High Velocity Clouds

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High Velocity Clouds
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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• 1956 Spitzer proposes the existence of a hot
  (106K) galactic corona
• 1961 Muench Zirin discover absorption lines at
  high velocity (20-60 km/s) due to interstellar
  clouds along lines of sight to relatively
  distant ( 1 kpc), high latitude stars
• 1957 Oort suggests that searches for counterparts
  of MZ high velocity clouds be carried out in
  the 21 cm line
• 1963 Mueller et al. report the first detections
  of HI HVCs
• Late 1960s to early 1980s major surveys carried
  out at Dwingeloo, Ohio State, Bell Labs, Hat
  Creek, Green Bank, Parkes, Villa Elisa (see Table
  1 of Wakker van Woerden 1997)
• Major Review Papers
• Oort (1966, BAN 18,421),
  
• Veschuur (1975, ARAA 13, 257),
• Wakker van Woerden (1997, 35,
  217)
• van Woerden et al. (2004, ApSS
  vol 312)

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Wakker van Woerden 1997, ARAA 35,217
HIPASS and ALFALFA
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What does high mean in HVC?
Initially, by Oorts fiat, a distinction was made
between - Intermediate Velocity Clouds
(IVC), defined as having VLSRlt 70 km/s
?
to Leiden - High Velocity Clouds (HVC),
defined as having VLSRgt 70
km/s
? to Groningen
Objects with velocities as high as -450 and 250
km/s are known. The distinction between IVCs and
HVCs has faded, and high velocity is now
referred to the degree of departure from the
range of velocities expected, at the given (l,b),
from a differentially rotating disk of given V(R).
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l360
l0
GB300 HVSurvey Giovanelli 1980, AJ 85,1155
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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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- Galactic Rotation plays an important role -
North and South are dissimilar
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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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HV Gas Distance Measurements
If a cloud is bracketed between two stars
angularly very close, and absorption at the
clouds velocity is seen in the spectrum of star
1, but not in that of star 2, the clouds
distance can be constrained.
Observer
1
2
Caution fine structure in clouds and ability to
find probes of high angular proximity restrict
determination of star 2 as foreground object.
Technique has worked for a few l.o.s.
Note that linear size, HI mass and density scale
respectively like D, D2 and D-1
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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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Dust in HVCs-1

• 0. No IRAS detections of HVCs
• Select a MIPS/XFLS Spizter field with high
  quality HI mapping data (Lockman Condon GBT)
• Make HI maps (3degx3deg) of region, windowing,
  respectively, local HI, two IVC features and the
  HVC emission of Complex C (-132 to -210 km/s)
• Cross-correlate HI maps with 24, 60, 100 and 160
  micron MIPS maps
• Where the sum is over the 4 components (local,
  IVC1, IVC2, HVC)
• Solve for a coefficients

Miville-Deschenes et al. 2005 ApJ 631, L57
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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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• They are parts of nearby Supernova shells

Idea strongly supported by Berkeley school
(Weaver, then Heiles). While HV gas is certainly
associated with SNRs, and some of the HVCs are
parts of SN shells, these objects should be
concentrated to te galactic plane, exhibit both
approaching and receding parts along any given
l.o.s. and be associated with strong halpha
emission. (see IC443, next slide) Most HVCs do
not fit those requirements.
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SNR IC443
Giovanelli Haynes 1979 ApJ 230,404
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2.They are condensations formed in a gaseous
corona of high temperature

• Oort excluded this hypothesis on physical
  grounds
• for HVCs to be in pressure equilibrium with a hot
  (106 K) corona (Spitzer 1956), the density of
  the coronal gas would have to be 0.01 cm-3
• At a lower coronal density, clouds would
  evaporate
• ? clouds could be in the corona, but as
  transients
• At density 0.01, the coronal emission would be
  very high and clouds would never achieve high
  velocity, due to coronal friction.

3. They have been ejected from the Galactic
nucleus
Kinematics just doesnt work.
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4. They have been ejected as cool clouds from the
Galactic disk and are the principal constituents
of the corona
Alive and well galactic fountain model
(Shapiro Field 1976, ApJ 205, 762 Bregman
1979, ApJ 229, 514). Fits well some of the lower
velocity gas.
5. They are due to intergalactic gas accreted by
the Galactic system 6.They are small satellites
of the Galactic system, or independent galaxies
in the Local Group
Excluded by Oort on grounds of angular
extent. Alive and well for smaller objects Braun
Butler-Burton (1999, AA 341,447) Blitz et al.
(1999, ApJ 514, 818).
And then there is the Magellanic Stream, which
fits none of the above explanations but whose
origin we know best
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The Magellanic Stream-1
Mathewson, Cleary Murray 1974, ApJ 190, 291
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Magellanic Stream the HIPASS view
Bruens et al. 2005 AA 432,45
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Magellanic Stream the HIPASS view
Magellanic angle (along Stream)
Bruens et al. 2005 AA 432,45
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HI column density
Magellanic Stream the HIPASS view
Velocity field (LSR,GSR)
Left Velocity field in LMC rest frame Right
Velocity dispersion
Bruens et al. 2005 AA 432,45
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Magellanic Stream the HIPASS view
Bruens et al. 2005 AA 432,45
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Magellanic Stream the HIPASS view
Bruens et al. 2005 AA 432,45
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Modelling the MC/MS/MW System-1
Connors, Kawata Gibson 2006, MN 371,108
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Modelling the MC/MS/MW System-2
Connors, Kawata Gibson 2006, MN 371,108
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Modelling the MC/MS/MW System-3
Connors, Kawata Gibson 2006, MN 371,108
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Modelling the MC/MS/MW System-4
Connors, Kawata Gibson 2006, MN 371,108
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Modelling the MC/MS/MW System-5
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Modelling the MC/MS/MW System-6
Connors, Kawata Gibson 2006, MN 371,108
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Modelling the MC/MS/MW System-7
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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