Transcript Observational Constraints on Hot Gas Accretion

Observational Constraints on Hot
Gas Accretion
Joel Bregman
University 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)
Missing Baryons in Galaxies
Dai et al. (2010)
McGaugh et
al. 2010
Smooth continuation of baryon loss from clusters through galaxies
Ellipticals may be more baryon poor than spirals (weak lensing)
“Average” spiral (like M33) is missing 90% of baryons
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
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•
•
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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
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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
Anderson, M. E. and Bregman, J. N. 2011, ApJ, 737, 22
NGC 1961 is one of the largest spiral galaxies in the local Universe:
HI rotation curve, from Haan+ 2008
Inclination-corrected, I = 43o
DSS
image
each box is 17’ (280 kpc) on a side
NGC 1961 X-Ray Surface Brightness Profile
(with smoothed background)
95% confidence bounds
smoothed
backgrounds
NGC 1961 Results
< 50 kpc
(measured)
Gas Mass (Msun)
Luminosity (erg/s)
(unabsorbed, 0.6-2 keV)
< 500 kpc
(extrapolated)
4.9-5.2 x 109
1.4-2.6 x 1011
3.4-3.9 x 1040
5.6-11.5 x 1040
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
UGC 12591
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
vmax is nearly 500 km s-1
Early-type spiral (S0/Sa)
as opposed to NGC 1961 (Sc)
SDSS
Decomposing and fitting the surface
brightness profile
hot halo emission
XRB emission
stellar emission
Χ2/dof= 4.6/6
maximum
flattened
profile
(inner 25 kpc)
0.1
data+model
0.01
APEC model:
10− 3
2
0
·
Rate (cnts s− 1 keV− 1)
UGC 12591 Spectrum
−2
−4
residual
0.5
1
Observed− Frame Energy (keV)
Model: (APEC + PL) x (PHABS+PHABS)
2
UGC 12591 Results
< 50 kpc
(measured)
Gas Mass (Msun)
Luminosity (erg/s)
(unabsorbed, 0.6-1.4 keV)
< 500 kpc
(extrapolated)
4.1-4.7 x 109
0.45-2.3 x 1011
2.2-2.5 x 1040
2.5-7.1 x 1040
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.
< 500 kpc
(extrapolated)
1.4-2.6 x 1011
5.6-11.5 x 1040
Turnover in the BTF?
Ordinary Galaxies: The ROSAT Stacking Project
Anderson, Dai & Bregman (2012)
K-band absolute magnitude
N=756
N=1695
Isolated spirals and ellipticals
Distance
Spiral + elliptical galaxies
Radius = 100 pix = 500 kpc
Fit: A “beta” surface brightness component,
a point source (< 5 kpc) + background
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
Observed
Extrapolated
to Virial
Radius
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
Anderson and
Bregman 2010
Star-rich
galaxies
Gas-poor
Gas-rich
Star-poor
galaxies
the fraction of stars unrelated to baryon fraction.
Star-poor galaxies don’t have enough SN energy to drive a wind
SN unlikely to drive out the baryons (some galaxies very star-poor).
Stark et al. (2009), McGaugh (2005) data
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