High Velocity Clouds2

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High Velocity Clouds-2
Ive looked at clouds from both sides now From
up and down and still somehow Its cloud
illusions I recall I really dont know clouds at
all Joni Mitchell, Both Sides Now
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Much of the HV gas appears associated with
complexes
HV HI gas can be found to cover a fraction f of
sky, to column density limit N(gt) f0.15
N(gt) 2x1018 cm-2 f0.37 N(gt) 8x1017
cm-2 Lockman et al. 2002, ApJ 140,331
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Typical velocity widths of HVC are lt40
km/s. Giovanelli Brown (1973 ApJ 182,755) first
noted bimodal behavior of HVC linewidths
Giovanelli Haynes (1977 AA 54,909) related
narrow linewidth components with small-scale
spatial structure
Tkin lt 103 K
In cores
nHI (10-100/Dkpc) cm-3
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M 2-5 kpc
C dgt2.4 kpc
H dgt4 kpc
AIV 4-10 kpc
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Metallicity of HVCs

• Very uncertain
• In a few cases in which SII lines were detected
  (S is thought not to be depleted by accretion
  onto dust and SII is dominant ionization stage),
  abundances of order of 0.1-0.6 of solar inferred
• Metallicities based on OI/HI range from 0.1 to
  0.25 solar
• Multiple l.o.s. through complex C show a wide
  range of metallicities gas not well mixed
  alpha-elements (O, S, Si) enhanced wrt to N and
  Fe
• CaII is the most often detected ion in HVCs
  because it can be depleted by accretion onto dust
  and because it may not be the dominant ionization
  stage under range of plausible environments,
  abundances are unreliable. Measurements are
  compatible with solar.

For recent references see Collins et al. 2003 ApJ
585, 336
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Miville-Deschenes et al. 2005 ApJ 631, L57
Dust in HVCs-2
Significant correlation claimed, esp. at longer
wavelengths ? cold dust
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Dust in HVCs-3
Issues raised by reported detection of dust
emission

• Null detection surveys of molecular emission in
  HVCs
• Extremely few (lt5, all at low b) detections of HI
  in absorption in HVCs
• 160 micron emissivity per H atom in HVC component
  appears to be greater than that in local HI
• Allowing for possible low metallicity of HV gas
  in complex C (Z/Zsolar0.2/-.1 according to
  Tripp et al. 2004), the HI column density in HVC
  required to explain dust emissivity is 5 times
  large than observed
• Suggested solution dust emission arises from
  very dense, opaque molecular clumps in a
  multiphase medium
• If that explanation is correct, gas masses of
  HVCs would be much larger than implied by HI
  observations
• Caveat emptor

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Hot Gas in HVCs

• C IV high velocity halo gas emission first
  reported by Sembach et al (1996 ApJ 451, 616)
• O VI high velocity halo gas emission first
  reported by Sembach et al (2000, ApJ 538, L31)
• For recent O VI survey, see Fox et al (2004 ApJ
  602, 738)

• - 5 l.o.s. trough complex C sampled with FUSE and
  HST
• High velocity O VI emission detected in each
  case, matching the velocity of the HI
• Si IV/O VI, C IV/O VI, N V/O VI
  inconsistent with photoionization by
  extragalactic flux or by radiation from Galactic
  disk more easily explained by model whereby
  emission arises from conductive interfaces
  between cold HI gas and a hot (106 K) corona

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Angular diameter, deg line HPFW, km/s
Distance, kpc
Useful scales

• tz0 time to reach z0 ? typically (5-10)Dkpc
  Myr
• tcr time to cross its diameter, or double its
  size if cloud is not gravitating
• Dgrav distance at which cloud would be
  self-gravitating

• A 10o cloud of 25 km/s linewidth has tcr 34
  Myr at D 1 kpc
• A 1o cloud of 25 km/s linewidth has tcr 3.4
  Gyr at D 1 Mpc

? A 1o cloud of 25 km/s linewidth with peak
brightness temperature of 1 K and HI mass
fraction f0.1 would be self-gravitating at Dgrav
225 kpc
Fraction of mass in HI peak brightness
temperature, K
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Useful scales
4. Characteristic HI Mass

• A 10o cloud, 10K peak brightness, at 10 kpc has
  HI mass of 107 solar
• A 1o cloud, 1K peak brightness, at 1 Mpc has HI
  mass of 108 solar

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What are High Velocity Clouds?
Oort (1966, BAN 18, 421)

• They are parts of nearby Supernova shells
• They are condensations formed in a gaseous corona
  of high temperature
• They have been ejected from the Galactic nucleus
• They have been ejected as cool clouds from the
  Galactic disk and are the principal constituents
  of the corona
• They are due to intergalactic gas accreted by the
  Galactic system
• They are small satellites of the Galactic system,
  or independent galaxies in the Local Group

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The Magellanic Stream-1
Mathewson, Cleary Murray 1974, ApJ 190, 291
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Modelling the MC/MS/MW System-3
Connors, Kawata Gibson 2006, MN 371,108
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HVCs an Intergalactic Population? 1
So, the Magellanic Stream is an extragalactic
(tidal) feature. Are other components of the HVC
population also extragalactic?

• Idea ventilated by Oort (1966), who did not rule
  it out
• Revisited by Mathewson et al. (1975 ApJ 195,L97),
  who claimed that some HVCs were associated with
  Sculptor Group galaxies NGC55 and NGC300 proved
  wrong by Haynes Roberts (1979, ApJ 227,767
  MPHs PhD thesis)
• RGs paper Against the Intergalactic
  Interpretation of High Velocity Clouds of 1977
  (AA 55, 395) did cost him a postdoc
• Idea revisited in 1981 (Giovanelli 1981 AJ 86,
  1468), after he already had a solid job

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HVCs an Intergalactic Population? 2
Can the population of Very High Velocity Clouds
be Intergalactic?
Giovanelli 1981, AJ 86, 1468
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HVCs an Intergalactic Population? 3
Can the population of Very High Velocity Clouds
be Intergalactic?
Maybe. But in that case they are more likely to
be associated wit the MS, rather than being
independent members of the LG.
Giovanelli 1981, AJ 86, 1468
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HVCs an Intergalactic Population? 4
Idea revived by 2 influential papers -
1. Blitz et al. 1999 ApJ 514, 818 - 2.
Braun Burton 1999 AA 341, 437
HVCs are large clouds, with typical diameters of
25 kpc, containing 3x107 solar of neutral gas
and 3x108 solar of dark matter, falling towards
the barycenter of the Local Group altogether
the HVCs contain 1010 solar of neutral
gas.(1) The typical HVC is placed at a
distance of 1 Mpc, metallicity expected to be
low (lt0.2 solar) and Halpha emission to be weak
and consistent with the metagalactic radiation
field. Accretion of clouds onto the MW, M31 and
M33 should be expected to occur large angular
size complexes would be such beasts. Braun
Butler burton refined the concept by postulating
that the so-called Compact HVCs (CHVC) are the
prototypical LG dweller. They suggested the CHVCs
represent the missing satellites, low mass
halos.
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HVCs an Intergalactic Population? 5
de Heij, Braun Burton (2002 AA 392, 417) and
Putman et al. (2002 AJ123, 873) Have produced
catalogs of CHVCs from respectively the HIPASS
and the Leiden/Dwingeloo Survey (LDS). CHVCs are
defined as isolated objects isolation is
assessed on 10degx10deg maps within those, an
object makes the cut if its 1.5x1018 cm-2
column density contour closes within the 10x10deg
map and such contour is not elongated towards a
nearby extended region of emission. Objects must
also exhibit a deviation of 70 km/s in the LSR
from the velocity a conventionally rotating
Galaxy would exhibit in the given l.o.s. The
combined sample contains a few hundred objects,
mostly in the southern galactic hemisphere
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HVCs an Intergalactic Population? 6
Space distribution of the CHVC catalog of de
Heij, Braun Burton (2002) Red symbols
vlsrgt0 Black vlsrlt0 S hemisphere
shaded light in top image Grey contours NHI
integrated between -450 to 400 km/s
(excluding local HI) Lower image smoothed sky
density of CHVCs
LG center
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HVCs an Intergalactic Population? 7
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HVCs an Intergalactic Population? 8
Predominantly southern hemisphere objects
This is the old 300sin(l)cos(b)
These dont fit obscuration by Galactic HI
blamed
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HVCs an Intergalactic Population? 9
Burton, Braun Chengalur (2001, AA 369, 616)
mapped 10 CHVCs at AO. Here are 2.
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HVCs an Intergalactic Population? 10

• Typical properties of CHVCs
• Angular diameter lt 1deg
• FWHM linewidth 25 km/s ? if thermal Tkin
  104 K
• Core-halo morphological structure
• Central column density 4.1/-3.2x1019 cm-2
• Exponential scale length 420/-90 arcsec

Distance Estimate 1 -They assume pressure
equilibrium b/w cold (CNM) cores but they dont
detect narrow linewidths! and warm (WNM) halos
they argue that P/k near the plane is 2000
cm-3 K, but at zgt25 kpc pressure should drop to
100 cm-3 K (Wolfire et al. 1995). Then,
assuming a spherical cloud

which yields
D distance H scale length P pressure
CHVC distances ? 150-850 kpc
Distance Estimate 2 Assume the scale length h
is the same as for dwarf galaxies (1.17 kpc
according to Swaters 1999). Then
CHVC distances ? 320-730 kpc
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HVCs an Intergalactic Population? 11
If HVCs (or CHVCs) are bona fide LG members, they
should also exist in other groups. Pisano et
al.(2006, astro-ph/0603626) used ATCA to obtain
deep maps of 6 nearby, loose groups of galaxies,
obtaining a census of HI-bearing members down to
107 solar LGG 93, 106, 180, 293, 478, HIPASS
Group 3 -- 64 objects were detected, all have
optical counterparts ? NO HVCs
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HVCs near M31? -1
Thilker et al. 2004 ApJ 601, L39 GBT
Survey of M31
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HVCs near M31? -2
Thilker et al. 2004 ApJ 601, L39 GBT
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HVCs near M31? -3
Westmeier, Braun and Thilker 2005 AA 436,115
WSRT fields ?
GBT map
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HVCs near M31? -4
Westmeier, Braun and Thilker 2005 AA 436,115
HVC 2-12
HVC 1
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HVCs near M31? -5
Westmeier, Braun and Thilker 2005 AA 436,115
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HVCs near M31? -6
Best case for rotation?
Westmeier, Braun and Thilker 2005 AA 436,115
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HVCs near M31? -7
Westmeier, Braun and Thilker 2005 AA 436,115
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HVCs near M31? -8
Westmeier, Braun and Thilker 2005 AA 436,115
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Blitz et al. (1999, ApJ 514, 818) Braun Burton
(1999, AA 341, 447) Bregman (1979, ApJ 229,
514) Bruens et al. (2005, AA 432, 45) Burton,
Braun Chengalur (2001, AA 369, 616) Collins et
al (2003, ApJ 585, 336) Connors, Kawata Gibson
(2006, MN 371, 108) Fox et al. (2004, ApJ 602,
738) Giovanelli (1977, AA 55, 395) Giovanelli
(1981, AJ 86, 1468) Giovanelli (1980, AJ 85,
1155) Giovanelli Brown (1973, ApJ
182,755) Giovanelli Haynes (1977, AA 54,
909) Giovanelli Haynes (1979, ApJ 230,
404) Haynes Roberts (1979, ApJ 227, 767) De
Heij, Braun Burton (2002, AA 392,
417) Mathewson, Cleary Murray (1974, ApJ 190,
291) Mathewson et al. (1975, ApJ 195,
L97) Miville-Deschenes et al. (2005, ApJ 631
L57) Oort (1966, BAN 18,421),
Pisano et al. (2006, astro-ph/0603626) Putman et
al. (2002, AJ 123, 873) Sembach et al. (1996, ApJ
451, 616) Sembach et al. (2000, ApJ 538,
L31) Shapiro Field (1976, ApJ 205, 762) Thilker
et al. (2004, ApJ 601, L39) Verschuur (1975, ARAA
13, 257), Wakker van Woerden (1997, 35,
217) Westmeier, Braun Thilker (2005, AA 436,
115) van Woerden et al. (2004, ApSS vol 312)
Reference List
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