A Dist ant Echo of M ilky Way Cent ral A ct ivity closes t he Galaxy s B aryon Census

Transcription

1 A Dist ant Echo of M ilky Way Cent ral A ct ivity closes t he Galaxy s B aryon Census F. Nicastro 1,2,F. Senatore [1,3], Y. Krongold 4,S. Mathur [5,6], M. Elvis 2 A B ST RACT We report on the presence of large amounts of million-degree gas in the Milky Way s interstellar and circum-galactic Nicastro medium. (OAR-INAF) This gas (1) permeatesboth the Galactic plane and F. the Senatore, halo, (2) Y. extends Krongold, to distances M. Elvis, larger S. Mathur than kpc from the center, and (3) its mass is sufficient to close the Galaxy s baryon census. Moreover, we show that a vast, 6 kpc radius, spherically-symmetric central region of the Milky Way above and below the 0.16 kpc thick plane, has either been emptied of hot gas or the density of thisgas within the cavity has a peculiar 7/4/2016 Mykonos 2016 (F. Nicastro) 1 profile, increasing from the center up to a radius of 6 kpc, and then decreasing

2 Outline The Galaxy roaming baryon problem All the Milky Way gaseous components, as seen in X-rays The million degree gas against Low Galactic Latitude (LGL: XRBs) and High Galactic Latitude (HGL: AGNs) lines of sight: Reconstructing the Galaxy s density profiles through OVII column densities + comparison with EM Three main results: Two components: disk + halo Presence of central cavity in the halo Hot gas in the halo sufficient to close the Galaxy s baryon census 7/4/2016 Mykonos 2016 (F. Nicastro) 2

3 The Galaxy Missing Baryons Problem Cosmological Baryon Fraction f b = The baryon deficit is more severe in smaller galaxies Less massive objects are unable to retain baryons (and metals?) due to feedback processes (winds)? Are these baryons in the surrounding CGM/IGM? 7/4/2016 Mykonos 2016 (F. Nicastro) 3

4 The Milky Way s Baryon Problem (M b ) Obs = 6.5x M (McMillian & Binney, 2012) M DM = (1-2)x10 12 M (Boylan f b = (The Plank Collaboration, 2015) (Boylan-Kolchin+12) M b /(M b ) Obs M b Missing (1.5-3)x10 11 M 7/4/2016 Mykonos 2016 (F. Nicastro) 4

5 All the X-ray Spectral Signatures of the Milky Way Nicastro+16a /4/2016 Mykonos 2016 (F. Nicastro) 5

6 The Optimal OVII Sample Differs from previous (e.g. Gupta+12, Miller&Bregman13): - HGL + LGL for the first time - Complete to SNRE>10 at 22 Å - Remove N OVII -b Degeneracy 18/20 (90%) LGL have OVII, but only 14 known distance (1 contaminated) 51 HGL; 9 contaminated + 3 Instr.Feat.; 34/39 (87%) have OVII, but only 18 Kα & Kβ 13 LGL (XRBs) + 18 HGL (AGNs) = 31 LoS 7/4/2016 Mykonos 2016 (F. Nicastro) 6

7 ensity n 0 (in cm 3 ) and the core n(r) =n 6 0 e R radius R s /RR c (in kpc): ; n(r) =n 0 e R R s /R c ; nsity n 0 (in Adopted cm 3 ) and the core Density radiusthe R c fit(in is performed Profiles kpc): by integrating, for a given se exponential profile, characterized each line of bysighthree direction, parameters: the density profile n(r) theal y n 0 (in cm 3 ) andn(r) the =n core 0 eradius R R s /R c lanar d CS exponential core ξ(r) radius = LoS profile, ρcoordinate, c ; (in kpc): c (incharacterized kpc) with [with and R 2 the = by ξ 2 + vertical three R 2 2ξRparameters: cos(b)cos(l)] core-height andthe comparing hnorm c ( e coplanar core radius ρ c (in kpc) CS exponential profile, n(r) =n characterized 0 e R density and R s /Rby with the c ; three the vertical core-height h c (in k parameters: the normali coplanar coren(r) radius=n ρ c (in 0 e (Exponential observed value. Sphere) All parameters are th (ρ/ ρ kpc) and at c ) 2 +(z h s )/h c ) the vertical 2 n(r) = n core-height ; 0 e (ρ/ ρ c each iteration, ) 2 +(z h s )/h c the χ 2 is evaluated h for c (in the entire kpc): s ) 2 ; exponential profile, characterized fit. When by three the full ranges parameters: of the valuesnorm have b lanar core radius n(r) ρ= c (in n 0 ekpc) (ρ/ ρ c ) and 2 +(z h s )/h c ) ofile, with with the the three three parameters completely n the vertical 2 00,, R c evaluated, and; (Exponential the the core-height index set of parameter Disk) β βcharacteriz values h c that (in kp the ofile: profile: le, with the three parameters n(r) = n 0 e (ρ/ ρ c n) 2 0 +(z, R c h s and )/h c ) the index β characterizing n(r) = n 0 [1 + (R R s ) 2 /Rc] 2 ; 3β/ 2 ; e profile: n(r) =n 0 [1 + (R R s ) 2 /R 2 c] 3β/ 2 ; distributions - β 1/ 4) and structures (peaks and vall d CS -profile, with the three n(r) with parameters = the n 0 [1 four + (R parameters n 0, R s R ) 2 /Rn c and c] 2 3β/ the 2 0 ), ρ c ;, h index c and β: -profile, with the four parameters 0 ), c, h c andβ β: characterizi ofile: CS -profile, n(r) with the = nfour 0 [1 + parameters ρ 2 / ρ 2 n 0 ), ρ c, h c and β: n(r) n(r) = =n n 0 [1 + (R R s ) 2 /Rc] 2 3β/ 2 0 [1 + ρ 2 / ρ 2 c +(z c +(z h s) 2 /h 2 h s) 2 c /h; 2 ] 3β/ 2. (β-cylinder) c ] 3β/ 2. Given the one-dimensional nature of our observab n(r) = n 0 [1 + ρ 2 / ρ 2 c +(z unknown h maximum line of sight integration distance s) 2 /h 2 c ] 3β/ 2. n-profile, in the analytical with the four expressions, Rparameters for each n 0 ), of ρ these c, h c and fourβ: models we also a s and h s = Offset Radius and Height of an additional parameter (R s for the SS profiles and h s for the CS he analytical expressions, for each of these four models we al in the 7/4/2016 analytical expressions, for Mykonos each 2016 of these (F. Nicastro) four models we also 7allow β/ 2 (β-sphere) 2.4. Det erminat ion of H alo Ex

8 Separate This preprint was prepared LGL with the & AAS L A HGL TEX macros v5.2. N OVII -(l,b)) Fits Table 1: Separat e H GL and LGL Fit s Model Model n 0 R c or ρ c h c R s Halo Size Mass chi 2 (dof ) Name Type (10 2 cm 3 ) (kpc) (kpc) (kpc) (kpc) (10 9 M HGL M1 Exp-SS N/ A > (15) LGL M2 Exp-CS N/ A N/ A (10) LGL: χ 2 flat(dof) = 12.8(10) vs χ 2 Sph(dof) = 22.5(11) LGL to HLG: χ 2 (dof) = 373(18)!!! HGL to LGL: χ 2 (dof) = 147(13)!!! LGLs trace the Galaxy s Disk: M OVII (Disk) 1.4x10 8 M 2 distinct components or compromising solution 7/4/2016 Mykonos 2016 (F. Nicastro) 8

9 Combined LGL+HGL N OVII -(l,b)) Fits Table 2: Simult aneous LGL+ H GL Fit s Model Model n 0 R c β R s Halo Size Mass chi 2 (dof ) Name Type (10 2 cm 3 ) (kpc) (kpc) (kpc) (10 11 M A β-ss > (27) B M2 + β-ss / 8 > M3 M2 + β-ss (frozen) 0 (frozen) > M4 β-ss (frozen) 0 (frozen) > (27) (29) (29) Model A: peak density at 6 kpc Model B: empty central 6-kpc sphere P(A;M4) = 99.8% ; P(A;M3) = 91.3% 7/4/2016 Mykonos 2016 (F. Nicastro) 9

10 Why an Offset Radius? Model A Model A Model M4 Model M4 Model B Model B Model M3 Model M3 HGL (AGNs) LGL (XRBs) HGL (AGNs) LGL (XRBs) HGL (AGNs) LGL (XRBs) HGL (AGNs) LGL (XRBs) /4/2016 Mykonos 2016 (F. Nicastro) 10

11 OVII Emission Measure In 1-sr around l=90 0, b=60 0 : EM = cm - 6 pc (McCammon+02) Toward the Galactic Center at b <20 <20 0 and across the Fermi Bubbles: EM = ( ) 0.3) cm - 6 pc (Kataoka+15) 7/4/2016 Mykonos 2016 (F. Nicastro) 11

12 The 6-kpc Central Cavity (other pieces of evidence) Size and shape similar to Fermi Bubbles (gamma-rays) and Plank Haze (microwave): Dobler+10; Su+10 Seen in Hα and IR emission: Bland-Hawthorn+03 Seen in modulation of X-ray EM across edges of Fermi Bubbles: Kataoka+15 Resolved in moderate-ionization double-peaked (near- and far-side of the bubble) metal absorption suggesting expansion v 1000 km/s: Fox+15 7/4/2016 Mykonos 2016 (F. Nicastro) 12

13 A Distant Echo of Central Activity A fast moving piston powered by a period of activity of our supermassive black hole moves outwards and compresses the ambient gas while pushing (a fraction of) it outwards (i.e. Davè+11; Faucher-Giguère & Quataert, 2012, Lapi+05) Nuclear energy-conserving outflows with v in km/s produce shock traveling at v s 1000 km/s (Faucher- Giguère & Quataert, 2012) At this rate it would take 6 Myrs for the OVII-traced shock to reach its current position at 6 kpc (6 ± 2) Myr is also the age of 2 stellar disks in the innermost 0.2 pc of the Milky Way, thought to be a relic of a gaseous accretion disk (Paumard+06; Levin, (Paumard+06; Levin, Beloborodov 03) 7/4/2016 Mykonos 2016 (F. Nicastro) 13

14 ambient medium (e.g. Lapi, Cavaliere & Menci, 2005), we can estima ss contrast between the gas contained in the bubble and the ave. According to Lapi, Cavaliere & Menci (2005), and for a simpleisothe of the blast wave. According mass to Lapi, within Cavaliere one virial & Menci radius, (2005), equals and a eass virial contrast radius, between equals theabout gas contained in the bubble and the total ho sphere, the Checking mass contrast between kinetic half of the energy the energy Energetic gas in contained the outflow contrast in the (provided E/E bubble in one the virial outflow radius, (provided equals about by half of the energy contrast E/E between y mass in thewithin outflow one (provided virial radius, by the inthe equals central thecentral equilibrium AGN) about AGN) half andhot and of the the gas. the total energy From total energy contra our en res umkinetic hot gas. energy From in the our outflow best-fitting rium hot gas. From our best-fitting as(provided inmodel a simple model by A (whose isothermal A central (whose AGN) density densitysphere) profile and decr the pro For a simple isothermal sphere ΔM/M 1/2ΔE/E le isothermal in equilibrium sphere) hot the gas. amount amount From M Bubble of gas our of = gas currently best-fitting 3.4 currently +1.8 contained model contained A (whose the bubb ind as (Lapi+05) in10 a M. The con 8 simple M. The isothermal contrast sphere) filling between the athis virial-radius amount this mass mass and of gas sphere the and currently hotthe isbaryon then hot cont in mb l-radius M Bubble sphere = 3.4 is then 10 in in 8 Mthe. energy range The( M/M contrast ( M/M ) Bubble between is ) Bubble E/E this mass and Thu 0. 0 ast filling is a virial-radius sphere 0.1. The is then total inenergy the range in the( equilibrium M/M ) Bubble hot g is E/E EThe 2 total 10energy 61 ((kt 0 Hot in)/kev) the equilibriu 5/ 2 ergs ( 61 ((kt energy Hot )/kev) contrast 5/ 2 is ergs E/E (Lapi, Cavaliere & Menci, The total 2005), energy where int Hot thei (kt ofe the For Hot )/kev) gas, 2 T Hot which 10 (0.6-1)x10 5/ 2 ergs (Lapi, temperature 61 ((kt we 6 Cavaliere of & the Menci, gas, which 2005), K E ( )x100.4)x10 59 we wher ass Hot assume )/kev) in 5/ the 2 ergs range (Lapi, T Hot Cavaliere (0.6 1) & Menci, ergs 10 6 K(w 200 ction the temperature gas, peaks which in gas of the we in collisional gas, assume the which in OVII equilibrium). we the ΔE (0.1-4)x10 57 assume range fraction in Thus, T the Hot peaks Erange (0.6 in gas ergs = 1/2ΔM<v 2 (0.1 T > Hot 1) in 0.4) col (0.6 1 ionthe peaks (0.1 OVII 4) in fraction gas inpeaks ergs collisional in1/ ergs, gas 2 Min equilibrium). vand s 2 collisional which E gives (0.1 equilibrium). v Thus, s 4) 150E Thus, (0.1 ergs kme ement ergs, (0.1with and 4) the E v10 s 57 = ( ergs <v> ) km in 1/ s good needed Mergs vagreement km/s to1/ take 2 Mthe vs 2 shock which front gives from s 2 which gives with v the s v s 150 = 100 v s r to in its good current ΔE<<E agreement position AGN (Δt with of R Δt=4-8 the s 6v kpc, Myr) (2.5-5)x105)x10 58 s = 1000 in 6 Myr. km swe 1 needed also note ergs tothat takethe the a ent with the v s = 1000 km Galaxy s 1 needed center to to its take current the position shock fr o he Galaxy kinetic center energy to of its the current outflow, estimate position E of (0.1 ofr the s 4) kinetic 6 kpc, inenergy ergs, 6 Myr. isof signific o its current position of R We theal o nd so fully consistent with) the s 6 kpc, in 6 Myr. We also note th maximum amount of energy that can be m lower than (and so fully consistent wit estimate 7/4/2016 of the kinetic energy Mykonos of the 2016 outflow, (F. Nicastro) E (0.1 4)

15 Closing the Galaxy s Baryon Census Sizes: R(A)>61 kpc R(B)>190 kpc Masses at R vir : M(A)=( )x10 11 M M(A)=( )x10 11 M Model B Actual Solution probably in between Model A 7/4/2016 Mykonos 2016 (F. Nicastro) 15

16 Conclusions Million-degree Gas permeates both the Disk and Halo of our Galaxy A spherically symmetric structure in the density profile of the million-degree halo gas tracks the current position of a shock-front generated 6 Million years ago by an energetic outflow powered by an AGN-like accretion episode The Mass of the OVII-bearing Gas may be sufficient to close the Galaxy s Baryon Census 7/4/2016 Mykonos 2016 (F. Nicastro) 16