Nearby Lynx-Cancer Void Research Articles

XMP gas-rich dwarfs in nearby voids: results of BTA spectroscopy

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.

XMP gas-rich dwarfs in nearby voids: candidate selection

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.

Very gas-rich extremely metal-poor blue void dwarfs

AbstractHalf-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] objects, 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 MB range of –9.5m to –14m. 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.

Nearby void dwarf galaxies: recent results, the ongoing project and prospects

AbstractProperties of dwarf galaxies formed and evolved in the lowest density environment remain largely unexplored and poorly understood. Especially this concerns the low-mass end (Mbar < 109M⊙). We overview the results of systematic study of a hundred void dwarfs from the nearby Lynx-Cancer void. We describe the ongoing project aiming to form Nearby Void galaxy sample (R < 25 Mpc) over the whole sky. 1354 objects with distances less than 25 Mpc fall within 25 voids delineated by 460 luminous galaxies/groups. The void major sizes range from 13 to 37 Mpc. 1088 of 1354 void galaxies reside deeply in voids, having distances to the nearest luminous neighbour of 2–11 Mpc. 195 nearest void galaxies reside in the Local Volume. We summarize the main statistical properties of the new sample and outline the prospects of study of both, the void dwarf properties and the fine structure of voids.

Unusual void galaxy DDO 68: implications of theHST-resolved photometry

DDO68 (UGC5340) is an unusual dwarf galaxy with extremely low gas metallicity (12+log(O/H) = 7.14) residing in the nearby Lynx-Cancer void. Despite its apparent isolation, it shows both optical and HI morphological evidence for strong tidal disturbance. Here, we study the resolved stellar populations of DDO68 using deep images from the HST archive. We determined a distance of 12.75+-0.41 Mpc using the tip of the red giant branch (TRGB). The star formation history reconstruction reveals that about 60 per cent of stars formed during the initial period of star formation, about 12-14 Gyr ago. During the next 10 Gyr DDO68 was in the quenched state, with only slight traces of star formation. The onset of the most recent burst of star formation occurred about 300 Myr ago. We find that young populations with ages of several million to a few hundred million years are widely spread across various parts of DDO68, indicating an intense star formation episode with a high mean rate of 0.15 Msun/yr. A major fraction of the visible stars in the whole system (~80 per cent) have low metallicities: Z = Zsun/50 - Zsun/20. The properties of the northern periphery of DDO68 can be explained by an ongoing burst of star formation induced by the minor merger of a small, gas-rich, extremely metal-poor galaxy with a more typical dwarf galaxy. The current TRGB-based distance of DDO68 implies a total negative peculiar velocity of ~500 km/s.

Study of galaxies in the Lynx-Cancer void

Context. Void population consists mainly of late-type and low surface brightness (LSB) dwarf galaxies whose atomic hydrogen is the main component of their baryonic matter. Therefore, observations of void galaxy HI are mandatory in order to understand their evolution and dynamics. Aims. Our aim was to obtain integrated HI parameters for a fainter part of the nearby Lynx-Cancer void galaxy sample (total of 45 objects) with the Nancay Radio Telescope (NRT) and to conduct the comparative analysis of all the 103 void galaxies with known HI data with a sample of similar galaxies residing in denser environments of the Local Volume. Methods. For HI observations we used the NRT with its sensitive antenna/receiver system FORT and standard processing. The comparison of the void and control samples on the parameter M(HI)/L_B is conducted with the non-parametric method `The 2x2 Contingency Table test'. Results. We obtained new HI data for about 40% of the Lynx-Cancer galaxy sample. Along with data from the literature, we use these new data for further analysis of 103 void objects. The proxy of the evolutional parameter M(HI)/L_B of the void sample is compared with that of 82 galaxies of morphological types 8--10 residing in the Local Volume (LV) groups and aggregates. Conclusions. At the confidence level of P = 0.988, we conclude that for the same luminosity, these void galaxies are systematically gas-richer, on average by ~39%. This result is consistent with the authors' earlier conclusion on the smaller gas metallicities and evidence for the slower low-mass galaxy evolution in voids.

UGC 3672: an unusual merging triplet of gas-rich galaxies in the Lynx–Cancer void

We present HI 21cm and optical observations of UGC 3672 which is located near the centre of the nearby Lynx-Cancer void. We find that UGC 3672 consists of an approximately linearly aligned triplet of gas rich dwarfs with large scale velocity continuity along the triplet axis. The faintest component of the triplet is extremely gas-rich (MHI/LB ~ 17) and also extremely metal deficient (12+log(O/H) ~ 7.0). The metallicity of this dwarf is close to the 'floor' observed in star forming galaxies. Low resolution HI images show that the galaxy triplet is located inside a common HI envelope, with fairly regular, disk like kinematics. At high angular resolution however, the gas is found to be confined to several filamentary tidal tails and bridges. The linear alignment of the galaxies, along with the velocity continuity that we observe, is consistent with the galaxies lying along a filament. We argue that the location of this highly unusual system in an extremely low density environment is not a coincidence, but is a consequence of structure formation proceeding more slowly and also probing smaller scales than in regions with average density. Our observations also indicate that wet mergers of galaxies flowing along filaments is a possible pathway for the formation of gas rich disks. The UGC 3672 system provides an interesting opportunity to study the kind of interactions typical between high redshift extremely gas rich unevolved small systems that lie at base of the hierarchical galaxy formation model.

Study of the Lynx-Cancer void galaxies. – V. The extremely isolated galaxy UGC 4722

We present a detailed study of the extremely isolated Sdm galaxy UGC4722 (M_B = -17.4) located in the nearby Lynx-Cancer void. UGC4722 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 ionised and neutral gas in UGC4722 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_HI/L_B ~4.3). We estimate the oxygen abundance for both galaxies to be 12+log(O/H) ~ 7.5-7.6, which is 2-3 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.

Study of galaxies in the Lynx-Cancer void. IV. Photometric properties

We present the results of a photometric study of 85 objects from the updated sample of galaxies residing in the nearby Lynx-Cancer void. We perform our photometry on u, g, r, and i-band images of the Sloan Digital Sky Survey. We determine model-independent galaxy parameters such as the integrated magnitudes and colors, effective radii and the corresponding surface brightness values, optical radii and Holmberg radii. We analyze the radial surface brightness profiles to determine the central brightness values and scale lengths of the model disks. We analyze the colors of the outer parts of the galaxies and compare them with model evolutionary tracks computed using the PEGASE 2 software package. This allowed us to estimate the time T SF elapsed since the onset of star formation, which turned out to be on the order of the cosmological time T 0 for the overwhelming majority of the galaxies studied. However, for 13 galaxies of the sample the time T SF does not exceed T 0/2 ∼ 7 Gyr, and for 7 of them T SF ≲ 3.5 Gyr. The latter are mostly unevolved objects dominated by low-luminosity galaxies with M B > −13.2. We use the integrated magnitudes and colors to estimate the stellar masses of the galaxies.We estimate the parameter M(H I)/L B and the gas mass fractions for void galaxies with known HI-line fluxes. A small subgroup (about 10%) of the gas-richest void galaxies with M(H I)/L B ≳ 2.5 has gas mass fractions that reach 94–99%. The outer regions of many of these galaxies show atypically blue colors. To test various statistical differences between void galaxies and galaxies from the samples selected using more general criteria, we compare some of the parameters of void galaxies with similar data for the sample of 195 galaxies from the Equatorial Survey (ES) based on a part of the HIPASS blind HI survey. The compared samples have similar properties in the common luminosity interval −18.5<M g < −13.5. The faintest void galaxies differ appreciably from the ES survey galaxies. However, the ES survey also contains about 7% of the so-called “inchoate” galaxies with highM(H I)/L B ratios, most of which are located far from massive neighbors and are probably analogs of void galaxies.

Properties of the most metal-poor gas-rich LSB dwarf galaxies SDSS J0015+0104 and J2354−0005 residing in the Eridanus void

SDSS J0015+0104 is the lowest metallicity low surface brightness dwarf (LSBD) galaxy known. The oxygen abundance in its HII region SDSS J001520.70+010436.9 (at ~1.5 kpc from the galaxy centre) is 12+log(O/H)=7.07 (Guseva et al.). This galaxy, at the distance of 28.4 Mpc, appears to reside deeply in the volume devoid of luminous massive galaxies, known as the Eridanus void. SDSS J235437.29-000501.6 is another Eridanus void LSBD galaxy, with parameter 12+log(O/H)=7.36 (also Guseva et al.). We present the results of their HI observations with the Nancay Radio Telescope revealing their high ratios of M(HI)/L_B ~2.3. Based on the Sloan Digital Sky Survey images, we derived for both galaxies their radial surface brightness profiles and the main photometric parameters. Their colours and total magnitudes are used to estimate the galaxy stellar mass and ages. The related gas mass-fractions, f_g ~0.98 and ~0.97, and the extremely low metallicities (much lower than for their more typical counterparts with the same luminosity) indicate their unevolved status. We compare these Eridanus void LSBDs with several extreme LSBD galaxies residing in the nearby Lynx-Cancer void. Based on the combination of all their unusual properties, the two discussed LSBD galaxies are similar to the unusual LSBDs residing in the closer void. This finding presents additional evidence for the existence in voids of a4 figures, sizable fraction of low-mass unevolved galaxies. Their dedicated search might result in the substantial increase of the number of such objects in the local Universe and in the advancement of understanding their nature.

Study of galaxies in the Lynx-Cancer void - III. New extreme low surface brightness dwarf galaxies

(Abridged) We present the results of the complex study of the low surface brightness dwarf (LSBD) gas-rich galaxies J0723+3621, J0737+4724 and J0852+1350, which reside in the nearby Lynx-Cancer void. Their ratios M(HI)/L_B, according to HI data obtained with the NRT, are respectively ~3.9, ~2, ~2.6. For the two latter galaxies, we derived oxygen abundance corresponding to the value of 12+log(O/H) <~7.3, using spectra from the Russian 6m telescope and from the SDSS database. We found two additional blue LSB dwarfs, J0723+3622 and J0852+1351, which appear to be physical companions of J0723+3621 and J0852+1350 situated at the projected distances of ~12--13 kpc. The companion relative velocities, derived from the BTA spectra, are dV = +89 km/s and +30 km/s respectively. The geometry and the relative orientation of orbits and spins in these pairs indicate, respectively, prograde and polar encounters for J0723+3621 and J0852+1350. The NRT HI profiles of J0723+3621 and J0723+3622 indicate a sizable gas flow in this system. The SDSS u,g,r,i images of the five dwarfs are used to derive the photometric parameters and the exponential or Sersic disc model fits. For three of them, the (u-g),(g-r),(r-i) colours of the outer parts, being compared with the PEGASE evolutionary tracks, evidence for the dominance of the old stellar populations with ages of T ~(8-10)+-3 Gyr. For J0723+3622 and J0737+4724, the outer region colours appear rather blue, implying the ages of the oldest visible stars of T <~1-3 Gyr. The new LSB galaxies complement the list of the known most metal-poor and `unevolved' dwarfs in this void, including DDO 68, SDSS J0926+3343 and others. This unique concentration of 'unevolved' dwarf galaxies in a small cell of the nearby Universe implies a physical relationship between the slow galaxy evolution and the void-type global environment.

Study of galaxies in the Lynx-Cancer void. II. Element abundances

In the framework of study of the evolutionary status of galaxies in the nearby Lynx-Cancer void, we present the results of the SAO RAS 6-m telescope spectroscopy for 20 objects in this region. The principal faint line [Oiii]λ4363 Å, used to determine the electron temperature and oxygen abundance (O/H) by the classicalmethod, is clearly detected in only about 2/3 of the studied objects. For the remaining galaxies this line is either faint or undetected. To obtain the oxygen abundances in these galaxies we as well apply the semi-empirical method by Izotov and Thuan, and/or the empirical methods of Pilyugin et al., which are only employing the intensities of sufficiently strong lines. We also present our O/H measurements for 22 Lynx-Cancer void galaxies, for which the suitable Sloan Digital Sky Survey (SDSS) spectra are available. In total, we present the combined O/H data for 48 Lynx-Cancer void galaxies, including the data adopted from the literature and our own earlier results. We make a comparison of their locations on the (O/H)-MB diagram with those of the dwarf galaxies of the Local Volume in the regions with denser environment. We infer that the majority of galaxies from this void on the average reveal an about 30% lower metallicity. In addition, a substantial fraction (not less than 10%) of the void dwarf galaxies have a much larger O/H deficiency (up to a factor of 5). Most of them belong to the tiny group of objects with the gas metallicity Z <Z⊙/20 or 12+log(O/H)≲7.35. The surface density of very metal-poor galaxies (Z <Z⊙/10) in this region of the sky is 2–2.5 times higher than that, derived from the emission-line galaxy samples in the Hamburg-SAO and the SDSS surveys. We discuss possible implications of these results for the galaxy evolution models.

Study of galaxies in the Lynx-Cancer void - I. Sample description

The evolution of galaxies is influenced by the environment in which they reside. This effect should be strongest for the least-mass and -luminosity galaxies. To study dwarf galaxies in extremely low density environments we have compiled a deep catalogue of dwarf galaxies in the nearby Lynx-Cancer void. This void hosts some of the most metal-poor dwarfs known to date. It borders the Local Volume at the negative supergalactic Z (SGZ) coordinates and has the size of more than 16 Mpc. With a distance to its centre of only 18 Mpc it is close enough to allow the search for the faintest dwarfs. Within the void 75 dwarf (-11.9 > M_B > -18.0) and 4 subluminous (-18.0 > M_B > -18.4) galaxies have been identified. We present the parameters of the void galaxies and give a detailed analysis of the completeness of the catalogue as a function of magnitude and surface brightness. The catalogue appears almost complete to M_B < -14 mag, but misses part of the fainter low surface brightness (LSB) face-on galaxies. This sample of void galaxies builds the basis of forthcoming observational studies that will give insight into the main stellar population, HI-mass-to-light ratio, metallicity and age for comparison with dwarfs in higher density regions. We briefly summarize the information on the unusual objects in the void and conclude that their concentration hints that voids are environments that are favourable for finding and studying unevolved dwarf galaxies.

H I in extremely metal deficient galaxies - III. Giant Metrewave Radio Telescope observations of blue compact galaxy HS 0822+3542

We present the results of Giant Metrewave Radio Telescope H i 21-cm line observations of the extremely metal deficient (XMD) blue compact galaxy (BCG) HS 0822+3542. HS 0822+3542 is the smallest known XMD galaxy; from Hubble Space Telescope (HST) imaging, it has been suggested that it actually consists of two still smaller (∼100 pc sized) ultra-compact dwarfs that are in the process of merging. The brighter of these two putative ultra-compact dwarfs has an ocular appearance, similar to that seen in galaxies that have suffered a penetrating encounter with a smaller companion. From our H i imaging, we find that the gas distribution and kinematics in this object are similar to that of other low-mass galaxies, albeit with some evidence for tidal disturbance. On the other hand, the H i emission has an angular size ∼25 times larger than that of the putative ultra-compact dwarfs. The optical emission is also offset from the centre of the H i emission. HS 0822+3542 is located in the nearby Lynx–Cancer void, but has a nearby companion low surface brightness dwarf galaxy SAO 0822+3545. In light of all this, we also consider a scenario where the optical emission from HS 0822+3542 comes not from two merging ultra-compact dwarfs but from multiple star-forming regions in a tidally disturbed galaxy. In this model, the ocular appearance of the brighter star-forming region could be the result of triggered star formation.

Starburst in HS 0822+3542 induced by the very blue LSB dwarf SAO 0822+3545

(Abridged). One of the most metal-deficient blue compact galaxies (BCGs) HS 0822+3542 (Z=1/34 Zsun), is also one of the nearest such objects. A trigger mechanism for its current SF burst has remained unclear. We report the discovery of a very blue ((B-V)tot=0.08 and (V-R)tot=0.14) LSB (mu_B^0 > 23.4 arcsec^-2) dwarf irregular (dIrr) galaxy, named SAO 0822+3545. Its small relative velocity and projected distance of ~11 kpc from the BCG imply their physical association. For this LSB galaxy, we present spectroscopic results, total B,V,R magnitudes, the effective radii and surface brightness (SB), and we describe its morphological properties. We compare the very blue colours of this dwarf with PEGASE.2 models of the colour evolution of a Z=1/20 Zsun stellar population, and combine this analysis with the data on the LSBD EW(Ha) values. The models best describing all available observational data depend on the relative fraction of massive stars in the IMF used. For a Salpeter IMF with Mup = 120 Msun, the best model includes a single stellar population (SSP) with an age of ~10 Myr and an SSP with the age of ~0.2--10 Gyr. The mass ratio of the old to young components should be in the range of 10 to 30. The role of interaction in triggering the galaxies major SF episodes during the last ~100-200 Myr is discussed. For the BCG, based on the spectroscopy with the 6m telescope, we estimate the physical parameters of its SF region and present the first evidence of an ionized gas supershell. This pair of dwarfs lies deep within the nearby Lynx-Cancer void, with the nearest bright (L > L*) galaxies at distances > 3 Mpc. This is probably one of the main factors responsible for the unevolved state of HS 0822+3542.