Active Galactic Nuclei Abundance in Cosmic Voids

The abundance of active galactic nuclei (AGN) in cosmic voids is relatively unexplored in the literature, but can potentially provide new constraints on the environmental dependence of AGN activity and the AGN-host coevolution. We investigated AGN fractions in one of the largest samples of optically selected cosmic voids from Sloan Digital Sky Survey Data Release 12 for redshift range 0.2–0.7 for moderately bright and bright AGN. We separated inner and outer void regions based on the void size, given by its effective void radius. We classified galaxies at a distance <0.6 R eff as inner void members and galaxies in the interval 0.6 < R/R eff < 1.3 as outer void galaxies. We found higher average fractions in the inner voids (4.9 ± 0.7)% than for their outer counterparts (3.1 ± 0.1)% at z > 0.42, which clearly indicates an environmental dependence. This conclusion was confirmed upon further separating the data in narrower void-centric distance bins and measured a significant decrease in AGN activity from inner to outer voids for z > 0.42. At low redshifts (z < 0.42), we find very weak dependence on the environment for the inner and outer regions for two out of three bins. We argue that the higher fraction in low-density regions close to void centers relative to their outer counterparts observed in the two higher-redshift bins suggests that more efficient galaxy interactions may occur at a one-to-one level in voids that may be suppressed in denser environments due to higher velocity dispersions. It could also indicate less prominent ram pressure stripping in voids or some intrinsic host or void environment properties.

Properties of simulated galaxies and supermassive black holes in cosmic voids

ABSTRACT Cosmic voids, the underdense regions of the cosmic web, are widely used to constrain cosmology. Voids contain few, isolated galaxies, presumably expected to be less evolved and preserving memory of the pristine Universe. We use the cosmological hydrodynamical simulation Horizon-AGN coupled to the void finder vide to investigate properties of galaxies in voids at z = 0. We find that, closer to void centres, low-mass galaxies are more common than their massive counterparts. At a fixed dark matter halo mass, they have smaller stellar masses than in denser regions. The star formation rate of void galaxies diminishes when approaching void centres, but their specific star formation rate slightly increases, suggesting that void galaxies form stars more efficiently with respect to their stellar mass. We find that this cannot only be attributed to the prevalence of low-mass galaxies. The inner region of voids also predominantly hosts low-mass black holes (BHs). However, the BH mass-to-galaxy mass ratios resemble those of the whole simulation at z = 0. Our results suggest that even if the growth channels in cosmic voids are different from those in denser environments, voids grow their galaxies and BHs in a similar way. While a large fraction of the BHs have low Eddington ratios, we find that $\text{$\sim$} 20{{\ \rm per\ cent}}$ could be observed as active galactic nuclei with $\log _{10} L_{\rm 2\!-\!10 \, keV}=41.5\!-\!42.5 \, \rm erg\, s^{-1}$. These results pave the way to future work with larger next-generation hydro-simulations, aiming to confirm our findings and prepare the application on data from upcoming large surveys such as Prime Focus Spectrograph, Euclid, and Wide Field Infrared Survey Telescope.

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

ABSTRACT In the framework of an ongoing project aimed at searching for and studying eXtremely Metal-Poor (XMP) very gas-rich blue dwarfs in nearby voids, we conducted spectroscopy with the 11-m Southern African Large Telescope (SALT) of 26 candidates, preselected in the first paper of this series (PEPK19). For 23 of them, we detected oxygen lines, allowing us to estimate the gas O/H ratio. For 10 of them, the oxygen abundance is found to be very low, in the range of 12 + log (O/H) = 6.95–7.30 dex. Of those, four void dwarfs have 12 + log (O/H) &lt; 7.19, or Z &lt; Z⊙/30. For the majority of observed galaxies, the faint line [O iii] λ4363 Å used to estimate O/H with the direct Te method appeared either too noisy or was not detected. We therefore use the semi-empirical method of Izotov & Thuan for these spectra, or, when applicable, the new ‘Strong line’ method of Izotov et al. We present and discuss the results for all void dwarfs observed in this work. We also compare their O/H values with O/H values of ∼140 void galaxies available from our recent papers. We address the properties of the newly found unusual void XMP dwarfs and compare them with those for 10 known prototype void XMP objects. The latter small group is outstanding based on their very small mass fraction of stars (only 0.01–0.02 of the baryonic mass), the blue colours of stars in the outer body indicating a non-cosmological age for the main star-forming episode), and the low gas metallicity (several times lower than expected for their luminosity).

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.

Deficit of luminous and normal red galaxies in cosmic voids

ABSTRACT We construct a sample of 10 680 wall galaxies and 3064 void galaxies with MR ≲ −20 by cross-referencing a void catalogue from literature with Baryon Oscillation Spectroscopic Survey (BOSS) CMASS and WiggleZ galaxies, where the CMASS survey targets redder galaxies and the WiggleZ survey targets bluer galaxies. Comparing the density profiles of the red and blue galaxies as a function of the void radius, we find that the number ratio of red-to-blue galaxies increases with distances from the void centres, suggesting a deficit of luminous and normal red galaxies in voids. We find a mean (g – r) magnitude colour of 1.298 and 1.210 for the wall and void galaxies, respectively, when considering the combined red and blue samples, which is found to be a significant difference. However, when considering the blue and red samples separately, we find no significant colour difference. We conclude that the constituents galaxies of each population, rather than intrinsic colour difference, is the main driver in the apparent average colour difference of galaxies in voids and walls, indicating a deficit of luminous and normal red galaxies in voids. Our analysis suggests that the primary environmental-dependence effect on galaxy evolution for normal and luminous galaxies between void and wall regions is manifested in the number of red galaxies, which depends on the environmental-dependent merger history. Using a semi-analytic simulation model, we can successfully reproduce the apparent colour difference between the void and wall galaxies.

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

Properties 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.