KRS 1

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Interactions of High Velocity Clouds and the
Galactic Corona
Ken Sembach Space Telescope Science Institute
Galaxy Formation and Evolution as Revealed by
Cosmic Gas Irvine, April 2008
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Three types of gas relevant to this talk

• Neutral (H I) HVCs
• 21 cm emission (NHI gt 1018 cm-2)
• Some low ionization absorption (C II, Si II)
• T 104 K
• Highly Ionized (H) HVCs
• No detectable 21cm emission (NHI lt 1018 cm-2)
• O VI and/or C IV absorption
• T 104 K (photoionized - C IV) or T 105 K
  (collisional - O VI)
• Hot Galactic corona
• Detected in O VII (X-ray) absorption
• T (1-2)x106 K

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The High Velocity H I O VI Sky
Complex C
Covering fraction (102 sight lines) 60 85
of sky covered at N(H) 1018 cm-2 (for SMC
metallicity)
Magellanic Stream
H I 21cm Emission FUSE O VI Absorption
Sembach, Wakker, Savage, et al. (2003, ApJS, 146,
165)
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Photoionization at z 0

• Photoionization by the
• extragalactic UV background
• is not a viable production
• mechanism for the O VI HVCs around the Galaxy.
• Cloud sizes too large
• Densities too low
• No low ionization gas
• expected with O VI,
• but low ions are observed

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High Velocity Cloud Complex C
Metallicity Z/Zo 0.1 0.25 (FUSE HST
21cm) (Wakker et al. 1999 Gibson
et al. 2002 Collins et al. 2003 Sembach et al.
2004) Distance d 6 -11 kpc from the Galactic
plane (Wakker et al. 2007) Mass MHI
few x 106 - 107 Mo No dust or H2 low N
abundance high D/H (2.2x10-5) ? chemically
young
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O VI Absorption in Complex C
Complex C
Complex C
Complex C

• Nine Complex C sight lines observed, all show O
  VI absorption
• lt N(O VI) / N(H I) gt 1.8x10-6 gt 106 Mo of H
  in O VI region
• (equivalent to 10 thickness layer around 107
  Mo H I complex)
• Kinematics Ionization gt interfaces with
  Galactic halo/corona
• (Sembach et al. 2003 Fox et al. 2004
  Collins et al. 2004)

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Highly Ionized Gas in the Magellanic Stream
Much of MS is located 20 to 80 kpc from the Milky
Way disk gt O VI in this case is well beyond
confines of normal Galactic halo (hOVI 2.5
kpc), but probably within an extended Galactic
corona
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H? Emission in the Magellanic Stream

• H? emission is detected along the leading edge
  of the Magellanic Stream
• Weiner Williams 1996 Putman et al. 2003
• Emission is too strong (hundreds of mR) to be
  due to photoionization
• gt Interaction with Galactic corona

Hawthorn, Sutherland, Agertz, Moore (2008, ApJ
670, L109)
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Head-Tail Structures of H I HVCs
Compact HVCs around MW are spatially isolated and
tend to have H I extents of 1 and baryonic
masses of 105 Mo per cloud. Some show
head-tail structures indicative of interaction
with an external medium. Absence of detectable
HVCs in other groups indicate that they are
probably located within 150-200 kpc of large
galaxies (Zwaan, Pisano, Kovac, and others).
Konz et al. (2002, AA, 391, 713)
Peek et al. (2007, ApJ, 656, 907)
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Interactions with the Hot Galactic Corona
O VI in HVCs located at large distances (e.g.,
Mag. Stream) Head-tail structures of H I HVCs
Tidal models of Stream favor weak drag forces on
Stream gas Strong H? emission in the Stream
Local Group dSph galaxies have low gas mass
Stars and gas in streams are separated
Hot Galactic Corona
R gt 70 kpc T gt 106 K nH lt 10-4 cm-3 M 109 Mo
Hot corona
O VI at interface
Quilis Moore 2001
Grebel, Gallagher, Harbeck 2003
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Density Evolution of a Warm Cloud in a Hot Corona
Temperatures Tcloud 104 K Tcorona
106K Density contrast ncloud / ncorona
100 Initial velocity vcloud 0
Time
Cloud decelerates and begins to fragment near vt
Cs
Murray Lin (2004, ApJ, 615, 586)
Large reservoir of coronal (T 106 K) gas should
be visible in X-ray absorption N(O VII) nH L
fOVII (O/H)o Z/Zo N(O VII) 1.5x1016 (n / 10-4
cm-3) (L / 100 kpc) (Z / Zo) cm-2 at T 106 K
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O VI HVCs and O VII Corona are Not the Same Gas
XMM-Newton RGS Spectra of Local O VII Absorption

• O VI HVCs have associated low ion
• absorption gt multiphase
• Models having the O VI and O VII in the same
  collisionally ionized
• plasma simply are not realistic.
• Non-equilibrium models dont help.
• A range of ionization stages will still
• be present.

N 2x1016 cm-2
N 1x1016 cm-2
N 4x1015 cm-2
Rasmussen et al. (2003, astro-ph/0301183)
CIE Sutherland Dopita (1993) Abundances
Anders Grevesse (1989)
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Does the Coronal Gas Fill the Local Group?
Mass of a smooth Local Group medium with T 106 K
M (4/3 p R3) (1.32 mH nH) (1) M (1012
Mo) (R / Mpc)2 (NOVII / 1016 cm-2) (Z /
Zo)-1 (2)

• If metallicity is less than solar, then M
  increases as inverse of metallicity.
• This strains dynamical/gravitational models of
  the Local Group, which
• indicate that MLG 2x1012 Mo (e.g., Peebles
  1995).
• Furthermore, if the hot gas has a normal dark
  matter component, then we
• need to multiply Eq. (2) by a factor of Wm/Wb
  6 (e.g., Spergel et al. 2003).
• The above argument has been made by Collins,
  Shull, Giroux (2005) and
• others to rule out a hot Local Group medium as
  the sole source of the O VII
• absorption.

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3C273
Some O VII X-ray Absorption Traces Local Gas
Mrk 421
PKS 2155-304
3C 273 intersects North Polar Spur Chandra X-ray
O VII absorption (Fang et al. 2003, ApJ, 586,
L49)
ROSAT All-Sky Survey (Snowden et al. 1997, ApJ,
485, 125)
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Yao Wang 2005
Multiple detections of O VII and Ne IX toward
local LMXB.
Objects with height 1 kpc Temperature is
2x106 K Density is 6x10-3 cm -3
Interstellar gas
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Detection of O VII toward LMC X-3 with N(O VII)
2x1016 cm-2 demonstrates that the O VII
absorption has dominant component within 50 kpc
(Wang et al. 2005).
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Wish List
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Summary

• If you remember just one thing from this talk,
  remember this
• Ionized gas should not be ignored in discussions
• of HVC interactions or accretion!
• Hot (106 K) gas envelops the Galaxy. The coronal
  gas is revealed directly
• through O VII X-ray absorption. Most evidence
  indicates that the coronal
• gas is associated with the Milky Way rather
  than distributed throughout
• the Local Group.
• Neutral and ionized high velocity clouds in/near
  the Galaxy are interacting
• with this hot, extended corona. When they do,
  O VI is formed at the
• hot/warm gas interfaces.
• Something to keep in mind for later talks Some
  of the IGM O VI
• absorbers resemble the O VI HVCs.