Galaxy Formation. Physics 463, Spring 07

Transcription

1 Physics 463, Spring 07 Lecture 9 Galaxy Formation We see that scattered through space! out to in"nite distances! there exist similar systems of stars! and that all of creation! in the whole extent of its in"nite grandeur! is everywhere organized into systems whose members are in relation with one another # Immanuel Kant! $%&& Galaxy Light in the Universe today

2 galaxies change with environment galaxies change with wavelength long, cold The Milky Way galaxies change with time short, hot galaxy properties change with type most galaxies consist of two components, a disk and a bulge. Stars in the bulge are typically older (redder light) early type galaxies live in denser regions

3 basic issues in galaxy formation why is there a characteristic mass for galaxies? why is star formation so inefficient (why aren t baryons mostly stars?) what sets the very tight correlations between galaxy properties? galaxy scaling relations, bulge BH what role does environment play? how do galaxies at one epoch relate to those observed at another? observational targets luminosity functions, colors [N(m), N(z)] cold gas (H I, H 2 ), hot gas (X-ray) metallicities of stars, cold & hot gas structural/kinematic properties (radius, V c(r),!(r)) spatial clustering star formation rates all of the the above as function of wavelength (UV-submm), redshift, galaxy type, environment correlations/scaling relations (e.g. Tully-Fisher, fundamental plane) gravitationally bound structures, and hence galaxies occur at the peaks in the density distribution Collapsed protogalactic clouds RADIATION 5 density Kauffmann, Diaferio, Colberg & White 1999 position

4 two-stage galaxy formation Gas cools in virialized dark matter halos. Physics of halos is nonlinear, but primarily gravitational. galaxy formation is not equally efficient at all masses Complicated gastrophysics (star formation, supernovae enrichment, etc.) mainly determined by local environment (i.e., by parent halo), not by surrounding halos. inflation primordial power spectrum gravity collisional heating radiative cooling star formation stellar feedback chemical enrichment stellar populations dust absorption & emission BH formation AGN activity feedback galaxy observables simulation: solve equations of physics (e.g., gravity, thermo, hydro, etc.) using particles or mesh cells semi-analytic: trace bulk quantities using approximations. usually Monte-Carlo based no spatial information unless you combine with simulations

5 If elliptical galaxies and spiral bulges are linked to galaxy created in these events, too. H ole masses are strongly correlatmergers, then it follows that supermassive black holes may be ed with the mass of the surrounding elliptical galaxy or bulge; created in these events, too. H ole masses are strongly correlatthey are not correlated with the mass of the spiral disk. M erged with the mass of the surrounding elliptical galaxy or bulge; er models have been extended to incorporate supermassive they are not correlated with the mass of the spiral disk. M ergholes and therefore A G Ns. The abundant gas that is funneled er models have been extended to incorporate supermassive toward the center during a merger could revive a dormant black holes and therefore A G Ns. The abundant gas that is funneled toward the center during a merger could revive a dormant black en t i fi c Ame r ican, June 2000]. In heftier galaxies such as the T hese results are intriguing because astronomers have often M ilky W ay, star formation occurs at a more constant rate. hypothesized that the mass of a galaxy determines its fertility. T hese results are intriguing because astronomers have often In lightweight galaxies, supernova explosions can easily disrupt hypothesized that the mass of a galaxy determines its fertility. or even rid the system of its gas, thus choking off star formain lightweight galaxies, supernova explosions can easily disrupt tion. Even the smallest perturbation can have a dramatic effect. or even rid the system of its gas, thus choking off star formait is this sensitivity to initial conditions and random events tion. Even the smallest perturbation can have a dramatic effect. It is this sensitivity to initial conditions and random events HOW RELAXING HOW RELAXING its shape and density profile. (An analogous equilibrium determines separating light from dark AN INTERNAL STATE OF EQUILIBRIUM is what makes a galaxy a distinct object rather than merely an arbitrary patch of space. This AN INTERNAL STATE OF EQUILIBRIUM is what makes a galaxy a equilibrium determines the overall properties of the galaxy, such as distinct object rather than merely an arbitrary patch of space. This ORDINARY MATTER equilibrium determines the overall properties of the galaxy, such as the size and temperature of stars.) The ordinary matter and dark its shape and density profile. (An analogous equilibrium determines matter attain equilibrium by different means. the size and temperature of stars.) The ordinary matter and dark matter attain equilibrium by different means. t1 t2 t3 t4 The First Structures ORDINARY MATTER 1 ordinary matter: 2 *hydrostatic+ 3equilibrium 1 pressure forces 3 gravity 2 of the gas balance The ordinary matter predominantly hydrogen gas starts off moving every The ordinary matter predominantly which way. Its density varies randomly. hydrogen gas starts off moving every which way. Its density varies randomly. DARK MATTER The gas particles bang into one another, redistributing energy and The gasaparticles into onegravity. generating pressurebang that resists another, redistributing energy and generating a pressure that resists gravity. Eventually the gas settles down into hydrostatic equilibrium, with the Eventually into density highestthe neargas thesettles centerdown of gravity. hydrostatic equilibrium, with the density highest near the center of gravity. 2 3 DARK MATTER Mvir,Vvir, rvir c, " start with a merger tree. DON DIXON DON DIXON t5 1 1 Initially the dark matter has the same arrangement as ordinary matter. The Initially the dark matter has the collide. same difference is that the particles do not arrangement as ordinary matter. The difference is that the particles do not collide. As the particles move around, the gravitational field changes, which As the particles move the causes particles to gain oraround, lose energy. gravitational field changes, which causes particles to gain or lose energy. Gradually the system settles down into virial equilibrium, in which the Graduallyfield the no system down into gravitational longersettles fluctuates. virial equilibrium, in which the gravitational field no longer fluctuates. dark matter: 2 *virial+ equilibrium 3 kinetic energy of the particles balances gravity m.com COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. SCIENTIFIC AMERICAN SCIENTIFIC AMERICAN Risa H' Wechsler! Spring ())& Compton Lectures Gas cooling halos are spherically symmetric hot gas initially follows the dark matter distribution gas is shock heated to virial temperature collisional equilibrium assumed Cooling rate from collisional ionization is strong function of temperature and metallicity, so cooling rate is function of position in halo (~ radius from centre) Cooling by Bremsstrahlung continuum dominates at T>108 K, metal line-cooling important at K Cooling rate defines time; since rate depends on radius, cooling time depends on radius Gas cools within cooling radius : radius where cooling time = tuniverse White & Frenk 1991

6 for an assumed gas density profile, can solve for cooling radius cooling depends on the metalicity of the gas r cool cooling radius circular velocity Somerville & Primack 99

7 Cooling catastrophe Prescription for cooling straightforward, but there is a problem: cooling times in centres of massive halos extremely short (T ~ m 2/3 ) Prescription leads to massive, luminous central galaxies which are NOT observed Solution: switch-off cooling BY HAND in all halos with!gm vir /R vir > 350 km/s or, come up with some other way to do it... simple picture of Hubble type Two parts to galaxies: bulge and disk Ellipticals all bulge, Sd galaxies basically all disk clumps gain angular momentum from interactions and tidal torques gas collapses to form a disk Disk Formation

8 t 1 star formation Key: t 2 dark matter hot gas cold gas t 3. # * = C # 1.5 g above critical threshold note: this simplified model is almost certainly wrong! t 4 Kennicutt 1989, 1998; Martin & Kennicutt 2001 SFR $ = % & cold /t dyn = % & cold (10V vir /R vir ) dm * /dt = m cold /[) 0 t dyn (1+z) * ] (V 0 /V c ) % Note t dyn = R vir /10V vir ~ 0.01/H ~ 0.01 t Universe because GM/R = V 2 so (4'G/3)(3M/4'R 3 ) =(4'G/3)200( crit =(V/R) 2 But ( crit =(3H 2 /8'G) so (10H) 2 =(V/R) 2 Fudge factor % = % 0 (V vir /220 km/s) % 1 Makes star formation efficiency depend on halo circular velocity and redshift (at fixed V, R is smaller at high z, so SFR $ is larger) ) 0 set by fitting present-day luminosity and gas fraction degeneracies in scaling with redshift can be broken by high redshift observations e.g. Kauffmann et al. 1999; rss & Primack 1999, Kauffmann & Haehnelt 2000; Cole et al. 2001; rss, Primack & Faber 2001

9 feedback Energy input from stars which form and then explode as SNae will heat gas, preventing further cooling: +M reheat =, (4/3) (- SN E SN /V vir 2) +M star Uncertainties - SN : number of SNae per solar mass in stars, depends on IMF (~0.0063/M sun ) E SN : energy released per SN (~10 51 ergs),: efficiency of process(!) Is reheating local? Global? Does energy leave halo (e.g., SN winds may exceed escape velocity of low mass halos)? Ejection vs. Retention Retention: shocked material is reheated to virial temperature, and is then again available for cooling Ejection: +M back = * M eject (V/R) +t (ejected gas falls back on a timescale determined by *; mainly purpose is to remove some of gas from the cooling reservoir) Winds: dm wind /dt= c$ (wind strength scales with SFR ~ observed) in presence of UV ionizing background, halos with virial temp < background radiation field are unable to accrete gas (! < km/s) gas can be boiled out of halos (!<20 km/s) cooling function modified (cooling suppressed at low T) eg. Somerville 2002 Benson et al. 2002

10 t mrg /t dyn = orbit (M H /M s ) /ln(/ c ) where ln(/ c ) = ln(m H /M s ), dynamical friction. orbit = [J/J c (E)] % [r c (E)/r vir ] 2 %= van den Bosch et al. 1999; Colpi et al mergers can trigger starbursts disk regrows after a merger major merger: trigger big burst, destroy disks, form a spheroid minor merger: trigger little burst e burst = (m 1 /m 2 ) %

11 dust absorption and emission inclination dependence: slab model energy absorbed = energy emitted optical depth of dust proportional to column density of metals in disk Z gas N H increasing L bol star formation efficiency SN feedback efficiency chemical yield stellar IMF dust optical depth normalization adjusted to fit a subset of observations then left fixed Star formation (% 0 ~0.1; % 1 ~2.5): changes have weak effect on L, but strong effect on correlation between gas fraction and mass Return fraction (R~0.35): changes have small effect on metallicity Feedback efficiency (,~0.35; c~5): increase feedback! decrease L, with bigger effect at small mass Efficiency (per solar mass of gas to stars) of metal production (Y~0.04): more metals! more cooling in small mass halos! more L at small mass Timescale for reincorporating ejected gas: if long (*~0.1) then this is effectively like feedback first generation merger tree SAMs: did not attempt to model galaxy sizes or internal velocities (e.g., Cole et al. 1994; Kauffmann et al. 1998; 1999; rss & Primack 1999) disk models: monolithic formation or smooth accretion assumed (e.g. Mo, Mao & White 1998; van den Bosch 2001) merging neglected second generation merger tree SAMs: detailed modeling of sizes of disks and spheroids, rotation curves and velocity dispersions (e.g. Cole et al. 2000; rss in prep.) within merger trees

12 pure SAM: dark matter merger tree constructed using extended Press-Schechter hybrid SAM+N-body, a posteriori galaxies associated with halos in N-body at output redshift merging histories obtained using EPS hybrid SAM+N-body, a priori structural merger trees halo merger histories extracted from N-body more on heating and cooling AGN feedback models sizes & structural parameters of galaxies galaxy evolution + environmental trends