Gustavo Yepes Universidad Autónoma de Madrid KNAW International Colloquium on COSMIC VOIDS Amsterdam 2006

COLLABORATORS Matthias Hoeft (IU. Bremen) Stefan Gottlöber (AIP) Volker Springel (MPG)

CDM Mass function in void regions High-resolution N-body Simulations Gottlöber et al 2003

The missing dwarf galaxy problem in void regions ● If  CDM comology is correct there should be a lot of small halos (Vc < 20 km/s ) in voids. Same problem as satellites of galaxies: too much substructure in CDM models. ● No galaxies brighter than M B =-11 (Karanchentsev talk) found in local voids. ● What happens with baryons of small halos in voids? – Are they visible but very faint?. Magnitude, colors. (Red Dwarfs) – Are they just baryonless dark halos? ● What are the physical mechanisms ? – Gas evaporation by UV photoionization – Supernova feedback (e.g Dekel & Silk) ● What is the typical halo mass for this to happen?

GALAXY FORMATION IN VOIDS M. Hoeft, G. Yepes, S. Gottlober and V. Springel, MNRAS 2006, 371, 401 And Matthias Hoeft’s talk tomorrow

HIGH RESOLUTION  CDM VOID SIMULATIONS MULTIMASS TECHNIQUE Multi-mass technique to achieve high resolution: Re-Simulated void areas from large computational boxes by resampling particles of incresing mass away from the refined region: Original intial conditions set up to particles in a big (50-80 Mpc) box. S. Gottloeber’s void finding algorithm: Spherical void regions are selected as maximum spheres that do not contain any large (> 2x10 11 M  )halo

(G)ASTROPHYSICAL PROCESSES ● To study in detail the galaxy formation process we take into account: ● Radiative and Compton cooling ● UV-photoionization ● Multiphase ISM. ● Star Formation. ● Star-Gas back-reactions – SN’s thermal Feedbacks: Cloud Evaporation and gas reheating – Stelar Winds ● Springel-Hernquist (2003) implementation of multiphase SPH modeling in GADGET-2.

VOIDS FROM A 80/h Mpc Box effective particle in void region M gas = M gas = 5.5  10 6 M  M dark = M dark = 3.4  10 7 M  Smoothing= 2 kpc Smoothing= 2 kpc 20/h Mpc Simulations done with GADGET2 Primordial Cooling Photoionization Multiphase medium Star formation Feedback Thermal Thermal Kinetic (Winds) Kinetic (Winds)

15/h Mpc

Void with effective particles (7 million particles ) –M gas = 1.5  10 6 M  –M dark = 8.2  10 6 M  Spatial smoothing= 2 kpc Same void was resimulated with full resolution ( 43 million particles in total) –M gas  2  10 5 M  –M dark  10 6 M  –Spatial smoothing= 0.5 kpc comoving (ULTRA)HIGH-RESOLUTION SIMULATIONS OF VOIDS 50/h Mpc Box 10/h Mpc Different feedback parameters

List of simulations Hoeft et al 2006, MNRAS Hoeft et al 2006, MNRAS 371, Simulations with Kinetic Feedback (stellar winds) With different values of  (fraction of E SN that goes into wind  + Simulations with Kinetic Feedback (stellar winds) With different values of  (fraction of E SN that goes into wind 

Halo Mass function in Voids

Baryon fraction Halos below few times 10 9 M sun are baryon-poor Characteristic mass scale depends on redshift

Characteristic Mass of UV evaporation

Characteristic mass Characteristic mass M c baryon-rich baryon-poor M c rises significantly with z Halo may start baryon-rich and become later baryon-poor

T entry What is the characteristic Mass? Listen to Matthias Hoeft’s talk tomorrow Density –Temperature phase diagram Cold mode of galactic gas accretion: gas creeps along the equilibrium line between heating and Cooling (Keres et al. 04) For gas to be able to enter the instabilty branch, need to be heated above Tentry temperature. Mc can be viewed as the critical mass for which the virial temperature of gas inside is above the entry temperature of the thermal unstable regime.

New Filtering Mass Estimate (M. Hoeft’s talk tomorrow) Smoothing of baryonic Fluctuations due to counteracting pressure gradiants M f = 2  a/k f ) 3 M f = 4/3  2  a/k f ) 3

Mass accretion histories and gas condensation in void halos McMcMcMc

Age of stars in void halos In small halos stars can only be formed at high redshift

Stellar mass function

Thermal feedback Mass-Luminosity function z=0 Bruzual & Charlot 03 SPSM

Thermal feedback Strong wind model Mass-Luminosity function z=0

Cum. Luminosity function High Resolution Basic Small Winds

Luminosity function Wind 0.15 Wind 0.05 Wind 0.4 Wind 0.25 Kinetic Feedback: Energy in winds

Luminosity function Effects of UV flux UV=0UV=0 UV*100UV*100 UV*0.1UV*0.1 UV*10UV*10 UV*0.01UV*0.01

COLOR-MAGNITUDE

COLOR-MAGNITUDE UV Normalization

COLOR-MAGNITUDE Wind Energy

Filtering Mass at z=0 McMc

Mean age of stars Mean Metallicity of stars

DWARF GALAXIES IN GROUPS: Same resolution than void simulations ( effective) 11.5 million total particles 4.5 million gas 4.5 million high-res dark

Dwarfs in voids and groups

CONCLUSIONS At present, halos below M lim ~ 7x10 9 M  (v c ~ 27 km/s) are photo-evaporated and have few baryon content, either cold gas or stars. This mass scale decreases with redshift. Very small dependence of UV flux. Halos can condense baryons and form stars early than reionization redshift. Seems enough for them to shine today. UV-photoheating not able to suppress most of the small galaxies: Does not solve the problem of excess of substructure of  CDM. Thermal feedback (as implemented in the simulations) does not play a significant role in keeping gas out of small halos. Kinetic feedback (winds) is very efficient in suppressing star-formation provided that a substantial fraction of ESN goes into wind energy. Need more work on feedback modeling. Dwarfs ending up in other environments (e.g groups) have similar properties as long as they have not experience substantial interactions with the host halo.

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