Similar Galaxies Research Articles

Interactions, Starbursts, and Star Formation

We study how interactions between galaxies affect star formation within them by considering a sample of almost 1500 of the nearest galaxies, all within a distance of ∼45 Mpc. We use the far-IR emission to define the massive star formation rate (SFR), and then normalise the SFR by the stellar mass of the galaxy to obtain the specific star formation rate (SSFR). We explore the distribution of (S)SFR with morphological type and with stellar mass. We calculate the relative enhancement of SFR and SSFR for each galaxy by normalising them by the median SFR and SSFR values of individual control samples of similar non-interacting galaxies. We find that both the median SFR and SSFR are enhanced in interacting galaxies, and more so as the degree of interaction is higher. The increase is moderate, reaching a maximum of a factor of 1.9 for the highest degree of interaction (mergers). While the SFR and SSFR are enhanced statistically by interactions, in many individual interacting galaxies they are not enhanced at all. Our study is based on a representative sample of nearby galaxies and should be used to place constraints on studies based on samples of galaxies at larger distances.

The effects of environment on the intrinsic shape of galaxies

We measure the effect of the environment on the intrinsic shapes of spiral and elliptical galaxies by finding the 3D shape distribution and dust extinction that fits better the projected shape of galaxies in different environment. We find that spiral galaxies in groups are very similar to field spirals with similar intrinsic properties (magnitudes, sizes and colours). But for spirals in groups, those in denser environments or closer to the centre of the group tend to have a more circular disc than similar galaxies in less dense environments or far from the group centres. Also we find that central spiral galaxies in their groups tend to be thinner than other similar spirals. For ellipticals, we do not find any important dependence of their shape on their position in a group or on the local density. However, we find that elliptical galaxies in groups tend to be more spherical than field ellipticals with similar intrinsic properties. We find that, once in groups, the shape of member galaxies do not depend on group mass, regardless of their morphological type.

The massive end of the stellar mass function

We derive average flux corrections to the \texttt{Model} magnitudes of the Sloan Digital Sky Survey (SDSS) galaxies by stacking together mosaics of similar galaxies in bins of stellar mass and concentration. Extra flux is detected in the outer low surface brightness part of the galaxies, leading to corrections ranging from 0.05 to 0.32 mag for the highest stellar mass galaxies. We apply these corrections to the MPA-JHU (Max-Planck Institute for Astrophysics - John Hopkins University) stellar masses for a complete sample of half a million galaxies from the SDSS survey to derive a corrected galaxy stellar mass function at $z=0.1$ in the stellar mass range $9.5<\log(M_\ast/M_\odot)<12.0$. We find that the flux corrections and the use of the MPA-JHU stellar masses have a significant impact on the massive end of the stellar mass function, making the slope significantly shallower than that estimated by Li \& White (2009), but steeper than derived by Bernardi et al. (2013). This corresponds to a mean comoving stellar mass density of galaxies with stellar masses $\log(M_\ast/M_\odot) \ge 11.0$ that is a factor of 3.36 larger than the estimate by Li \& White (2009), but is 43\% smaller than reported by Bernardi et al. (2013).

Interacting galaxies in the nearby Universe: only moderate increase of star formation

We investigate the influence of interactions on the star formation by studying a sample of almost 1500 of the nearest galaxies, all within a distance of ~45 Mpc. We define the massive star formation rate (SFR), as measured from far-IR emission, and the specific star formation rate (SSFR), which is the former quantity normalised by the stellar mass of the galaxy, and explore their distribution with morphological type and with stellar mass. We then calculate the relative enhancement of these quantities for each galaxy by normalising them by the median SFR and SSFR values of individual control populations of similar non-interacting galaxies. We find that both SFR and SSFR are enhanced in interacting galaxies, and more so as the degree of interaction is higher. The increase is, however, moderate, reaching a maximum of a factor of 1.9 for the highest degree of interaction (mergers). The SFR and SSFR are enhanced statistically in the population, but in many individual interacting galaxies they are not enhanced at all. We discuss how those galaxies with the largest SFR and/or SSFR enhancement can be defined as starbursts. This study is based on a representative sample of nearby galaxies, including many low-mass and dwarf/irregular galaxies, and we argue that it should be used to place constraints on studies based on samples of galaxies at larger distances, beyond the local Universe.

GRB hosts through cosmic time

[Abridged] We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 GRB-selected galaxies at 0.1<z<3.6, the largest sample of GRB host spectroscopy available to date. Most of our GRBs were detected by Swift and 76% are at 0.5<z<2.5 with a median z~1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFRs), visual attenuations, oxygen abundances (12+log(O/H)), and emission-line widths. We find a strong change of the typical physical properties of GRB hosts with redshift. The median SFR, for example, increases from ~0.6 M_sun/yr at z~0.6 up to ~15 M_sun/yr at z~2. A higher ratio of [OIII]/[OII] at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the BPT diagram. Oxygen abundances of the galaxies are distributed between 12+log(O/H)=7.9 and 12+log(O/H)=9.0 with a median of 12+log(O/H)~8.5. The fraction of GRB-selected galaxies with super-solar metallicities is around 20% at z<1 in the adopted metallicity scale. This is significantly less than the fraction of star formation in similar galaxies, illustrating that GRBs are scarce in high-metallicity environments. At z~3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z ~< 0.5 Z_sun. Together with a high incidence of galaxies with similar metallicity in our sample at z~1.5, this indicates that the metallicity dependence observed at low redshift will not be dominant at z~3.

ALFALFA Hα Reveals How Galaxies Use Their Hi Fuel

AbstractAtomic hydrogen traces the raw material from which molecular clouds and stars form. With 565 galaxies from the ALFALFA Hα survey, a statistically complete subset of the ALFALFA survey, we examine the processes that affect galaxies' abilities to access and consume their Hi gas. On galaxy-wide scales, Hi gas fractions correlate only weakly with instantaneous specific star formation rates (sSFRs) but tightly with galaxy color. We show that a connection between dust and Hi content, arising from the fundamental mass-metallicity-Hi relation, leads to this tight color correlation. We find that disk galaxies follow a relation between stellar surface density and Hi depletion time, consistent with a scenario in which higher mid-plane pressure leads to more efficient molecular cloud formation from Hi. In contrast, spheroids show no such trend. Starbursts, identified by Hα equivalent width, do not show enhanced Hi gas fractions relative to similar mass non-starburst galaxies. The starbursts' shorter Hi depletion times indicate more efficient consumption of Hi, and galaxy interactions drive this enhanced star formation efficiency in several starbursts. Interestingly, the most disturbed starbursts show greater enhancements in Hi gas fraction, which may indicate an excess of Hi at early merger stages. At low galaxy stellar masses, the triggering mechanism for starbursts is less clear; the high scatter in efficiency and sSFR among low-mass galaxies may result from periodic bursts. We find no evidence for depleted Hi reservoirs in starbursts, which suggests that galaxies may maintain sufficient Hi to fuel multiple starburst episodes.

Interactions and star formation

AbstractWe investigate the influence of interactions on the star formation by studying a sample of almost 1500 of the nearest galaxies, all within a distance of ~45 Mpc. We define the massive star formation rate (SFR), as measured from far-IR emission, and the specific star formation rate (SSFR), which is the former quantity normalized by the stellar mass of the galaxy, and explore their distribution with morphological type and with stellar mass. We then calculate the relative enhancement of these quantities for each galaxy by normalizing them by the median SFR and SSFR values of individual control populations of similar non-interacting galaxies. We find that both SFR and SSFR are enhanced in interacting galaxies, and more so as the degree of interaction is higher. The increase is, however, moderate, reaching a maximum of a factor of 1.9 for the highest degree of interaction (mergers). The SFR and SSFR are enhanced statistically in the population, but in most individual interacting galaxies they are not enhanced at all. We discuss how those galaxies with the largest SFR and/or SSFR enhancement can be defined as starbursts. We argue that this study, based on a representative sample of nearby galaxies, should be used to place constraints on studies based on samples of galaxies at larger distances.

H-ATLAS/GAMA and HeViCS – dusty early-type galaxies in different environments

The Herschel Space Observatory has had a tremendous impact on the study of extragalactic dust. Specifically, early-type galaxies (ETG) have been the focus of several studies. In this paper we combine results from two Herschel studies - a Virgo cluster study HeViCS and a broader, low-redshift H-ATLAS/GAMA study - and contrast the dust and associated properties for similar mass galaxies. This comparison is motivated by differences in results exhibited between multiple Herschel studies of early-type galaxies. A comparison between consistent modified blackbody derived dust mass is carried out, revealing strong differences between the two samples in both dust mass and dust-to-stellar mass ratio. In particular, the HeViCS sample lacks massive ETG with as high a specific dust content as found in H-ATLAS. This is most likely connected with the difference in environment for the two samples. We calculate nearest neighbour environment densities in a consistent way, showing that H-ATLAS ETG occupy sparser regions of the local Universe, whereas HeViCS ETG occupy dense regions. This is also true for ETG that are not Herschel-detected but are in the Virgo and GAMA parent samples. Spectral energy distributions are fit to the panchromatic data. From these we find that in H-ATLAS the specific star formation rate anticorrelates with stellar mass and reaches values as high as in our Galaxy. On the other hand HeViCS ETG appear to have little star formation. Based on the trends found here, H-ATLAS ETG are thought to have more extended star formation histories and a younger stellar population than HeViCS ETG.

THE SCATTER IN THE HOT GAS CONTENT OF EARLY-TYPE GALAXIES

Optically similar early-type galaxies are observed to have a large and poorly understood range in the amount of hot, X-ray-emitting gas they contain. To investigate the origin of this diversity, we studied the hot gas properties of all 42 early-type galaxies in the multiwavelength ATLAS3D survey that have sufficiently deep Chandra X-ray observations. We related their hot gas properties to a number of internal and external physical quantities. To characterize the amount of hot gas relative to the stellar light, we use the ratio of the gaseous X-ray luminosity to the stellar K-band luminosity, ; we also use the deviations of from the best-fit ?LK relation (denoted ). We quantitatively confirm previous suggestions that various effects conspire to produce the large scatter in the observed relation. In particular, we find that the deviations are most strongly positively correlated with the (low rates of) star formation and the hot gas temperatures in the sample galaxies. This suggests that mild stellar feedback may energize the gas without pushing it out of the host galaxies. We also find that galaxies in high galaxy density environments tend to be massive slow rotators, while galaxies in low galaxy density environments tend to be low mass, fast rotators. Moreover, cold gas in clusters and fields may have different origins. The star formation rate increases with cold gas mass for field galaxies but it appears to be uncorrelated with cold gas for cluster galaxies.

Circumnuclear and infalling H i gas in a merging galaxy pair at z = 0.123

Using long-slit optical spectra obtained with the 2-m telescope at IUCAA Girawali Observatory, we show that the radio source J094221.98+062335.2 (z=0.123) is associated with a galaxy pair undergoing a major merger. Its companion is a normal star-forming galaxy infalling with a velocity of 185 km/s at a projected separation of 4.8 kpc. Using the Westerbork Synthesis Radio Telescope (WSRT) and Giant Metrewave Radio Telescope (GMRT) we detect a strong HI 21-cm absorption at the systemic redshift of the radio galaxy with N(HI)~9x10^21 cm^{-2} for an assumed spin-temperature of 100 K. Such a strong HI 21-cm absorption is rare and has been seen only in a few compact radio sources associated with similar merging galaxy pairs. Milliarcsecond resolution Very Long Baseline Array (VLBA) observations resolve the radio source into a compact symmetric object with the hotspot separation of 89 pc. The 21-cm absorption is detected in the VLBA spectra towards both the radio lobes albeit with a strong optical depth gradient. We show that the strong 21-cm absorption is consistent with it being arising from a clumpy circumnuclear disk/torus. We also detect two weaker absorption lines redshifted with respect to the radio source in the WSRT/GMRT spectrum. They probably represent cold (i.e. T< 10^4 K) HI gas falling into the radio source. The presence of high concentration of HI gas in the circumnuclear regions and signature of infalling cold gas allows us to conjecture that the young radio source may have been triggered by the gas infall due to the ongoing merger.

ZOOMING IN ON THE PROGENITORS OF SUPERLUMINOUS SUPERNOVAE WITH THEHST

We present Hubble Space Telescope (HST) rest-frame ultraviolet imaging of the host galaxies of 16 hydrogen-poor superluminous supernovae (SLSNe), including 11 events from the Pan-STARRS Medium Deep Survey. Taking advantage of the superb angular resolution of HST, we characterize the galaxies' morphological properties, sizes and star formation rate (SFR) densities. We determine the supernova (SN) locations within the host galaxies through precise astrometric matching, and measure physical and host-normalized offsets, as well as the SN positions within the cumulative distribution of UV light pixel brightness. We find that the host galaxies of H-poor SLSNe are irregular, compact dwarf galaxies, with a median half-light radius of just 0.9 kpc. The UV-derived SFR densities are high (<Sigma_SFR> ~ 0.1 M_sun/yr/kpc^2), suggesting that SLSNe form in overdense environments. Their locations trace the UV light of their host galaxies, with a distribution intermediate between that of long-duration gamma-ray bursts (LGRBs) (which are strongly clustered on the brightest regions of their hosts) and a uniform distribution (characteristic of normal core-collapse SNe), though cannot be statistically distinguished from either with the current sample size. Taken together, this strengthens the picture that SLSN progenitors require different conditions than those of ordinary core-collapse SNe to form, and that they explode in broadly similar galaxies as do LGRBs. If the tendency for SLSNe to be less clustered on the brightest regions than are LGRBs is confirmed by a larger sample, this would indicate a different, potentially lower-mass progenitor for SLSNe than LRGBs.

The UV, Lyman α, and dark matter halo properties of high-redshift galaxies

We explore the properties of high-redshift Lyman-alpha emitters (LAE), and their link with the Lyman-Break galaxy population (LBG), using a semi-analytic model of galaxy formation that takes into account resonant scattering of Lya photons in gas outflows. We can reasonably reproduce the abundances of LAEs and LBGs from redshift 3 to 7, as well as most UV LFs of LAEs. The stronger dust attenuation for (resonant) Lya photons compared to UV continuum photons in bright LBGs provides a natural interpretation to the increase of the LAE fraction in LBG samples, X_LAE, towards fainter magnitudes. The redshift evolution of X_LAE seems however very sensitive to UV magnitudes limits and EW cuts. In spite of the apparent good match between the statistical properties predicted by the model and the observations, we find that the tail of the Lya equivalent width distribution (EW > 100 A) cannot be explained by our model, and we need to invoke additional mechanisms. We find that LAEs and LBGs span a very similar dynamical range, but bright LAEs are about 4 times rarer than LBGs in massive halos. Moreover, massive halos mainly contain weak LAEs in our model, which might introduce a bias towards low-mass halos in surveys which select sources with high EW cuts. Overall, our results are consistent with the idea that LAEs and LBGs make a very similar galaxy population. Their apparent differences seem mainly due to EW selections, UV detection limits, and a decreasing Lya-to-UV escape fraction ratio in high SFR galaxies.

CONNECTING GRBs AND ULIRGs: A SENSITIVE, UNBIASED SURVEY FOR RADIO EMISSION FROM GAMMA-RAY BURST HOST GALAXIES AT 0 &lt;z&lt; 2.5

Luminous infrared galaxies and submillimeter galaxies contribute significantly to stellar mass assembly and the frequency of GRBs in these systems provides an important test of the connection between the gamma-ray burst rate and that of overall cosmic star-formation. We present sensitive 3 GHz radio observations using the Karl G. Jansky Very Large Array of 32 uniformly-selected GRB host galaxies spanning a redshift range from 0 < z < 2.5, providing the first fully dust- and sample-unbiased measurement of the fraction of GRBs originating from the Universe's most bolometrically luminous galaxies. Four galaxies are detected, with inferred radio star-formation rates ranging between 50-300 Msun/yr. Three of the four detections correspond to events consistent with being optically-obscured "dark" bursts. Our overall detection fraction implies that between 9-23% of GRBs between 0.5 < z < 2.5 occur in galaxies with S_3GHz > 10 uJy, corresponding to SFR >50 Msun/yr at z~1 or >250 Msun/yr at z~2. Similar galaxies contribute approximately 10-30% of all cosmic star-formation, so our results are consistent with a GRB rate which is not strongly biased with respect to the total star-formation rate of a galaxy. However, all four radio-detected hosts have modest stellar masses (~few x 10^10 Msun), significantly lower than IR/submillimeter-selected field galaxies of similar luminosities. We suggest that the GRB rate may be suppressed in metal-rich environments but independently enhanced in intense starbursts, producing a strong efficiency dependence on mass but little net dependence on bulk galaxy star-formation rate.

Two spectroscopically confirmed galaxy structures atz= 0.61 and 0.74 in the CFHTLS Deep 3 field

Adami et al. (2010) have detected several cluster candidates at z>0.5 as part of a systematic search for clusters in the Canada France Hawaii Telescope Legacy Survey, based on photometric redshifts. We focus here on two of them, located in the D3 field: D3-6 and D3-43. We have obtained spectroscopy with Gemini/GMOS and measured redshifts for 23 and 14 galaxies in the two structures. These redshifts were combined with those available in the literature. A dynamical and a weak lensing analysis were also performed, together with the study of X-ray Chandra archive data. Cluster D3-6 is found to be a single structure of 8 spectroscopically confirmed members at an average redshift z=0.607, with a velocity dispersion of 423 km/s. It appears to be a relatively low mass cluster. D3-43-S3 has 46 spectroscopically confirmed members at an average redshift z=0.739. It can be decomposed into two main substructures, having a velocity dispersion of about 600 and 350 km/s. An explanation to the fact that D3-43-S3 is detected through weak lensing (only marginally, at the ~3sigma level) but not in X-rays could be that the two substructures are just beginning to merge more or less along the line of sight. We also show that D3-6 and D3-43-S3 have similar global galaxy luminosity functions, stellar mass functions, and star formation rate (SFR) distributions. The only differences are that D3-6 exhibits a lack of faint early type galaxies, a deficit of extremely high stellar mass galaxies compared to D3-43-S3, and an excess of very high SFR galaxies. This study shows the power of techniques based on photometric redshifts to detect low to moderately massive structures, even at z~0.75.

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.

The environment of low-redshift quasar pairs

We investigate the properties of the galaxy environment of a sample of 14 low redshift (z $<$ 0.85) quasar physical pairs extracted from SDSS DR10 archives. The pairs have a systemic radial velocity difference $\Delta V_\parallel \leqslant$ 600 $km \ s^{-1}$ (based on [OIII]5007 \AA \ line) and projected distance $ R_\bot \leqslant$ 600 kpc. The physical association of the pairs is statistically confirmed at a level of $\sim$ 90 %. For most of the images of these quasars we are able to resolve their host galaxies that turn out to be on average similar to those of quasars not in pairs. We also found that quasars in a pair are on average in region of modest galaxy overdensity extending up 0.5 Mpc from the QSO. This galaxy overdensity is indistinguishable from that of a homogeneous sample of isolated quasars at the same redshift and with similar host galaxy luminosity. These results, albeit derived from a small (but homogeneous) sample of objects, suggest that the rare activation of two quasars with small physical separation does not require any extraordinary environment.

Stellar content and distances to the isolated spiral galaxies NGC 6503 and NGC 6946

Based on archival Hubble Space Telescope images, we have performed stellar photometry of several fields in the isolated spiral galaxies NGC 6503 and NGC 6946 with high peculiar velocities. Based on the TRGB method, we have determined the distances to the galaxies: D = 6.30 ± 0.10 Mpc for NGC 6503 and D = 6.72 ± 0.15 Mpc for NGC 6946. The current stellar content of the galaxies does not differ from that of other similar galaxies. The metallicity for young stars in NGC 6503 is Z = 0.02 (corresponding to the solar metallicity), while the metallicity for stars in NGC 6946 reaches Z = 0.05. Very few old globular clusters have been found in NGC 6946, while they have not been found at all in NGC 6503. The number density distribution of stars with different ages in NGC 6503 does not differ from the analogous distributions in other galaxies. The large sizes of the thick disk in NGC 6503, which is clearly seen up to 6 kpc from the galactic disk plane and whose possible extension is noticeable up to 8.6 kpc from the plane, are a difference. The sizes of the region occupied by red giants of the disk in NGC 6503 are 51 × 17 kpc, which are not much larger than the sizes of this galaxy from H I radio observations.

Milky Way mass galaxies with X-shaped bulges are not rare in the local Universe

Abstract Boxy/Peanut/X-shaped (B/P/X) bulges are studied using the 3.6 μm images from the Spitzer Survey of Stellar Structure in Galaxies, and the Ks-band images from the Near-IR S0 galaxy Survey. They are compared with the properties of barlenses, defined as lens-like structures embedded in bars, with sizes of ∼50 per cent of bars and axial ratios of ∼0.6–0.9. Based on observations (extending Laurikainen et al.) and recent simulation models, we show evidence that barlenses are the more face-on counterparts of B/P/X-shaped bulges. Using unsharp masks 18 new X-shaped structures are identified, covering a large range of galaxy inclinations. The similar masses and red B−3.6 μm colours of the host galaxies, and the fact that the combined axial ratio distribution of the host galaxy discs is flat, support the interpretation that barlenses and X-shapes are physically the same phenomenon. In Hubble types −3 ≤ T ≤ 2 even half of the bars contain either a barlens or an X-shaped structure. Our detailed 2D multicomponent decompositions for 29 galaxies, fitting the barlens/X-shape with a separate component, indicate very small or non-existent classical bulges. Taking into account that the structures we study have similar host galaxy masses as the Milky Way (MW), our results imply that MW mass galaxies with no significant classical bulges are common in the nearby Universe.

Supernova Remnants Interacting with Molecular Clouds: X-Ray and Gamma-Ray Signatures

The giant molecular clouds (MCs) found in the Milky Way and similar galaxies play a crucial role in the evolution of these systems. The supernova explosions that mark the death of massive stars in these regions often lead to interactions between the supernova remnants (SNRs) and the clouds. These interactions have a profound effect on our understanding of SNRs. Shocks in SNRs should be capable of accelerating particles to cosmic ray (CR) energies with efficiencies high enough to power Galactic CRs. X-ray and gamma-ray studies have established the presence of relativistic electrons and protons is some SNRs and provided strong evidence for diffusive shock acceleration as the primary acceleration mechanism, including strongly amplified magnetic fields, temperature and ionization effects on the shock-heated plasmas, and modifications to the dynamical evolution of some systems. Because protons dominate the overall energetics of the CRs, it is crucial to understand this hadronic component even though electrons are much more efficient radiators and it can be difficult to identify the hadronic component. However, near MCs the densities are sufficiently high to allow the gamma-ray emission to be dominated by protons. Thus, these interaction sites provide some of our best opportunities to constrain the overall energetics of these particle accelerators. Here we summarize some key properties of interactions between SNRs and MCs, with an emphasis on recent X-ray and gamma-ray studies that are providing important constraints on our understanding of cosmic rays in our Galaxy.

Observations of dwarfs in nearby voids: implications for galaxy formation and evolution

AbstractThe 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 their evolutionary status. We derive directly the hydrogen mass M(HI), the ratio M(HI)/LB 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 fg). 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 fg ∼(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.