Void Dwarf Galaxies Research Articles

Searching for Dwarf Hα Emission-line Galaxies within Voids. I. Survey Methods and First Observations

The population density of dwarf galaxies in low-density voids is likely determined by the dark matter halo mass function and how galaxy formation proceeds in smaller halos. This depends on the nature of dark matter itself, making the dwarf galaxy population a tracer of its properties. While dwarfs have been found in smaller, closer voids, they have proven difficult to find in larger, more distant voids through magnitude-limited spectroscopic surveys. This is because these surveys detect an overwhelmingly large number of objects behind the voids that must be verified spectroscopically, making void surveys prohibitively inefficient and expensive in terms of large-telescope time. Narrowband imaging for emission lines such as Hα reduces the number of background objects, although the overall number remains large. If imaging is done through a filter set with overlapping transmission wings, then object redshift can be estimated from photometry alone. The precision possible is an order of magnitude greater than single-band photometry, with the caveat that the captured line must be identified through other means. Broadband photometry can be used to reject enough objects with emission of an unwanted type to make obtaining spectra of the remaining objects feasible. In this study, we present an Hα survey for dwarf galaxies with M r ′ fainter than −14 mag through the center 4.3 square degrees of the void FN8. Using Sloan photometry, we exclude enough [O ii] and [O iii] emitters that follow-up spectra of only a few dozen objects are required to statistically estimate the void population density.

Mass models of gas-rich void dwarf galaxies

ABSTRACT We construct mass models of eight gas rich dwarf galaxies that lie in the Lynx–Cancer void. From NFW fits to the dark matter halo profile, we find that the concentration parameters of haloes of void dwarf galaxies are similar to those of dwarf galaxies in normal density regions. We also measure the slope of the central dark matter density profiles, obtained by converting the rotation curves derived using 3D (fat) and 2D (ROTCUR) tilted ring fitting routines, into mass densities. We find that the average slope (α = −1.39 ± 0.19), obtained from 3D fitting is consistent with that expected from an NFW profile. On the other hand, the average slope measured using the 2D approach is closer to what would be expected for an isothermal profile. This suggests that systematic effects in velocity field analysis have a significant effect on the slope of the central dark matter density profiles. Given the modest number of galaxies we use for our analysis, it is important to check these results using a larger sample.

Influence of the Void Environment on Chemical Abundances in Dwarf Galaxies and Implications for Connecting Star Formation and Halo Mass

Abstract We study how the void environment affects the chemical evolution of galaxies in the universe by comparing the oxygen and nitrogen abundances of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, we estimate the oxygen, nitrogen, and neon abundances of 889 void dwarf galaxies and 672 dwarf galaxies in denser regions. We use the Direct T e method for calculating the gas-phase chemical abundances in the dwarf galaxies because it is best suited for low-metallicity, low-mass (dwarf) galaxies. A substitute for the [O ii] λ3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the Direct T e method. We find that void dwarf galaxies have about the same oxygen abundance and Ne/O ratio as dwarf galaxies in denser environments. However, we find that void dwarf galaxies have slightly higher neon (∼10%) abundances than dwarf galaxies in denser environments. The opposite trend is seen in both the nitrogen abundance and N/O ratio: void dwarf galaxies have slightly lower nitrogen abundances (∼5%) and lower N/O ratios (∼7%) than dwarf galaxies in denser regions. Therefore, we conclude that the void environment has a slight influence on dwarf galaxy chemical evolution. Our mass–N/O relationship shows that the secondary production of nitrogen commences at a lower stellar mass in void dwarf star-forming galaxies than in dwarf star-forming galaxies in denser environments. We also find that star-forming void dwarf galaxies have higher H i masses than the star-forming dwarf galaxies in denser regions. Our star-forming dwarf galaxy sample demonstrates a strong anti-correlation between the sSFR and N/O ratio, providing evidence that oxygen is produced in higher-mass stars than those which synthesize nitrogen. The lower N/O ratios and smaller stellar mass for secondary nitrogen production seen in void dwarf galaxies may indicate both delayed star formation as predicted by ΛCDM cosmology and a dependence of cosmic downsizing on the large-scale environment. A shift toward slightly higher oxygen abundances and higher H i masses in void dwarf galaxies could be evidence of larger ratios of dark matter halo mass to stellar mass in voids compared with denser regions.

Influence of the Void Environment on Chemical Abundances in Dwarf Galaxies and Implications for Connecting Star Formation and Halo Mass

AbstractWe study how the void environment affects the chemical evolution of galaxies by comparing the metallicity of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from SDSS DR7, we estimate oxygen and nitrogen abundances of 889 void dwarf galaxies and 672 dwarf galaxies in denser regions. A substitute for the [OII] λ3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the direct method. We find that void dwarf galaxies have about the same oxygen abundances and slightly lower N/O ratios than dwarf galaxies in denser environments. The lower N/O ratios seen in void dwarf galaxies may indicate both delayed star formation and a dependence of cosmic downsizing on the large-scale environment. Similar oxygen abundances in the two dwarf galaxy populations might be evidence of larger ratios of dark matter halo mass to stellar mass in voids.

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.

Angular momentum of dwarf galaxies

Mass and specific angular momentum are two fundamental physical parameters of galaxies. We present measurements of the baryonic mass and specific angular momentum of 11 void dwarf galaxies derived from neutral hydrogen (H{\sc i}) synthesis data. Rotation curves were measured using 3D and 2D tilted ring fitting routines, and the derived curves generally overlap within the error bars, except in the central regions where, as expected, the 3D routines give steeper curves. The specific angular momentum of void dwarfs is found to be high compared to an extrapolation of the trends seen for higher mass bulge-less spirals, but comparable to that of other dwarf irregular galaxies that lie outside of voids. As such, our data show no evidence for a dependence of the specific angular momentum on the large scale environment. Combining our data with the data from the literature, we find a baryonic threshold of $\sim 10^{9.1}~M_{\odot}$ for this increase in specific angular momentum. Interestingly, this threshold is very similar to the mass threshold below which the galaxy discs start to become systematically thicker. This provides qualitative support to the suggestion that the thickening of the discs, as well as the increase in specific angular momentum, are both results of a common physical mechanism, such as feedback from star formation. Quantitatively, however, the amount of star formation observed in our dwarfs appears insufficient to produce the observed increase in specific angular momentum. It is hence likely that other processes, such as cold accretion of high angular momentum gas, also play a role in increasing the specific angular momentum.

Milgromian dynamics and dwarf galaxies in galactic voids

We use kinematic data of 103 dwarf galaxies, obtained from the Sloan Digital Sky Survey catalog, to test the Milgromian dynamics (MOND) inside a galactic void. From this data, we compute the line-of-sight velocity dispersions of the dwarf galaxies in the frameworks of MOND and Newtonian dynamics without invoking any dark matter. The prediction for the line-of-sight velocity dispersions from MOND of 53 selected dwarf galaxies is compared with their measured values. For appropriate mass-to-light ratios in the range 1 to 5 for each individual dwarf galaxy, our results for the line-of-sight velocity dispersions predicted by MOND are more compatible with observations than those predicted by Newtonian dynamics.

Large-scale Environmental Dependence of the Abundance Ratio of Nitrogen to Oxygen in Blue, Star-forming Galaxies Fainter than L*

Abstract We examine how the cosmic environment affects the chemical evolution of galaxies in the universe by comparing the N/O ratio of dwarf galaxies in voids with that of dwarf galaxies in denser regions. Ratios of the forbidden [O iii] and [S ii] transitions provide estimates of a region’s electron temperature and number density. We estimate the abundances of oxygen and nitrogen using these temperature and density estimates and the emission-line fluxes [O ii] , [O iii] , and [N ii] with the direct T e method. Using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, we are able to estimate the N/O ratio in 42 void dwarf galaxies and 89 dwarf galaxies in denser regions. The N/O ratio for void dwarfs ( ) is slightly lower ( ) than for dwarf galaxies in denser regions. We also estimate the nitrogen and oxygen abundances of 2050 void galaxies and 3883 galaxies in denser regions with . These somewhat brighter galaxies (but still fainter than L *) also display similar minor shifts in the N/O ratio. The shifts in the average and median element abundance values in all absolute magnitude bins studied are in the same direction, suggesting that the large-scale environment may influence the chemical evolution of galaxies. We discuss possible causes of such a large-scale environmental dependence of the chemical evolution of galaxies, including retarded star formation and a higher ratio of dark matter halo mass to stellar mass in void galaxies.

DETERMINING THE LARGE-SCALE ENVIRONMENTAL DEPENDENCE OF GAS-PHASE METALLICITY IN DWARF GALAXIES

ABSTRACT We study how the cosmic environment affects galaxy evolution in the universe by comparing the metallicities of dwarf galaxies in voids with dwarf galaxies in more dense regions. Ratios of the fluxes of emission lines, particularly those of the forbidden [O iii] and [S ii] transitions, provide estimates of a region’s electron temperature and number density. From these two quantities and the emission line fluxes [O ii] λ3727, [O iii] λ4363, and [O iii] λλ4959, 5007, we estimate the abundance of oxygen with the direct Te method. We estimate the metallicity of 42 blue, star-forming void dwarf galaxies and 89 blue, star-forming dwarf galaxies in more dense regions using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, as reprocessed in the MPA-JHU value-added catalog. We find very little difference between the two sets of galaxies, indicating little influence from the large-scale environment on their chemical evolution. Of particular interest are a number of extremely metal-poor dwarf galaxies that are less prevalent in voids than in the denser regions.

THE METALLICITY OF VOID DWARF GALAXIES

The current Lambda CDM cosmological model predicts that galaxy evolution proceeds more slowly in lower density environments, suggesting that voids are a prime location to search for relatively pristine galaxies that are representative of the building blocks of early massive galaxies. To test the assumption that void galaxies are more pristine, we compare the evolutionary properties of a sample of dwarf galaxies selected specifically to lie in voids with a sample of similar isolated dwarf galaxies in average density environments. We measure gas-phase oxygen abundances and gas fractions for eight dwarf galaxies (M_r > -16.2), carefully selected to reside within the lowest density environments of seven voids, and apply the same calibrations to existing samples of isolated dwarf galaxies. We find no significant difference between these void dwarf galaxies and the isolated dwarf galaxies, suggesting that dwarf galaxy chemical evolution proceeds independent of the large-scale environment. While this sample is too small to draw strong conclusions, it suggests that external gas accretion is playing a limited role in the chemical evolution of these systems, and that this evolution is instead dominated mainly by the internal secular processes that are linking the simultaneous growth and enrichment of these galaxies.

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.

Dwarf Galaxies in Voids: Dark Matter Halos and Gas Cooling

Galaxy surveys have shown that luminous galaxies are mainly distributed in large filaments and galaxy clusters. The remaining large volumes are virtually devoid of luminous galaxies. This is in concordance with the formation of the large‐scale structure in the universe as derived from cosmological simulations. However, the numerical results indicate that cosmological voids are abundantly populated with dark matter haloes which may in principle host dwarf galaxies. Observational efforts have in contrast revealed that voids are apparently devoid of dwarf galaxies. We investigate the formation of dwarf galaxies in voids by hydrodynamical cosmological simulations. Due to the cosmic ultraviolet background radiation low‐mass haloes show generally a reduced baryon fraction. We determine the characteristic mass below which dwarf galaxies are baryon deficient. We show that the circular velocity below which the accretion of baryons is suppressed is approximately 40 kms−1. The suppressed baryon accretion is caused by the photo‐heating due to the UV background. We set up a spherical halo model and show that the effective equation of the state of the gas in the periphery of dwarf galaxies determines the characteristic mass. This implies that any process which heats the gas around dwarf galaxies increases the characteristic mass and thus reduces the number of observable dwarf galaxies.

Dwarf galaxies in voids: suppressing star formation with photoheating

We study the structure formation in cosmological void regions using high-resolution hydrody-namical simulations. Despite being significantly underdense, voids are populated abundantly with small dark matter haloes which should appear as dwarf galaxies if their star formation is not suppressed significantly. We here investigate to which extent the cosmological ultraviolet (UV) background reduces the baryon content of dwarf galaxies, and thereby limits their cooling and star formation rates. Assuming a Haardt & Madau UV background with reionization at redshift z = 6, our samples of simulated galaxies show that haloes with masses below a characteristic mass of M c (z = 0) = 6.5 x 10 9 h -1 M ⊙ are baryon-poor, but in general not completely empty, because baryons that are in the condensed cold phase or are already locked up in stars resist evaporation. In haloes with mass M? M c , we find that photoheating suppresses further cooling of gas. The redshift- and UV-background-dependent characteristic mass M c (z) can be understood from the equilibrium temperature between heating and cooling at a characteristic overdensity of δ ≃ 1000. If a halo is massive enough to compress gas to this density despite the presence of UV-background radiation, gas is free to 'enter' the condensed phase and cooling continues in the halo, otherwise it stalls. By analysing the mass accretion histories of dwarf galaxies in voids, we show that they can build up a significant amount of condensed mass at early times before the epoch of reionization. Later on, the amount of mass in this phase remains roughly constant, but the masses of the dark matter haloes continue to increase. Consequently, photoheating leads to a reduced baryon fraction in void dwarf galaxies, endows them with a rather old stellar population, but still allows late star formation to some extent. We estimate the resulting stellar mass function for void galaxies. While the number of galaxies at the faint end is significantly reduced due to photoheating, additional physical feedback processes may be required to explain the apparent paucity of dwarfs in observations of voids.

The Dependence of Halo Clustering on Halo Formation History, Concentration, and Occupation

We investigate the dependence of dark matter halo clustering on halo formation time, density profile concentration, and subhalo occupation number, using high-resolution numerical simulations of a ?CDM cosmology. We confirm results that halo clustering is a function of halo formation time at fixed mass and that this trend depends on halo mass. For the first time, we show unequivocally that halo clustering is a function of halo concentration and show that the dependence of halo bias on concentration, mass, and redshift can be accurately parameterized in a simple way: b(M,c|z) = b(M|z)b(c|M/M). Interestingly, the scaling between bias and concentration changes sign with the value of M/M: high-concentration (early forming) objects cluster more strongly for M M, while low-concentration (late forming) objects cluster more strongly for rare high-mass halos, M M. We show the first explicit demonstration that host dark halo clustering depends on the halo occupation number (of dark matter subhalos) at fixed mass and discuss implications for halo model calculations of dark matter power spectra and galaxy clustering statistics. The effect of these halo properties on clustering is strongest for early-forming dwarf-mass halos, which are significantly more clustered than typical halos of their mass. Our results suggest that isolated low-mass galaxies (e.g., low surface brightness dwarfs) should have more slowly rising rotation curves than their more clustered counterparts and may have consequences for the dearth of dwarf galaxies in voids. They also imply that self-calibrating richness-selected cluster samples with their clustering properties might overestimate cluster masses and bias cosmological parameter estimation.

HS 2134+0400—a new very metal-poor galaxy, a representative of the void population?

We present the 6-m SAO telescope spectroscopy of HS 2134+0400, a blue compact galaxy (BCG) discovered within the framework of a dedicated Hamburg/SAO survey for low-metallicity BCGs (HSS-LM). Its very low abundance of oxygen (12 + log(O/H) = 7.44) and other heavy elements (S, N, Ne, Ar) allows this dwarf galaxy to be assigned to the group of eight lowest-metallicity BCGs among the several thousand BCGs known in the nearby Universe. The measured heavy-element abundance ratios (S/O, Ne/O, N/O, and Ar/O) are in good agreement with the typical values found for other very metal-poor BCGs. The spatial location of HS 2134+0400 is atypical of the majority of BCGs: it lies in the Pegasus void, a large region with a very low density of galaxies with normal or higher luminosities. In addition to HS 2134+0400, we found a dozen more very metal-poor galaxies in voids. Therefore, we discuss the hypothesis that this type of objects may be representative for the population of dwarf galaxies in voids.

On the Spectroscopic Properties of Star-Forming Dwarf Galaxies in Different Environments

view Abstract Citations (73) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Spectroscopic Properties of Star-Forming Dwarf Galaxies in Different Environments Vilchez, J. M. Abstract A study is presented on the spectroscopic properties of a sample of star- forming dwarf galaxies in extreme density environments, as part of an ongoing project intended to evaluate the influence of the environment on the evolution of dwarf galaxies. Subsets of dwarf galaxies in nearby voids, and in the field of the local supercluster have been selected to characterize lower density regions. Conversely, higher density environments have been characterized choosing a subset of galaxies in the direction of the core of the Virgo Cluster, as well as an isolated clump of galaxies. Our findings indicate that, overall, the spectra of those star-forming dwarf galaxies located in low-density regions tend to present higher excitations and ionization parameters, higher Hβ equivalent widths, and larger total Hβ luminosities than similar objects located in higher density environments. At the same time, only marginal evidence may be found supporting a trend between the gas metallicity and the density of the environment. The general metallicity luminosity relation for dwarf galaxies appears to be followed by most galaxies in the sample. An analysis of the spectroscopic ratios of the galaxies using recent evolutionary models of giant H II regions suggests a mixed star-formation history for most of the Virgo star- forming dwarfs, requiring continuous star formation in addition to some currently observable bursts. Publication: The Astronomical Journal Pub Date: September 1995 DOI: 10.1086/117589 Bibcode: 1995AJ....110.1090V Keywords: GALAXIES: PECULIAR; GALAXIES: EVOLUTION; GALAXIES: STELLAR CONTENT; STARS: FORMATION full text sources ADS | data products SIMBAD (58) NED (53)