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تسجيل مستخدم جديدXMP gas-rich dwarfs in nearby voids: results of BTA spectroscopy
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We present the second part of results of the on-going project of searching for and studying eXtremely Metal-Poor (XMP, adopted as those with Z(gas) <~ Zo/30, or with 12+log(O/H) <~ 7.21~dex) very gas-rich blue dwarfs in voids.They were first identified in course of the unbiased study of 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 preselected void XMP candidates, we obtained with the SAO 6-m telescope (BTA) spectra suitable for determination of O/H. For majority of the observed galaxies, the principal line [OIII]4363 used for the direct classical T_e 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 the most accurate empirical O/H estimator for the range of 12+log(O/H) < 7.4-7.5. For higher O/H objects, we use the semi-empirical method by Izotov and Thuan with our modification accounting for variance of the excitation parameter O32. Six of those 16 candidates are found to be the confident XMP dwarfs. In addition, eight studied galaxies are somewhat less metal-poor, with 12+log(O/H) = 7.24-7.33. They also can fall into the category of VYG candidates. With account of the recently published by us and previously known (9 prototype galaxies) XMP gas-rich void objects, the new findings increase the number of this type galaxies to the total of 19.
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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 (VYGs) defined recently in the model simulations by Tweed et al. (2018). We discuss the main properties of ten 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 (NVG) 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 HI 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.
The intermediate results of the ongoing study of deep samples of ~200 galaxies residing in nearby voids, are presented. Their properties are probed via optical spectroscopy, ugri surface photometry, and HI 21-cm line measurements, with emphasis on th
eir evolutionary status. We derive directly the hydrogen mass M(HI), the ratio M(HI)/L_B and the evolutionary parameter gas-phase O/H. Their luminosities and integrated colours are used to derive stellar mass M(*) and the second evolutionary parameter -- gas mass-fraction f_g. The colours of the outer parts, typically representative of the galaxy oldest stellar population, are used to estimate the upper limits on time since the beginning of the main SF episode. We compare properties of void galaxies with those of the similar late-type galaxies in denser environments. Most of void galaxies show smaller O/H for their luminosity, in average by ~30%, indicating slower evolution. Besides, the fraction of ~10% of the whole void sample or ~30% of the least luminous void LSB dwarfs show the oxygen deficiency by a factor of 2--5. The majority of this group appear very gas-rich, with f_g ~(95-99)%, while their outer parts appear rather blue, indicating the time of onset of the main star-formation episode of less than 1-4 Gyr. Such unevolved LSBD galaxies appear not rare among the smallest void objects, but turned out practically missed to date due to the strong observational selection effects. Our results evidense for unusual evolutionary properties of the sizable fraction of void galaxies, and thus, pose the task of better modelling of dwarf galaxy formation and evolution in voids.
Half-dozen of extreme representatives of void dwarf galaxy population were found in our study of evolutionary status of a hundred galaxies in the nearby Lynx-Cancer void. They are very gas-rich, extremely low-metallicity [7.0 < 12+log(O/H)< ~7.3] obj
ects, with blue colours of outer parts. The colours indicate the ages of the oldest visible stellar population of one to a few Gyr. They all are intrinsically faint, mostly Low Surface Brightness dwarfs, with M_B range of -9.5 to -14 mag. Thus, their finding is a subject of the severe observational selection. The recent advancement in search for such objects in other nearby voids resulted in doubled their total number. We summarize all available data on this group of unusual void dwarf galaxies and discuss them in the general context of very low metallicity galaxies and their possible formation and evolutionary scenarios.
We present the first results of a project aimed at searching for gas accretion events and interactions between late-type galaxies in the void environment. The project is based on long-slit spectroscopic and scanning Fabry-Perot interferometer observa
tions performed with the SCORPIO and SCORPIO-2 multimode instruments at the Russian 6-m telescope, as well as archival multiwavelength photometric data. In the first paper of the series we describe the project and present a sample of 18 void galaxies with oxygen abundances that fall below the reference `metallicity-luminosity relation, or with possible signs of recent external accretion in their optical morphology. To demonstrate our approach, we considered the brightest sample galaxy NGC 428, a late-type barred spiral with several morphological peculiarities. We analysed the radial metallicity distribution, the ionized gas line-of-sight velocity and velocity dispersion maps together with WISE and SDSS images. Despite its very perturbed morphology, the velocity field of ionized gas in NGC 428 is well described by pure circular rotation in a thin flat disc with streaming motions in the central bar. We also found some local non-circular gas motions clearly related to stellar feedback processes. At the same time, we revealed a circumnuclear inclined disc in NGC 428 and a region with significant residual velocities that could be considered as a result of a recent (<0.5 Gyr) accretion event. The observed oxygen abundance gradient does not contradict this conclusion.
We present the detection of molecular gas using CO(1-0) line emission and follow up Halpha imaging observations of galaxies located in nearby voids. The CO(1-0) observations were done using the 45m telescope of the Nobeyama Radio Observatory (NRO) an
d the optical observations were done using the Himalayan Chandra Telescope (HCT). Although void galaxies lie in the most under dense parts of our universe, a significant fraction of them are gas rich, spiral galaxies that show signatures of ongoing star formation. Not much is known about their cold gas content or star formation properties. In this study we searched for molecular gas in five void galaxies using the NRO. The galaxies were selected based on their relatively higher IRAS fluxes or Halpha line luminosities. CO(1--0) emission was detected in four galaxies and the derived molecular gas masses lie between (1 - 8)E+9 Msun. The H$alpha$ imaging observations of three galaxies detected in CO emission indicates ongoing star formation and the derived star formation rates vary between from 0.2 - 1.0 Msun/yr, which is similar to that observed in local galaxies. Our study shows that although void galaxies reside in under dense regions, their disks may contain molecular gas and have star formation rates similar to galaxies in denser environments.
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