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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

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Observational Constraints on Hot Gas Accretion

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  1. Observational Constraints on Hot Gas Accretion Joel Bregman University of Michigan Collaborators: Mike Anderson, Xinyu Dai

  2. 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

  3. Old enough to vote!

  4. 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?

  5. 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)

  6. 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)

  7. Missing Baryons in Galaxies Dai et al. (2010) McGaugh et al. 2010 Smooth continuation of baryon loss from clusters through galaxies Ellipticalsmay be more baryon poor than spirals (weak lensing) “Average” spiral (like M33) is missing 90% of baryons

  8. 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)

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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: DSS image each box is 17’ (280 kpc) on a side HI rotation curve, from Haan+ 2008 Inclination-corrected, I = 43o

  16. NGC 1961 X-Ray Surface Brightness Profile (with smoothed background) 95% confidence bounds smoothed backgrounds

  17. NGC 1961 Results < 50 kpc (measured) < 500 kpc (extrapolated) Gas Mass (Msun) 4.9-5.2 x 109 1.4-2.6 x 1011 Luminosity (erg/s) 3.4-3.9 x 1040 5.6-11.5 x 1040 (unabsorbed, 0.6-2 keV) M(flattened component) < 7.4 x 1011Msun 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 1011Msun

  18. 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

  19. Decomposing and fitting the surface brightness profile hot halo emission XRB emission maximum flattened profile stellar emission Χ2/dof= 4.6/6

  20. UGC 12591 Spectrum (inner 25 kpc) data+model APEC model: residual Model: (APEC + PL) x (PHABS+PHABS)

  21. UGC 12591 Results < 50 kpc (measured) < 500 kpc (extrapolated) Gas Mass (Msun) 4.1-4.7 x 109 0.45-2.3 x 1011 Luminosity (erg/s) 2.2-2.5 x 1040 2.5-7.1 x 1040 (unabsorbed, 0.6-1.4 keV) M(flattened component) < 3.5 x 1011Msun 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.

  22. < 500 kpc (extrapolated) 1.4-2.6 x 1011 5.6-11.5 x 1040 Turnover in the BTF?

  23. Ordinary Galaxies: The ROSAT Stacking ProjectAnderson, Dai& Bregman (2012) Distance K-band absolute magnitude N=756 N=1695 Isolated spirals and ellipticals

  24. Spiral + elliptical galaxies Radius = 100 pix = 500 kpc Fit: A “beta” surface brightness component, a point source (< 5 kpc) + background

  25. 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

  26. 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

  27. Baryon “poorness” unrelated to Star/Gas Ratio Anderson and Bregman 2010 Star-rich galaxies Star-poor galaxies Gas-poor Gas-rich 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

  28. 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

  29. 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

  30. What Prevents Infall? • This involves a visit to…. • (but just one visit)

  31. 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

  32. 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

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