Dwarfs in nearby voids: results of SALT spectroscopy

Voids represent the most rarefied and the largest volume elements of the Large-Scale Structure of the Universe. Despite galaxies residing in voids comprise a relatively small fraction of the whole realm of galaxies (van de Weygaert, 2016), they are the important ’bricks’ of our picture of the general galaxy evolution. Thus, Aragon-Calvo & Szalay (2013) predict the unusual properties of void structures and their galaxies due to the ’time machine and cosmic microscope’ effects of the void environment.

A more detailed modelling of the void galaxy evolution is necessary to compare their predicted properties with the observed ones. In particular, one needs the high resolution simulations in order to address the small dwarfs with the baryonic mass of 10⁶ – some 10⁸ M_⊙. Namely for this mass range, the observations (see below) demonstrate significant differences with the properties of galaxies in denser environments.

The effect of environment on evolution is expected to be stronger for the lower mass objects. Since all known wide-area spectral sky surveys have the similar limiting apparent magnitude of B (or $r$) of $\sim$18.5, to have a representative galaxy sample to, e.g., M_B = –10 to –12, one should explore the volume with the distances of R $\lesssim$ 20–25 Mpc. This choice differs substantially from many studies of void galaxy samples in more distant voids ($\sim$80–200 Mpc), which probe properties of only the upper part of the galaxy luminosity function (e.g., Rojas et al., 2005; Patiri et al., 2006; Kreckel et al., 2012, and references therein).

An alternative approach related to the study of galaxies in the nearby voids, was suggested and partly realised by our group. The first step in this direction was the formation and studying of the galaxy sample in the nearby void Lynx-Cancer (Pustilnik & Tepliakova, 2011; Pustilnik et al., 2016, and references therein). In particular, it was found that void galaxies as a whole are less evolved in average as compared to the reference sample of the Local Volume (LV) late-type galaxies of Berg et al. (2012). They have elevated, in average, ratio of gas mass to luminosity (M(Hi)/L_B) (Pustilnik, Martin, 2016) and the reduced gas metallicity (Pustilnik et al., 2016, 2021).

A surprising result of the above unbiased study of the whole Lynx-Cancer void sample was the discovery of several low-luminosity dwarfs with the eXtremely Metal-Poor (XMP) Hii regions and the extremely-high gas mass fraction, f_g = M_gas/M_baryon, up to 0.97–0.99 (Pustilnik et al., 2010, 2011; Chengalur & Pustilnik, 2013; Chengalur et al., 2017). Besides, an independently discovered XMP dwarf AGC198691 (Hirschauer et al., 2016; Aver et al., 2022) appeared to reside in the same void. We adopt here the definition of XMP galaxies as objects with 12+log(O/H) $\lesssim$ 7.21 dex, or with Z(gas) $\lesssim$ Z_⊙/30¹¹1 The solar value of 12+log(O/H) is adopted to be 8.69 dex after Asplund et al. (2009).. This metallicity is close to that of the prototype of such unusual galaxies, the famous blue compact dwarf IZw18 (Searle & Sargent, 1972).

In addition, these unusual dwarfs reveal blue colours of the outer parts of a galaxy (e.g. Perepelitsyna et al., 2014; Chengalur et al., 2017), where, in the framework of the common paradigm, the most aged stellar population resides. The colours, being compared to the evolutionary tracks from PEGASE2 package (Fioc & Volmerange, 1999), correspond to the ages of the related stellar population of about one to several Gyr. That hints, along with the extremely high gas mass fraction, to the early stages of galaxy evolution.

These properties, in turn, may connect the unusual void dwarfs with the predicted so-called Very Young Galaxies (VYG) (Tweed et al., 2018), defined as objects, formed more than a half of stellar mass within the last 1 Gyr. While the other their properties are not specified in simulations, the first attempts to find such a rare population are presented by Mamon et al. (2020). Meanwhile, the comprehensive study of a void XMP dwarf IZw18 and its companion IZw18C gives a clear indication on their VYG nature (Papaderos & Östlin, 2012).

The seach for and the study of XMP dwarf galaxies was in focus of many groups since the discovery of the record low metallicity of IZw18. We already discussed recently the continuing efforts and findings of such unusual galaxies in the previous papers of this series. Therefore we refer a reader to these texts. The only remarks we would add to this issue, are the recent results of the search for new XMP dwarfs within the nearby voids (Pustilnik et al., 2020b, 2021). They appeared very promising, since the number of known such objects was doubled thanks to our conducted program during several last years.

However, one should note that the prominent XMP dwarfs in the local Universe are occasionally found also in a less rarefied environments. The known examples include ’Little Cub’ galaxy (Hsyu et al., 2017) and ’Peekaboo’ galaxy (Karachentsev et al., 2023) with 12+log(O/H) = 7.11 and 6.98 dex, respectively. This is not clear yet, whether their evolutionary paths differ from those for the similar objects in voids.

The voids themselves are not simple and homogeneous. They have the fine sub-structure resembling the Large-Scale Structure of the Universe (e.g. Gottlöber et al., 2003; Aragon-Calvo & Szalay, 2013). This sub-structure contains nodes, filaments and sub-voids. Therefore, one expects that properties of void galaxies can vary, in particular due to the different effects of their in-void local environments.

The first results, briefly summarised above, appeal to a much more massive studying of void dwarf galaxies. To address the diversity of void galaxies and to have an opportunity to compare cosmological simulations with the real void objects, one needs properties for a sample of many hundreds to thousands galaxies within the limited volume, unbiased as much as possible.

The natural questions related to such studies are the following: a) how much properties of void galaxies can differ from those in a more typical environment? b) how often properties of void galaxies significantly differ from those of a more common population? c) what empirical relations exist between the properties of void galaxies and other factors. The answers to these questions can help us to understand possible differences in formation and evolution scenarios of void galaxies from those in a more common environment.

As a development of the observational study of dwarf void galaxies, in order to increase substantially the statistics of such objects with known evolutionary parameters, Z(gas) and gas mass fraction, we formed a sample of 25 voids in the volume with R $<$ 25 Mpc from the Local Group. We separate 1354 galaxies within this volume ($\sim$20 percent of the total number known in this volume) which fall within these nearby voids. We refer to this sample as the ’Nearby Void Galaxy’ (NVG) sample (Pustilnik, Tepliakova & Makarov, 2019, hereafter PTM19). The sample galaxies were collected mostly from the HyperLEDA data base (Makarov et al., 2014).

The main parameters of the parent NVG sample are as follows. The range of the absolute blue magnitudes is of M_B = –7.5 to –20.5 mag, with the median value of about –15.2 mag. The great majority of the NVG sample are the late-type galaxies (late spirals and dwarf irregulars), with the major fraction of the low surface brightness (LSB) galaxies. Similar to the wide range of luminosities, the distribution on the hydrogen mass in void galaxies is also very broad, with the full range of M(Hi) of 10^5.5 – 10^9.7 M_⊙, with the median M(Hi) $\sim$10^8.5 M_⊙. The parameter of gas richness, the ratio M(Hi)/L_B of about a thousand NVG sample galaxies with known Hi data, spreads from 0.03 to 26, with the median of $\sim$1.

In the procedure of separation of void galaxies, all objects, appeared in the preselected empty spheres, were assigned to the void sample. They were further divided to the ’inner’ void subsample ($\sim$80 %), and the ’outer’ subsample. The ’inner’ 1088 galaxies reside in the ’inner’ parts of voids. Their distances to the nearest luminous neighbours, delineating voids, D${}_{\rm NN}\geq$ 2.0 Mpc, with the median value of $\sim$3.4 Mpc.

The ’outer’ 242 galaxies have D${}_{\rm NN}<$ 2.0 Mpc, with the median of 1.65 Mpc. The separation of the void subsamples at D${}_{\rm NN}=$ 2.0 Mpc was chosen to be consistent with the definition of galaxies residing in the nearby Lynx-Cancer void from our previous study (Pustilnik & Tepliakova, 2011; Pustilnik et al., 2016). In that work, the border of D${}_{\rm NN}>$ 2.0 Mpc was intentively chosen so that to study the well defined void objects and to exclude possible interlopers. The study of both, the ’inner’ and the ’outer’ subsamples, will allow one to understand how many of the ’outer’ void galaxies represent on their properties an ’intermediate’ kind of the galaxy population in the context of transition between the lowest density regions and the walls, with a roughly mean matter density.

We then began two projects based on this sample. One of them is the search for new XMP void dwarfs. We selected 60 candidate XMP dwarfs from the whole NVG sample based on their known properties in the open data bases and in the literature (Pustilnik et al., 2020a). The results of spectroscopy of $\sim$3/4 of them are presented in papers Pustilnik et al. (2020b, 2021).

Another project is devoted to the study of the subsample of the NVG objects which fall within the LV. This consists of $\sim$260 objects from the NVG at the distances closer than 11 Mpc. First results on this part were presented in Pustilnik et al. (2022a, b). This is an analogue of our unbiased study of the galaxy sample in the nearby void Lynx-Cancer. The northern part of void galaxies in this project is studied at the SAO 6-m telescope BTA. The detailed description of the LV void sample, with some intermediate results, will appear in a paper in preparation.

Here we present the results of the Southern African Large Telescope (SALT; Buckley, Swart & Meiring, 2006; O’Donoghue et al., 2006) spectroscopy from the both projects. The first part includes eight of the remaining 14 dwarfs selected in Pustilnik et al. (2020a) as the void XMP candidates. The second part includes 49 galaxies selected as residing in voids within the LV, mainly the least luminous dwarfs. Also, a known XMP dwarf J0015+0104 from the Eridanus void (Guseva et al., 2009; Pustilnik et al., 2013) was observed to improve the accuracy of its O/H.

The content of the paper is arranged as follows: the description of the SALT spectral observations and data processing is presented in Sec. 2. In Sec. 3 we give the description of emission line measurements and methods used for O/H determination. The estimates of O/H for the observed galaxies are shown in Sec. 4. In Sec. 5 we discuss the obtained results along with other available information. In Sec. 6 we summarise and conclude. In the on-line supplementary materials we present the following data. In Appendix A we provide finding charts of all observed galaxies with the spectrograph long slit position superimposed. In Appendix B, plots of 1D spectra for the observed galaxies are presented. Appendix C presents the tables with line intensities, derived physical parameters and the values of 12+log(O/H).

For our spectral observations we used the Southern African Large Telescope (SALT; Buckley, Swart & Meiring, 2006; O’Donoghue et al., 2006) in service mode in the period from May 2019 to April 2023. Several of the 64 target galaxies were observed from two to three times in order to increase the S-to-N ratio of the ’strong’ lines in the potentially very low-metallicity objects. The obtained spectra were averaged for the subsequent analysis. See Tables 1, 2. The SALT Robert Stobie Spectrograph (RSS; Burgh et al., 2003; Kobulnicky et al., 2003) was used with VPH grating PG0900 with the long slit of 1.5″ by 8′. This option provides the range from 3600 Å to 6700 Å and the spectral resolution of FWHM$\sim$6.0 Å. For the obtained 2D spectra, we used a binning factor of four for the spatial scale and factor of two for the spectral coordinate. This gives us a final spatial sampling of 0$\aas@@fstack{\prime\prime}$51 pixel^-1 and spectral sampling of 0.97 Å pixel^-1 .

Spectral observations with the sufficiently small entrance hole or with the narrow slit are the subject of the differential atmospheric refraction (e.g., Filippenko, 1982). To minimise its effect on the wavelength-dependent light loss, the long slit position angle (PA) is recommended to keep close to the direction of the atmospheric refraction (parallactic angle). In many cases, this can substantially limit the efficiency of observations. The RSS is equipped with an Atmospheric Dispersion Compensator (ADC), what allowed us not to worry about the effect of atmospheric refraction at arbitrary long-slit PAs. Spectrophotometric standards were observed during every night as a part of the SALT standard calibrations program.

In order not to repeat literally the description of specifics of SALT observations and flux calibration, we refer a reader to Section 2 of our recent paper (Pustilnik et al., 2020b).

The majority of Hii-regions in the observed galaxies are faint and low-contrast. For the pointing, we used nearby offset stars. In most cases, we selected position angles (PAs, in degrees) of the slit so that to include a nearby offset star and to cover a faint Hii-knot in a program galaxy. See the journals of observation in Tables 1, 2. They present the main information on each observation: the dates of observations, exposure times, seeing $\theta$ (in arc seconds) and air mass.

Since many of the observed dwarfs are faint and of low surface brightness, an attempt to obtain their independent spectra can be a problem. To ease independent checks of our data, we present in Tables 1, 2 the used long slit position angles (PA). In Figures A1 – A4 of the on-line materials, we show their images with superimposed long slit positions. All but a few images are taken from the Legacy surveys database (Dey et al., 2019) The remaining several images which are unavailable in the Legacy database, are taken from the ESO Online archive (https://archive.eso.org/dss/dss).

Similar to our previous paper (Pustilnik et al., 2020b), the SALT science pipelines (Crawford et al., 2010) were used for the primary data reduction. For each CCD amplifier, they include bias and overscan subtraction, gain and cross-talk corrections and finally, mosaicing. The following long-slit reduction was conducted as described in the paper by Kniazev (2022).

The emission line fluxes obtained from 1D spectra were measured as described in detail in Kniazev et al. (2004) and briefly summarised in Pustilnik et al. (2020b). In order not to repeat this description, we refer a reader to these papers. Very briefly, the procedures include the robust drawing of continuum with the subsequent use of MIDAS²²2MIDAS is an acronym for the European Southern Observatory package – Munich Image Data Analysis System.-based programs for determination of parameters of emission lines.

Giving the line fluxes measured, we utilise an iterative procedure from Izotov et al. (1994), which accounts for the total sight-line dust extinction and the underlying Balmer line absorptions originating in the related young stellar clusters. As a result, it provides the simultaneous estimate of the equivalent width of absorption Balmer lines $EW(abs)$ and the extinction coefficient $C(H\beta)$. The relevant equation (1) from Izotov et al. (1994) was used:

Here $I(\lambda)$ is the intrinsic line flux corrected for the overall extinction (both in the Milky Way and internal to a particular galaxy or Hii region) and the underlying Balmer absorption, while $F(\lambda)$ is the measured line flux. Here, as in Izotov et al. (1994), $EW_{e}(\lambda)$ are equivalent widths of used emission lines. $EW_{a}(\lambda)$ is the adopted value of the underlying Balmer absorptions. The theoretical Balmer line ratios I($\lambda$)/I(H$\beta$) for Case B from Brocklehurst (1971) were used for iterative procedure with equation (1).

Following to Izotov et al. (1994), this term is used to estimate intrinsic fluxes only for Balmer emission lines. Similar to this paper, we adopt the reddening function $f(\lambda)$ from Whitford (1958). It is normalised so that $f(H\beta)$ = 0. For this $f(\lambda)$, there is a relation between the excess E($B-V$) and $C(H\beta)$: E($B-V$) = 0.68 $\times$ $C(H\beta)$.

In the classic (T_e) method of the oxygen abundance determination, they use the standard two-zone model and method of Aller (1984). Since the electron temperature T_e is different in high and low-ionisation Hii regions (e.g. Stasińska, 1990), to calculate abundances of ions O⁺⁺ and O⁺, one needs to know temperature T_e in both parts of an Hii region. The estimate of T_e(O⁺⁺) uses the flux ratio of [Oiii] lines: I($\lambda$4363)/I([$\lambda$4959 + $\lambda$5007) and the five-level atom model (Aller, 1984), with the electron density N_e derived from the line ratio of the [Sii] doublet: I($\lambda$6717)/I($\lambda$6731). Since in our spectra this doublet was outside the range, we adopted for further calculations N${}_{\rm e}=$ 10 cm^-3, a typical electron density in Hii regions of dIrr galaxies. To derive T_e(O⁺), we used the fit for the relation between T_e(O⁺) and T_e(O⁺⁺) from Izotov et al. (1994), based on the models of photoionised Hii regions by Stasińska (1990). This fit is read as follows:

t_e(Oii) = 0.243 + t_e(Oiii)[1.031 - 0.184 t_e(Oiii])] (2)

Here t_e = 10^-4 T_e. The resulting rms uncertainty of t_e(Oii) is transferred from the corresponding rms error of t_e(Oiii) as derived with use of the above relation. The ionic abundances of O⁺/H⁺ and O⁺⁺/H⁺ are calculated with formulae (3) and (5) from Izotov et al. (2006).

The major part of Hii regions in the studied galaxies are rather faint. Accordingly, the fluxes of their emission lines are low. The weak auroral line, [Oiii]$\lambda$4363 Å, which is used in the ’direct’ (T_e) method for the determination of the electron temperature T_e, was detected only in the minority of our targets. Therefore, for the estimate of O/H in the remaining galaxies, we applied the semi-empirical method of Izotov & Thuan (2007). However, as was shown in Pustilnik et al. (2021), this method provides the reliable estimates of T_e in rather limited range of parameters of Hii regions. We modified their method, as described in Pustilnik et al. (2021), in order to expand its applicability to the wider range of physical conditions.

The basement of method of Izotov & Thuan (2007) is the fitted empirical dependence between the values of the electron temperature T_e and parameter $R_{\mathrm{2}3}$. It was derived from the analysis of the grid of models of Hii-regions in Stasińska & Izotov (2003). As the authors find, for the large representative sample of extragalactic Hii-regions, which cover the whole range of observed O/H, the models approximate well the apparent relations of the strong line intensities versus EW(H$\beta$). The parameter $R_{\mathrm{2}3}$ is the ratio of the sum of fluxes of strong oxygen lines [Oii]$\lambda$3727 Å, [Oiii]$\lambda$4959 Å, [Oiii]$\lambda$5007 Å to that of H$\beta$. When T_e is estimated via $R_{\mathrm{2}3}$, the rest of the calculations of the ionic abundances of O⁺/H⁺ and O⁺⁺/H⁺ uses the standard equations of the classic T_e method, that is the same formulae (3) and (5) from Izotov et al. (2006).

As it became clear later, the good approximation of O/H, derived by this method, to O/H values, obtained by the direct method, has the limited application. This is because it was fitted from a sample of Hii-regions with a rather limited range of the excitation parameter $O_{\mathrm{3}2}$ (the ratio of line fluxes of [Oiii]$\lambda$5007 and [Oii]$\lambda$3727). This parameter is a proxy of the physical parameter $U$ - so-called the ionisation parameter, characterising the flux of ionising photons per unit area of the illuminated gas cloud surrounding the central source. While the original method of Izotov & Thuan (2007) works well for the intermediate values of $O_{\mathrm{3}2}$ ($\sim$ 2–6), it leads to the significant systematical bias in the T_e and O/H estimates at both, small and large values of $O_{\mathrm{3}2}$. The whole range of $O_{\mathrm{3}2}$ in known Hii regions spreads from $\lesssim$0.3 to $\sim$50.

This effect was examined in Pustilnik et al. (2021) on the large sample of Hii-regions with O/H derived via the direct method. The sample covers the broad range of parameter $O_{\mathrm{3}2}$ and O/H. The modified formula was suggested to estimate T_e, which includes the dependence of T_e on the both parameters, $R_{\mathrm{2}3}$ and $O_{\mathrm{3}2}$. The respective method is called the modified semi-empirical (mse). The mse method has the moderate internal scatter in log(O/H) of 0.09 dex, while allows one to eliminate the systematic bias of the original method of Izotov & Thuan (2007) for the extreme values of parameter $O_{\mathrm{3}2}$.

This is crucial for our project, since the typical Hii-regions in our void galaxy sample, in which the auroral line [Oiii]$\lambda$4363 was too noisy or undetected, have rather low excitation parameter, of $O_{\mathrm{3}2}\lesssim$ 1.0. The O/H estimates derived by this method are marked as (mse) in Column 9 of Table 3.

Recently Izotov et al. (2019) suggested a new empirical method, the best suited for Hii-regions with ’metallicity’ 12+log(O/H) $\lesssim$ 7.4 dex. This uses only the relative fluxes of strong Oxygen lines with respect of H$\beta$. Namely, their Equation (1) reads as:

This relation, calibrated on the large number of Hii-regions with O/H derived via the direct (T_e) method, empirically accounts for the large scatter in the ionisation parameter log(U) (from about –4 to –1) in various Hii-regions and, thus, reduces the relatively large internal rms scatter of the other empirical methods based on the strong oxygen lines, down to only $\sim$0.04 dex. The latter value of the rms scatter was obtained in Pustilnik et al. (2021) on the sample of $\sim$70 Hii-regions with the range of 7.0 $\lesssim$ 12+$\log$(O/H) $\lesssim$ 7.5 dex. However, its applicability is limited only to the range of 12+$\log$(O/H) $\lesssim$ 7.5. The above relation was slightly modified by Izotov et al. (2021) to account Hii regions with the highest observed values of O₃₂.

Among our galaxies there are 16 objects falling to this category. For them, we use this method. For derived this way value of O/H, we attach index (s) in Column 9 of Table 3, and denote them hereafter as O/H(s).

As shown in Pustilnik et al. (2021), there is a tiny offset in the zero-point of 12+log(O/H)(s) relative to that of 12+log(O/H)(T_e), of $\sim$0.01 dex. In Table 3, we present therefore the values of 12+log(O/H)(s) with the subtraction of 0.01 dex from the values obtained with the formula from Izotov et al. (2019). This allows us to compare them directly with the estimates of 12+log(O/H)(T_e) for other galaxies.

The contribution of errors in the strong line fluxes to the related uncertainty of 12+log(O/H)(s), $\sigma_{\rm log(O/H)}$, is rather modest. For the high S-to-N spectra with $\sigma_{\rm R23}$/$R_{\mathrm{2}3}<$0.02, the total $\sigma_{\rm log(O/H)}$ is close to the internal error of the method, namely, $\sigma_{\rm log(O/H)}\sim$0.04 dex. For the lowest S-to-N our spectra of $\sigma_{\rm R23}$/$R_{\mathrm{2}3}\sim$0.14, the error of $\log$(O/H)(s) increases to 0.12 dex. Intermediate S-to-N of $R_{\mathrm{2}3}$ results in the typical $\sigma_{\rm log(O/H)}$ = 0.05 – 0.06 dex.

As described in the Introduction, this work includes the observational results on the ionised gas metallicity from two different projects: a) the search for new XMP dwarfs among galaxies in the NVG sample, and b) the unbiased study of all void galaxies in the LV, that is about 260 the nearest galaxies of the whole NVG sample of 1354 objects. We summarise the respective results below.

In the previous papers (Pustilnik et al., 2020b, 2021) we presented results of spectroscopy for 46 objects from the sample of 60 preselected candidate XMP void objects from the fainter part ($M_{\rm B}\gtrsim-14.3$) of the Nearby Void Galaxies (NVG) sample. Ten of them were found to be the XMP objects, that is having 12+log(O/H) $\lesssim$ 7.21 dex. 13 more new void galaxies were found somewhat less metal-poor, with 12+log(O/H) $\lesssim$ 7.35 dex.

In this work we observed 8 galaxies of the remaining 13 XMP candidates and reobserved one XMP dwarf from Pustilnik et al. (2020b). Two new XMP dwarfs are found of these eight objects. Two additional dwarf galaxies with the Oxygen abundance of 12+log(O/H) $\lesssim$ 7.35 dex are found as well. Two known XMP void dwarfs were reobserved to improve the accuracy of their metallicity.

The rest 52 observed galaxies are selected to reside in the LV. They represent mostly a fainter part of the total subsample of the LV void galaxies. The currently updated list comprises of 243 LV void objects. Of 39 objects with the well detected strong lines, nine new void galaxies appeared to be very metal-poor, with 12+log(O/H) $\lesssim$ 7.39 dex. Of them, two more XMP dwarfs are found, with 12+log(O/H) = 7.07 and 7.20 dex.

Summarising the results, presented here and the related discussion, we draw the following conclusions:

1. 1.

   The spectroscopy of 8 remaining ’candidate’ XMP dwarfs from the sample of 60 objects residing in the nearby voids, results in the discovery of two additional XMP dwarfs, with 12+$\log$(O/H) $\sim$7.19 – 7.20 dex ($\sim$Z_⊙/30).

2. 2.

   The almost completed program of search for void XMP dwarfs results in 11 new objects with Z_gas = Z_⊙/50 – Z_⊙/30, that comprises $\sim$20 per cent of the selected candidates. In addition, 23 very low-metallicity dwarfs (Z_gas = Z_⊙/30 – Z_⊙/20) are found, that is $\sim$40 per cent of the initial list. This finding increases substantially the number of the known nearby very metal-poor dwarfs and allows us to conduct a deeper study of their individual and group properties and of their possible relation to Very Young Galaxies.

3. 3.

   The first results are presented for the on-going project of the studying a subsample of 243 NVG galaxies residing in the Local Volume. Of 52 observed objects only 37 appeared in real to reside in the nearby voids, while the rest 15 had the wrong radial velocities in HyperLEDA.

4. 4.

   The strong lines of Oxygen are detected and its abundance is estimated in 32 of these 37 NVG objects, with the total range of 12+log(O/H) of 7.07 dex to $\sim$8.0 dex.

5. 5.

   The addition of the other available data on the LV void dwarfs allows us to probe the relation between 12+log(O/H) and M_B on a sample of $\sim$90 objects. The general trend of the reduced metallicity for a given luminosity, already known from our earlier studies of the nearby void galaxies, is also well seen on the LV void galaxies. Besides, on this compilation of the LV void galaxies, we find a large scatter in O/H for a fixed M_B. Partly, the reason of this effect can be the presence in the LV void sample of galaxies in the ’bordering’ regions of voids.

6. 6.

   The studied void galaxies show a wide range of the star-forming activity. While the most typical dwarfs, dIrrs and late Spirals, show several SF regions, we find four new dTr galaxies with the substantial neutral gas reservoir and the absent or very subtle signs of the current/recent SF.

7. 7.

   Two Local Volume Hi objects, HIPASS J1614–72 and AGC208329 appear to have a mistaken optical identification. The search for their alternative optical counterparts did not reveal potential candidates. Since the full census of the LV population has the important implications for comparison of the LV with cosmological simulations, the understanding of the nature of these Hi objects will take special efforts, including their Hi-mapping and the improvement of their position accuracy.

This work is based on observations obtained with the Southern African Large Telescope (SALT), programs 2017-2-MLT-001, 2020-2-MLT-005, 2022-1-MLT-003 (PI: Kniazev). The reported study was funded by Russian Science Foundation according to the research project 22-22-00654. AYK acknowledges support from the National Research Foundation (NRF) of South Africa. We thank the anonymous reviewer for the useful comments and suggestions, which allowed us to improve the content and clarity of the paper. The use of the HyperLEDA database is greatly acknowledged. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We also acknowledge the great effort of the ALFALFA team which opened access to the nearby Universe gas-rich dwarfs with low or moderate SFR and thus helped us to identify the majority of very low metallicity galaxies of this study.

We also acknowledge the use of the SDSS and Legacy surveys databases. Funding for the Sloan Digital Sky Survey (SDSS) has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions.

The DESI Legacy Imaging Surveys consist of three individual and complementary projects: the Dark Energy Camera Legacy Survey (DECaLS), the Beijing-Arizona Sky Survey (BASS), and the Mayall z-band Legacy Survey (MzLS). DECaLS, BASS and MzLS together include data obtained, respectively, at the Blanco telescope, Cerro Tololo Inter-American Observatory, NSF’s NOIRLab; the Bok telescope, Steward Observatory, University of Arizona; and the Mayall telescope, Kitt Peak National Observatory, NOIRLab. NOIRLab is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. Pipeline processing and analyses of the data were supported by NOIRLab and the Lawrence Berkeley National Laboratory (LBNL). Legacy Surveys also uses data products from the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), a project of the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. Legacy Surveys was supported by: the Director, Office of Science, Office of High Energy Physics of the U.S. Department of Energy; the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility; the U.S. National Science Foundation, Division of Astronomical Sciences; the National Astronomical Observatories of China, the Chinese Academy of Sciences and the Chinese National Natural Science Foundation. LBNL is managed by the Regents of the University of California under contract to the U.S. Department of Energy.

The data underlying this article are available in Appendices A, B and C, which are available only in the on-line supplementary materials of the paper.