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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.


Void 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.


The universe within 1 billion light years
The Universe within 1 billion Light Years degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.


What is in a void
What is in a void? degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.

  • 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 degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.

  • 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


UZC degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.


Pscz redshift survey
PSCz Redshift Survey degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.


Void finding algorithm
Void Finding Algorithm degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.

  • 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 degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.

  • 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 degrees, or 10 percent of the sky, shows the distribution in space of some 2 million galaxies.

  • 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



Pscz uzc
PSCz UZC volume.

  • 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



Conclusion
Conclusion volume.

  • 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 volume.

Chooi Fei Ng and Aron Cooper


Outline
Outline volume.

  • 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


Motivation
Motivation volume.

  • 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 volume.

  • 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) volume.

  • 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 volume.

  • 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 volume.

  • 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 volume.

  • 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 volume. 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 volume.

  • 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 volume.

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) volume.

  • 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) volume.

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

Squares


Void galaxy properties
Void Galaxy Properties volume.

  • 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


Conclusions
Conclusions volume.

  • 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


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