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

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Presentation on theme: "Observational Constraints on Hot Gas Accretion Joel Bregman University of Michigan Collaborators: Mike Anderson, Xinyu Dai."— Presentation transcript:

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 dont replenish the gas, well run out soon – That would be a shame How long does it take to run out of gas? – Roberts Time: M gas /Star Formation Rate – 3 Gyr A 1990s 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 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 McGaugh et al Dai et al. (2010) Missing Baryons in Galaxies

8 Missing Baryon from Galaxies Galaxies are missing 70-95% of their baryons Were the baryons expelled from galaxies? – Maybe they didnt fall in to begin with Where are these missing galactic baryons? – A hot halo within R virial ? – Milky Way (Anderson and Bregman 2010) – What fraction of this missing 2x10 11 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 – t cool > t sound – t cool ~ t H – low density and hot – Natural Temperature = Dynamical T = 1-10x10 6 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 n e 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 n e = 5E-4 cm -3 NFW profile (concentration of 12) out to R virial = 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, M gas = 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 M sun of gas out to the LMC – 1E10 M sun 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 M sun Within R virial of MW (250 kpc), ~1E10 M sun 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 M sun yr -1 (unless special mechanism: Binney) the current inflow is dominated by stellar mass loss (1 M sun yr -1 ) and the Magellanic Stream

15 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 Inclination-corrected, I = 43 o DSSimage each box is 17 (280 kpc) on a side Anderson, M. E. and Bregman, J. N. 2011, ApJ, 737, 22

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

17 NGC 1961 Results M(flattened component) < 7.4 x M sun f b = (or for a flattened component) still seems to be missing 75% of its baryons! halo accretion rate (cooling) = 0.4 M sun / year NGC 1961 SFR = 6.0 M sun / year NGC 1961 M * = 3.1 x M sun < 50 kpc (measured) < 500 kpc (extrapolated) Gas Mass (M sun ) x x Luminosity (erg/s) (unabsorbed, keV) x x 10 40

18 UGC SDSS v max 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 Dai, X., Anderson, M. E. Bregman, J. N., and Miller, J 2011, in press, astro-ph D = 100 Mpc

19 Decomposing and fitting the surface brightness profile stellar emission XRB emission hot halo emission maximum flattened profile Χ 2 /dof= 4.6/6

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

21 UGC Results M(flattened component) < 3.5 x M sun 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 (M sun ) x x Luminosity (erg/s) (unabsorbed, keV) x x 10 40

22 < 500 kpc (extrapolated) x x Turnover in the BTF?

23 Ordinary Galaxies: The ROSAT Stacking Project Anderson, Dai & Bregman (2012) K-band absolute magnitude Distance 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 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

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 the fraction of stars unrelated to baryon fraction. Star-poor galaxies dont 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 Star-poor galaxies Star-rich galaxies Gas-richGas-poor Anderson and Bregman 2010

28 Galaxy Missing Baryons: Outflow or No Infall? No evidence that most of the baryons were expelled – Baryon Depletion Independent of M * /M gas, 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, R gas – Trivial to detect if R gas < 50 kpc – We now rule out R gas < R virial Causes too much emission and absorption – If metallicity is about 0.2 solar, R gas > 2-3 R virial Otherwise, OVII absorption would be widely seen – Missing baryons not in galaxy groups R gas > 1 Mpc (4 R virial ) Can gas get 1 Mpc away from a galaxy in 10 Gyr? – 100 km/sec (sound speed of 10 6 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; 5x10 6 K) 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 Dont 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 – 10 9 M of gas actually observed – Extrapolation to virial radius: M of gas – Never see more than 10% of missing baryons Missing hot baryons very extended, 3-4 R virial Missing baryons never fell in – Preheated by early population of massive stars


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