Observational Constraints on Hot Gas Accretion

Joel BregmanUniversity of Michigan

Collaborators: Mike Anderson, Xinyu Dai

Do We Actually Need Accretion Today?

• Usual Argument: – If we don’t replenish the gas, we’ll run out soon– That would be a shame

• How long does it take to run out of gas?– Roberts Time: Mgas /Star Formation Rate– 3 Gyr

• A 1990’s concept to the rescue

Old enough to vote!

How Long is Long Enough?• With stellar feedback, the gas depletion time

is about 5-15 Gyr– Really something like an e-folding time

• The Milky Way – Stellar mass-loss rate is about 1 Msun/yr– Star formation rate is 1-3 Msun/yr (similar)

• The star formation rate will slowly go down unless there is accretion

• Inconsistent with observations?

• Star formation rate is decreasing (for last 8 Gyr)

• E-folding time only 3 Gyr• If accretion = star

formation, rate turns back up! – (when including stellar

mass loss)

• Natural state of affairs is a decreasing star formation rate now

• There are not the “good old days”

Ouchi et al. (2009)

• But….– The gas depletion time is shorter in inner galaxy– That will use up the gas relatively faster in the

inner parts• It would lead to a local minimum in the gas

reservoir in the inner part of the disk• Observed in most spiral galaxies• Central gaseous hole should get bigger over

time• Meet back in 3 Gyr to find out (very long-term

funding)

Smooth continuation of baryon loss from clusters through galaxiesEllipticals may be more baryon poor than spirals (weak lensing)“Average” spiral (like M33) is missing 90% of baryons

McGaugh et al. 2010

Dai et al. (2010)

Missing Baryons in Galaxies

Missing Baryon from Galaxies• Galaxies are missing 70-95% of their baryons• Were the baryons expelled from galaxies?– Maybe they didn’t fall in to begin with

• Where are these missing galactic baryons?– A hot halo within Rvirial?– Milky Way (Anderson and Bregman 2010)– What fraction of this missing 2x1011 M of hot gas lies in

the Galactic Halo (3/4 of baryons “missing”)?– Baryons around massive spirals (Anderson et al. 2011)– Baryons around L* galaxies (Anderson et al. 2012)

Searching For A Hot Halo of Gas

• Why X-ray emitting gas?• Missing baryons– Hard to detect– Long-lived

• Should be in a stable configuration for Gyr• Rotational support: Disk (but we see that)• Dynamical support: Stars (we see that too)• Hydrostatic equilibrium

• Hydrostatic Equilibrium– tcool > tsound

– tcool ~ tH

– low density and hot– Natural Temperature = Dynamical T = 1-10x106 K

• Most astronomical objects have a characteristic gravitational T in the X-rays– OVII, OVIII, Fe L + continuum

• Models (sometime) tell us such hot gas is present• Hug an X-ray astronomer today

Constraints on Gas Around Milky Way• Limits on halo gas from pulsar dispersion measure• Dispersion measure: integral of ne along line of sight• Pulsars in LMC have a DM above that of the MW• Most of this could be due to the LMC environment• If due to path toward LMC ne = 5E-4 cm-3

• NFW profile (concentration of 12) out to Rvirial = 250 kpc

• 1.5E10 M

• 4% of the missing baryons• Constraint from the Galactic soft X-ray background• Use hotter component (3E6 K)• NFW Profile, Mgas = 6E9 M

• 2% of the missing baryons

Lower Limit to the Hot Halo• Dwarfs closer than about 280 kpc have had their gas

stripped (Blitz & Robishaw 2000; Grcevich et al. 2009)

– Ram pressure stripping – n = 2.5E-5 cm-3 at d = 250 kpc (about the virial radius of

the MW)– > 5E8 Msun of gas out to the LMC

– 1E10 Msun of gas within virial radius– Cooling time longer than Hubble time (but density likely

to rise at smaller radii)• Other constraints– pressure from halo clouds– Interaction of Magellanic Stream with environment

• Is there some other gas distribution possible?

• Kauffmann et al (2009): preheat gas so it has a shallow distribution

• n ~ r-0.9

• Reduces XRB, DM, etc.• MW halo can have 6-13% of

missing baryons• MW missing baryons not in a

hot halo• Less restrictive for external

galaxies

Milky Way Summary• Good evidence for an extended hot halo (OVII absorption)• Out to the LMC, mass is in range 0.5-3E9 Msun

• Within Rvirial of MW (250 kpc), ~1E10 Msun

• Not a significant fraction of the missing baryons• Cooling time can be less than Hubble time close to the

MW• Current cooling rate probably not much more than 0.2

Msun yr-1 (unless special mechanism: Binney)• the current inflow is dominated by stellar mass loss (1

Msun yr-1) and the Magellanic Stream

Detection of a Hot Gaseous Halo Around the Spiral Galaxy NGC 1961

NGC 1961 is one of the largest spiral galaxies in the local Universe:

HI rotation curve, from Haan+ 2008Inclination-corrected, I = 43o

DSS imageeach box is 17’ (280 kpc) on a side

Anderson, M. E. and Bregman, J. N. 2011, ApJ, 737, 22

NGC 1961 X-Ray Surface Brightness Profile(with smoothed background)

smoothedbackgrounds

95% confidence bounds

NGC 1961 Results

M(flattened component) < 7.4 x 1011 Msun

fb = 0.024-0.029 (or 0.051 for a flattened component) still seems to be missing 75% of its baryons!

halo accretion rate (cooling) = 0.4 Msun / year NGC 1961 SFR = 6.0 Msun / year NGC 1961 M* = 3.1 x 1011 Msun

< 50 kpc (measured)

< 500 kpc(extrapolated)

Gas Mass (Msun) 4.9-5.2 x 109 1.4-2.6 x 1011

Luminosity (erg/s)(unabsorbed, 0.6-2 keV) 3.4-3.9 x 1040 5.6-11.5 x 1040

UGC 12591

SDSS

vmax is nearly 500 km s-1

Early-type spiral (S0/Sa)as opposed to NGC 1961 (Sc)

XMM-Newton Observation of the Massive Galaxy UGC 12591”Dai, X., Anderson, M. E. Bregman, J. N., and Miller, J 2011, in press, astro-ph

D = 100 Mpc

Decomposing and fitting the surface brightness profile

stellar emission

XRB emission

hot halo emission

maximumflattened

profile

Χ2/dof= 4.6/6

UGC 12591 Spectrum

APEC model:

data+model

residual

Model: (APEC + PL) x (PHABS+PHABS)

(inner 25 kpc)

UGC 12591 Results

M(flattened component) < 3.5 x 1011 Msun

This galaxy is also missing ~75% of its baryons, and the accretion rate is also insufficient to assemble its stellar mass in a Hubble time.

< 50 kpc (measured)

< 500 kpc(extrapolated)

Gas Mass (Msun) 4.1-4.7 x 109 0.45-2.3 x 1011

Luminosity (erg/s)(unabsorbed, 0.6-1.4 keV) 2.2-2.5 x 1040 2.5-7.1 x 1040

< 500 kpc(extrapolated)

1.4-2.6 x 1011

5.6-11.5 x 1040

Turnover in the BTF?

Ordinary Galaxies: The ROSAT Stacking ProjectAnderson, Dai & Bregman (2012)

K-band absolute magnitude Distance

N=756N=1695

Isolated spirals and ellipticals

Spiral + elliptical galaxies

Radius = 100 pix = 500 kpc

Fit: A “beta” surface brightness component, a point source (< 5 kpc) + background

Observed

Extrapolated to Virial Radius

Good News: Detect Extended Hot Halos Around Spirals and Ellipticals

Gas mass significant (but not more than 10% of missing baryons)

Cooling rate 0.1 Msun/yr

Galaxy Missing Baryons: Outflow or No Infall?

• “Going-In” Expectation– Galaxies formed through accretion + merger– At one time they had their cosmological baryon

content– Starburst-driven galactic winds drive out most of the

baryons• Is there really enough energy to drive out >90% of

the baryons (some galaxies are mostly gaseous)?• If outflows due to stars, predict fewer missing

baryons in star-poor (gas-rich) galaxies

Baryon “poorness” unrelated to Star/Gas Ratio

the fraction of stars unrelated to baryon fraction.Star-poor galaxies don’t have enough SN energy to drive a windSN unlikely to drive out the baryons (some galaxies very star-poor).Stark et al. (2009), McGaugh (2005) data

Star-poor galaxies

Star-rich galaxies

Gas-richGas-poor

Anderson and Bregman 2010

Galaxy Missing Baryons: Outflow or No Infall?

• No evidence that most of the baryons were expelled – Baryon Depletion Independent of M*/Mgas, bulge

prominence (AGN)– Some outflows are driven by starbursts, but this is a

small amount of mass– Not enough energy to drive gas out in some cases

• No Infall is more consistent with the data

Anderson and Bregman 2010

Where Did The Gas Go?• Size of the missing baryon region, Rgas – Trivial to detect if Rgas < 50 kpc– We now rule out Rgas < Rvirial

• Causes too much emission and absorption

– If metallicity is about 0.2 solar, Rgas > 2-3 Rvirial

• Otherwise, OVII absorption would be widely seen– Missing baryons not in galaxy groups

• Rgas > 1 Mpc (4 Rvirial)

• Can gas get 1 Mpc away from a galaxy in 10 Gyr?– 100 km/sec (sound speed of 106 K gas)– Need early population of SN for heating

What Prevents Infall?

• This involves a visit to….• (but just one visit)

What Prevents Infall?• Preheating before the galaxy is formed• Preheating by High-Mass Population of stars– 2 < Z < 8– Before galaxy collapse– Entropy floor (preheating is 0.4 keV; 5x106K)

• Need about 1 SNe per 500 M of gas

• Other Consequences of this Population– Enrich the metals by distributed SNe

• 0.2 Solar metals is also 1 SNe per 500 M of gas• Widespread metal dispersal

– Solves the G-dwarf– Not all mass is retained by poor clusters– May lead to mass-metallicity relationship

Summary• Don’t really need accretion today to sustain star

formation in spirals• Typical galaxy is missing 90% of its baryons• Hot extended (70 kpc) halos detected around spirals

and ellipticals– 109 M of gas actually observed

– Extrapolation to virial radius: 1010 M of gas – Never see more than 10% of missing baryons

• Missing hot baryons very extended, 3-4 Rvirial

• Missing baryons never fell in– Preheated by early population of massive stars