarf of transition type residing in the void environment. Koda et al. present clear indications on its connection with Scd galaxy NGC 6503. This implies the distance to KK242 ...of ∼6.3 Mpc and its MB = −10.5 mag. Its radial velocity, known from the Effelsberg radio telescope H i observations, reveals, however, the difference with that of NGC 6503, ΔV ∼ 400 km s−1. If real, this fact implies the substantial constraints on its origin. To clear-up the issue of KK242 radial velocity, we obtained with the SAO 6-m telescope spectra of its faint star-forming (SF) complex. H α and H β emission is detected in two adjacent compact regions, the southern and northern, separated by ∼2 arcsec (∼60 pc). Their mean radial velocity is Vhel = −66 km s−1, ∼100 km s−1 lower than that of NGC 6503. We use the HST Legacy Archive images and photometry of individual stars from the Extragalactic Distance Database, available for KK242, to identify in the SF complex the exciting hot stars, the probable BHeB and RHeB stars and a supernova remnant. We address, based on the possible range of its gas metallicity, the probable evolutionary paths of KK242. Using package cloudy and parameters of the exciting B0V stars, we conclude that the observed flux ratio of the S ii doublet to H α is consistent with the value of 12+log (O/H) ∼7.35 ± 0.18 dex, expected for a stripped void dIrr galaxy.
We present a detailed study of the extremely isolated Sdm galaxy UGC 4722 (M
B = −17.4) located in the nearby Lynx-Cancer void. UGC 4722 is a member of the Catalogue of Isolated Galaxies, and has ...also been identified as one of the most isolated galaxies in the Local Supercluster. Optical images of the galaxy however show that it has a peculiar morphology with an elongated ∼14 kpc-long plume. New observations with the Russian 6-m telescope (BTA) and the Giant Metrewave Radio Telescope (GMRT) of the ionized and neutral gas in UGC 4722 reveal the second component responsible for the disturbed morphology of the system. This is a small, almost completely destroyed, very gas-rich dwarf (M
B = −15.2, M(H i)/L
B ∼ 4.3) We estimate the oxygen abundance for both galaxies to be 12 + log (O/H) ∼ 7.5–7.6 which is two to three times lower than what is expected from the luminosity–metallicity relation for similar galaxies in denser environments. The ugr colours of the plume derived from Sloan Digital Sky Survey (SDSS) images are consistent with a simple stellar population with a post starburst age of 0.45–0.5 Gyr. This system hence appears to be the first known case of a minor merger with a prominent tidal feature consisting of a young stellar population.
ABSTRACT
We introduce a project aimed at systematically searching for eXtremely Metal-Poor (XMP) very gas-rich blue dwarfs in voids in the nearby Universe. Several such galaxies were first identified ...in the course of an unbiased study of the galaxy population in the nearby Lynx-Cancer void. These very rare and unusual galaxies appear to be the best proxies for the so-called Very Young Galaxies defined recently in the model simulations by Tweed et al. (2018). We discuss the main properties of 10 prototype objects residing in nearby voids and formulate criteria to search for similar dwarfs in other voids. The recently published sample of 1354 Nearby Void Galaxies is used to identify a subsample of 60 void dwarf XMP candidates. We provide a list of these XMP candidates with their main parameters and finding charts. These candidates are the subjects of subsequent spectral, photometric, and H i studies in the accompanying papers. Looking ahead, with reference to the submitted accompanying papers, we find that this study results in the discovery of many new XMP dwarfs with 12+log (O/H) ∼ 7.0–7.3 dex.
This paper presents new results from the ongoing study of the unusual Lynx-Cancer void galaxy DDO 68, which has star-forming regions of record low metallicity 12+log (O/H) ~7.14. The results include ...the following. (i) A new spectrum and photometry have been obtained with the 6-m SAO RAS telescope (BTA) for the luminous blue variable (LBV = DDO68-V1). Photometric data sets were complemented with others based on the Sloan Digital Sky Survey (SDSS) and the Hubble Space Telescope (HST) archive images. (ii) We performed an analysis of the DDO 68 supergiant shell (SGS) and the prominent smaller Ha arcs/shells visible in the HST image coupled with kinematic maps in Ha obtained with the Fabry-Perot interferometer (FPI) at the BTA. (iii) We compiled a list of about 50 of the most luminous stars (-9.1 mag < MV < -6.0 mag) identified from the HST images associated with the star-forming regions with known extremely low O/H. This is intended to pave the path for the current science to be investigated with the next generation of giant telescopes. We have confirmed earlier hints of significant variation of the LBV optical light, deriving its amplitude as ... mag for the first time. New data suggest that in 2008-2010 the LBV reached MV = -10.5 mag and probably underwent a giant eruption. We argue that the structure of star-forming complexes along the SGS ('Northern Ring') perimeter provides evidence for sequential induced star-formation episodes caused by the shell gas instabilities and gravitational collapse. The variability of some luminous extremely metal-poor stars in DDO 68 can currently be monitored with medium-size telescopes at sites with superb seeing. (ProQuest: ... denotes formulae/symbols omitted.)
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