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Galaxy Voids. by Chooi Fei Ng and Aron Cooper. This false-color optical map, covering about 4300 square degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies. Void. Voids are the dominant feature and have a typical diameter of ~ 30Mpc.

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galaxy voids

Galaxy Voids

by Chooi Fei Ng and

Aron Cooper


This false-color optical map, covering about 4300 square degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.

  • Voids are the dominant feature and have a typical diameter of ~ 30Mpc.
  • Voids are very underdense region, δρ/ρ~0.95
  • Up to 40% of volume of the universe is occupied by voids
  • The largest void observed, Bootes void, has a diameter of about 124Mpc.
what is in a void
What is in a void?
  • A few unusually faint galaxy, void galaxies
  • Mostly Dark Matter – CDM model

Void Boundaries

  • Rather smooth, defined by galaxies with a broad range of luminosity, especially star forming galaxies
observation redshift surveys
Observation- Redshift surveys
  • PSCz Redshift Survey
    • Catalogue of detections with the Infra-Red Astronomical Satellite (IRAS)
    • 15411 galaxies with redshift
    • Covers 84% of the sky
  • UZC(Updated Zwicky Catalog)
    • Center for Astrophysics (CfA) Optical Redshift survey
    • 18633 galaxies with redshift
void finding algorithm
Void Finding Algorithm
  • Wall and field galaxies
    • Categorize each galaxy in the sample as a wall galaxy or a field (void) galaxy
    • Specify a length ln such that any galaxy that does not have n neighbors within a sphere of radius ln is classified as field galaxy. N = 3 for our choice
  • Place the wall galaxies onto a three dimensional grid
  • Beginning from the center of each empty grid cell, grow the largest possible sphere containing no wall galaxies and keep track of the radius of each hole
  • The largest hole found is automatically a void
  • Then, we test the second hole and if it overlaps the previous void by F% in volume, then it is a member of the first void rather than a new void. If not, it forms a separate void.
  • We continue like this for all holes with radii larger than 10h-1 Mpc
result from voids finding algorithm
Result from Voids Finding Algorithm
  • Shows the number of voids we find for the PSCz(dashed line) and the UZC(solid line) as a function of the overlap fraction for which the hole is still considered a separate void
recovering volume of voids
Recovering volume of voids
  • If the voids are highly elliptical, we will not detect them at the corners of the ellipse
  • Test this by generating data containing mock voids of known elliptical shape
  • Run the simulated data through our void finding algorithm and compare the volume obtained with the known volume of the void
If voids are spherical in shape we recover 100% of the volume.
  • The more elongated it becomes, the less of the volume we detect
pscz uzc
  • Show the super galactic coordinates (x,y) for different values of z.
  • Each panel shows a 10h-1Mpc slice
  • Shaded regions are the voids.
  • The points are the wall galaxies and the empty squares show the void centers
  • Different samples from the same survey yield the same voids
  • We detect the same voids in different redshift surveys
study of voids using n body simulations

Study of Voids Using N-Body Simulations

Chooi Fei Ng and Aron Cooper

  • Motivation for using simulations
  • Basic information about simulations used
  • Void characteristics as determined using a variety of analysis tools
    • Void Probability Function (VPF)
    • Nearest Neighbor Distances
    • Void Finder Algorithm (VF)
  • Characteristics of Void galaxy populations
  • Why use simulations to study voids?
    • Compare simulation results with observations
      • Verify/Constrain cosmological models
        • Structure formation
        • Cosmological parameters
        • Initial conditions
      • Give some indication of Galaxy formation process
        • Special void population of galaxies
      • Better understand galaxy bias
        • If no bias, galaxy mass distribution is equal to that of DM
n body simulations used
N-Body Simulations Used
  • Three used: GIF, GIF-II, and 5123
    • GIF and GIF-II only vary with initial conditions
    • Use Dark Matter Particles only
    • Cosmological Parameters Used:
    • “Special” methods applied to predict galaxy distribution and properties (details omitted)
void probability function vpf
Void Probability Function (VPF)
  • VPF: Probability that a random sphere of radius R will have no points/matter within it
  • Statistic is calculated for: DM, Galaxies, and random distribution of points with # density = n_gal
  • Results:
    • DM and galaxies have more larger voids due clustering than the random distribution of points
    • VPF for large R is much higher for galaxies, than for DM, i.e. more large voids in galaxies due to bias
      • Selection effects: small DM halos in voids may form galaxies, but they are not bright enough to meet brightness criteria
comparison to observation for vpf
Comparison to Observation for VPF
  • For R <~ 8 h^-1MPc:
    • Galaxy samples match observations
    • DM produce low VPFs with regards to observations
  • For R > 8 h^-1MPc:
    • DM VPF agrees with the observations
    • Galaxy samples over-predict observed VPF
  • However, both DM and galaxy VPFs are within the uncertainties of the observed data.
nearest neighbor distribution
Nearest Neighbor Distribution
  • Define:
      • Bright “ordinary” galaxies defined as those brighter than magnitude Mord
      • Faint “test” galaxies defined as those in a particular, fainter, magnitude range Mtest
      • Dt0 the distance from a test galaxy to the nearest ordinary galaxy
      • D00 the distance from an ordinary galaxy to the nearest ordinary galaxy
results from nearest neighbor
Results from Nearest Neighbor
  • D00 distribution shifts to larger average distances for brighter ordinary samples
    • These type of “ordinary” galaxies are generally found near the centers of massive DM halos (large clusters)
      • Rarely another galaxy of comparable magnitude found in cluster
    • Dt0 is generally smaller than D00 in this case.
  • Dt0 distribution shifts to larger average distances for faintest ordinary samples
    • Fainter test samples fill in voids defined by ordinary galaxies
    • Typical Dt0 being ~50% larger than typical D00
variation in void sizes from void finder
Variation in Void Sizes from Void Finder

512 survey incomplete

  • Rapid decline in # density of voids for larger R, as predicted by VPF
  • Still more larger voids in galaxies than there are in DM
  • Fractional overdensity in voids 0.1 or less.

Prediction by Sheth (2002)

examination of void population using vf
Examination of Void Population using VF
  • Examine the prevalence of “void” galaxies and structure surrounding VF candidates.
    • Determine Void Density Profiles:
      • First, determine number density of galaxies in concentric shells centered on center of void.
      • Scale all lengths using the void radius so voids of all sizes can be compared.
      • These number densities are summed for all voids of a particular range of radii
void density contrast profiles from vf
Void Density Contrast Profiles from VF

Open circles denote DM

  • Voids are highly underdense, more so for galaxies than for DM
  • Outside Threshold, density contrast becomes larger for galaxies than it is for DM.
  • Clear threshold corresponding to edge of void at about r/Rvoid = 1
    • Indicates VF works
  • Density contrast in voids:
    • Little variation
    • Population dependent
void galaxies vs field galaxies vf
Void Galaxies vs. Field Galaxies (VF)
  • Halo mass function
    • Much lower for Void galaxies in relation to that of the Field galaxies
  • Median occupied halo mass
    • 6 times lower for void galaxies
specific star formation rate vf
Specific Star Formation Rate (VF)

Open indicates only galaxies at halo centers

  • Star Formation Rate
    • Higher towards centers of voids
  • Outside voids there exist very massive halos w/numerous satellite galaxies
    • In model used these galaxies have lost their fresh gas
  • In model specific star formation rate is higher in lower mass (dimmer) stars
    • See filled squares v. open squares


void galaxy properties
Void Galaxy Properties
  • Void galaxies found to have systematically different properties
    • Fainter
    • More blue
    • Tend to be disk-dominated
    • Higher star formation rates
  • Reasons for property variation:
    • Lower DM halo mass function than field galaxies
    • More clustering of galaxies outside of voids
    • Less gas available to galaxies that have existed in clusters for a long period of time
  • A wealth of information about voids can be obtained using simulations
  • Better data for larger sky surveys needs to be analyzed for an accurate comparison to observations, i.e. SDSS, 2dFGRS
  • Galaxies in voids tend to have share similar characteristics
    • It is still unclear if these galaxies constitute their own population