The Underdense Universe in Simulations of Cosmic

Structure Formation

Jörg M. Colberg (CMU)with

Tiziana Di Matteo (CMU), Ravi Sheth (UPenn), Antonaldo Diaferio (Torino), et al.

“Cosmic Voids”, Amsterdam December 2006

Outline• Part I – Voids in the Virgo Simulations

– Studying large samples of voids from a set of simulations done by/with the Virgo Consortium

– Presenting/summarizing work from Colberg et al., 2005, MNRAS, 360, 216

• Part II – Supermassive Black Holes in low density regions of the Universe– Studying the evolution of black holes in the centers of

galaxies using hydrodynamical simulations, as a function of the density of their environments

– Extremely preliminary results (Di Matteo et al., in prep. and Colberg et al., in prep.)

Part I – Voids in Virgo Simulations• Take a bunch of cosmological N-body

simulations (all done with/by the Virgoans) and look for voids, study their properties

• See Colberg et al., 2005, MNRAS 360, 216 for details (or me, any time during the conference!)

Finding voids in the distribution of dark matter

• Since the simulation suite contains only dark matter, we define voids as regions of space in the density field:– Particles → density field thru adaptive

smoothing– Treat voids as negative primordial negative

overdensity fluctuations that grew gravitationally and that reach shell-crossing at present time. This gives δ=-0.8

– We do not want to require our voids to be spherical, so we merge spherical proto-voids.

Finding voids in the distribution of dark matter (cont’ed)

• Merging proto-voids such that voids can eventually look like lumpy potatoes but not like dumbells

… so that one gets1. Voids are only

spherical in the picture, but we define an reff

2. … and since they’re irregularly shaped, this region in reality is part of a void

3. There are no voids inside voids, this is a projection effect

4. Voids in more crowded regions are smaller

5. There are haloes inside voids

(this image shows the GIF2 simulation)

Properties of Voids I• Top panel: cumulative

volume function for all simulations agree pretty well (differences at larger volumes)

• Bottom panel: with a choice of δ=-0.8 at all redshifts, the void volume function evolves quite strongly with z (plot shows GIF2)

Properties of Voids II• Top panel: cumulative volume

fraction shows that with our definition of voids, small voids cover most of the volume (GIF2)

• Fraction of total volume (GIF2):

• Bottom panel: Clear difference between mass functions of all haloes and only those in voids (c.f. Stefan’s talk)

Properties of Voids III – Internal Structure

(The largest GIF void)

Properties of Voids III – Internal Structure (cont’ed)

• Inside reff, voids have a universal density profile:

• No dependence on cosmology, void size, or choice of δ

(VLS plus fit) (GIF)

Properties of Voids IV – Structure

Voids are have quite sharp boundaries

Part II - Supermassive Black Holes in low density regions of the Universe

• Cosmological SPH simulation (using Gadget2) that includes DM, Gas, Stars, and Black Holes

• Gas: Radiative cooling of gas within haloes• Stars: Star formation and feedback, subresolution

multiphase model for ISM (Springel & Hernquist 2003)• Black Holes (compare, e.g., Di Matteo et al. 2005):

– collisionless “sink” particles in centers of haloes/galaxies (seed mass 105 M☼; on-the-fly FOF group finder to detect haloes that host new BHs)

– swallow gas particles according to gas density of their neighbourhood (which estimates the unresolved accretion on the BH to the resolved gas distribution)

– generate energy feedback (injected into gas around BH f=5%)– merge at small separations and low relative speeds– In other words, use lots of simplifications for physical processes that

cannot be resolved

The “D6” Simulation• Simulation set up and run by Tiziana Di Matteo (CMU)• 2.(486)3 (≈2.150m) particles in box of size (33.75 Mpc/h)3

• Particle masses: mDM = 2.75.107 M☼, mgas = 4.24.106 M☼, mBH(initial) = 105 M☼

• Run on Cray X3T MPP supercomputer at PSC with 2,068 2.4-GHz AMD Opteron procs with 1GB memory each

• Run until z=1• Each time a BH is active, its activity is output → can

construct detailed merger trees for each black hole (with more than 20,000 progenitors in the trees of the largest BHs)

• → Simulation reproduces observed BH mass density

The “D6” Simulation

Black Holes from D6

(Di Matteo et al., in prep.)

Black Holes from D6

(Di Matteo et al., in prep.)

SMBH’s in D6

• Formation redshift zf = redshift at which most massive progenitor first had 50% of z=1 mass

z = 1

SMBH’s and their environments I

• We describe environment using an adaptive criterion: r13, the radius of a sphere that contains a mass of 1013 M☼

SMBH’s and their environments II

• More massive haloes live in denser environments, and the same is true for (most) SMBH

• Many low-mass SMBH in wide range of environments!• Lowest density environments contain SMBH

SMBH’s and their environments III

• For zf ≥ 2.5 trend unchanged• Still many low-mass SMBH in wide range of

environments!

SMBH’s and their environments IV

SMBH’s and their environments V• Growth histories

of low-mass SMBH appear to be independent of environment.

• Growth dominated by accretion of gas, for many cases, accretion shuts off at z<2 - for SMBH in voids and large haloes → supply of gas shut off

SMBH’s and their environments VI• The results in this part are very preliminary and

need a lot of follow-up work.• It is clear already that haloes in voids host

SMBH’s just like haloes in higher-density regions.

• It currently looks like that the formation histories/processes of SMBH in the mass ranges found in voids and elsewhere are independent of environment. They grow by slowly accreting gas, whose supply appears to get slowly shut off for lower z. So it looks like there is no distinctive void SMBH population.

Summary• Large cosmological simulations of structure formation

are ideal tools for studies of voids• Simulations containing only dark matter show that

– Just like voids in galaxy surveys, voids in the dark matter density field have steep edges

– With out choice of void finding, there is no peak in the distribution of void sizes (we need systematic comparison studies of void finders!)

– Voids have a universal density profile

• Sophisticated hydro simulations that include the formation of SMBH show (somewhat preliminary right now) that (assuming you believe the model that went into the simulation)– (mostly low-mass) black holes exist in quite underdense regions– if you fix the mass of the black holes such that you pick a sample

that covers a wide range of environments there does not appear to be any systematic trend with environment