The Hot ISM

1
The Hot ISM

• K.D.Kuntz
• The Henry Rowland Dept. of Physics
• The Johns Hopkins University
• and NASA/LHEA

2
What is the Hot ISM?

• Not identifiably a SNR
• Bubbles and Super-bubbles (SN and groups of SN
  that have lost their identities)
• Galactic Halo (hot gas that was originally
  produced by SN)
• IGM?

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Why study the Hot ISM?

• Grand unified theories of the ISM
• Contains bulk of the energy budget
• SN primary mechanism for injecting energy
• A. McKee-Ostriker (1977)
• hot gas surrounds cool clouds
• (appearance of ISM determined by balance between
  shock heating and radiative cooling)
• B. Cox-Smith (1974)
• cool clouds surround network of hot tunnels
• and bubbles

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Why study the Hot ISM?

• How much halo is there?
• A very important question for understanding
  enrichment of the IGM

Q.D.Wang (2001) NGC 4631 Strongly star-forming
galaxy
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!!!WARNING!!!

• Galaxies are not like clusters of galaxies.
• Typical virial temperatures 106K but
• Spitzer coronae not observed in the X-ray
• Benson et al. (2000)
• Toft et al. (2002)
• X-ray halos not observed except for strongly
  star-forming galaxies

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• X-ray more associated with star-formation

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Introductory Concepts

• The higher the energy, the further one can see!

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Historical Background

• Soft X-ray (lt2 keV) Astronomy
• Bowyer, Field, Mack (1968)
• Bunner et al (1969)
• Henry et al (1969)
• ? Expected soft extrapolation of EG emission
• ? Expected to see emission absorbed by disk
• ? Surprised by extra emission component
• A new instrumental background?

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Wisconsin Rocket Flights

• Large FOV (6 degrees)
• Anticorrelation
• Primarily thermal
• Copernicus - O VI

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• Contemporary thinking
• Copernicus observed OVI in all directions
• OVI is emitted by gas at temperatures of a few
  105K, cooler than the 106K gas that emits the
  soft X-rays.
• Perhaps the OVI emitting gas is at the interface
  between the X-ray emitting gas and the
  surrounding, cool, neutral gas.

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Three Models

• Absorption
• required unreasonable clumping of the ISM
• required emission in excess of that expected
  from the extrapolation of the hard X-ray spectrum
• emission in Galactic plane not explained
• high-b shadows not seen
• B. Interspersed
• many of the same problems as Absorption
• but fit well with the McKee-Ostriker model
• C. Displacement
• fit well with optical picture of local ISM

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Local ISM

• HI in the solar neighborhood is deficient Knapp
  (1975)

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Local ISM

• Frisch York (1983) determined the same thing
  with absorption line spectroscopy in the optical

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• The area around the sun is deficient in neutral
  cool material. This deficit has come to be known
  as The Local Cavity.
• The local region of X-ray emitting gas is now
  known as The Local Hot Bubble.
• The two things are not the same, but the Bubble
  must fit inside the Cavity (or else there would
  be detectable absorption of X-rays). In fact
  there are regions where the Bubble is much
  smaller than the Cavity and it is not clear what
  fills the gap.

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Local ISM

• Juda (1991)
• LB is empty!

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• Because the Be band is much softer than the B
  band, it is far more sensitive to absorption.
  Therefore, since the Be/B ratio is the same
  everywhere in the sky, there can be very little
  absorption within the X-ray emitting region.
• This has also been demonstrated with UV
  observations of local white dwarfs.

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ROSAT
ROSAT solved the question just months after
launch by observing the Draco molecular clouds at
relatively high galactic latitude.
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ROSAT Shadows

• Left map of column density, Right X-rays,
• There really is emission from outside the disk!

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Absorption Can Be Your Friend

• ItotIlocalIdiste-t

Thus, by measuring the aborption due to a
molecular cloud at a known distance, one can
determine the amount of foreground emission.
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• Since MBM 12 casts almost no shadow at ¼ keV, all
  of the local emission must be closer than the
  cloud.

0.25 keV
0.75 keV
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Absorption Can Be Your Friend

• Given a sufficient dynamic range of absorbing
  column can determine amount of emission behind
  and in front of absorption.
• If distance to absorption known can place
  limits on the distance to the emission.

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The ROSAT All-Sky Survey
0.25 keV
I100NH
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• The previous image was the ROSAT All-Sky Survey
  and a map of the neutral (absorbing) gas. One can
  use the anticorrelation of the two to map the
  local (Local Hot Bubble) and distant (Galactic
  Halo and IBM) emission.

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Whole Sky Decomposition

• The top panels are Snowdens map of the Galactic
  halo emission towards the galactic poles.

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Whole Sky Decomposition

• Snowdens image of the foreground (Local Hot
  Bubble ) emission from the ROSAT All-Sky Survey

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• Cross-sections of the Local Hot Bubble derived
  from the previous map.
• Note irregular, smaller in the Galactic plane
  than towards the poles.

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The ROSAT All-Sky Survey
0.75 keV
0.25 keV
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• Note the strong emission towards the poles in
  the 0.25 keV map is due to BOTH extragalactic
  emission AND the extension of the Local Hot
  Bubble perpendicular to the Galactic disk.

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Whole Sky Decomposition
Map of the local Galactic disk
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• Note about the previous image the X-ray emitting
  regions are not connected. The hot gas is not
  pervasive. The McKee-Ostriker model does not look
  like the local ISM.
• Now that we have a rough idea of the distribution
  of the local hot ISM, lets take a more detailed
  look at some of its principal components.

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Local Hot Bubble (LHB)

• Models
• Single SNR, Cox Anderson (1982)
• Reheating an old cavity with new SNR Smith Cox
  (2001)
• Adiabatic Expansion of hot gas into an old
  cavity, Breitschwerdt Smutzler (2001)
• Isolation of hot arm, Maiz-Apellaniz (2001)

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Local Hot Bubble (LHB)

• The Size Problem
• Path length proportional to Emission
• MBM 12 shadow sets distance scale
• MBM12 distance is changing!
• Hobbs (1986) 65pc (also Hipparchos)
• Luhman (2001) 275/-65 pc
• Anderson (2002) 360/-30 pc
• However, old scaling consistent with the newest
  measures of the local cavity, Sfeier (2001)

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Local Hot Bubble (LHB)
Sfeir et als map of the local cavity (thin
lines) Snowdenss map of the LHB (thick
lines) The two are consistent.
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Local Hot Bubble (LHB)

• The Pressure Problem
• Hot Gas
• T106 K, P/k15000 cm-3 K
• Partially Ionized Cloudlets within LHB
• T7500 K, P/k1400-3600, N1017-1018
• Lallement, Jenkins (1992)
• Total column lt few1018, Hutchinson (1998)

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Local Hot Bubble (LHB)

• The Spectrum Problem (1)
• Diffuse X-ray Spectrometer (DXS)
• energy range 0.15-0.31 keV
• resolution 4-14 eV
• Sanders et al. (2001)

FOV of the instrument
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DXS Spectrum of LHB (Sanders)

• The Spectrum Problem (1)
• Diffuse X-ray Spectrometer (DXS)
• energy range 0.15-0.31 keV
• resolution 4-14 eV
• Sanders et al. (2001)

Depleted models provide best fit, but not
good Line identification questionable for many
lines
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Local Hot Bubble (LHB)

• The Spectrum Problem
• Cosmic Hot Interstellar Plasma Spectrometer
• Hurwitz, Sasseen, Sirk (2005)
• 106 K plasma should have Fe VII-Fe XII lines near
  72 eV

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Local Hot Bubble (LHB)

• CHIPS Spectrum contains almost no lines!
• The EM is tightly constrained, but not the
  temperature.
• Depletion helps, but only by a factor of a few.

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Local Hot Bubble (LHB)

• The Spectrum Problem
• Bellm Vaillancourt (2005)
• no depletion can make all of the data consistent
• depletion makes the data less inconsistent

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Local Hot Bubble (LHB)

• The UV Problem
• O VI emission, Shelton (2003)
• EM is too small for BS model
• Allows only 3 interfaces per LOS
• O VI absorption, Oegerle (2005)
• some components seen nearby,
• LHB wall is not seen!
• Does this mean hot gas does not exist in LHB?
• No, some must exist to produce O VII.

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Local Hot Bubble (LHB)

• Models
• Single SNR, Cox Anderson (1982) would produce
  too much O VI
• Reheating an old cavity with new SNR Smith Cox
  (2001) still viable
• Adiabatic Expansion of hot gas into an old
  cavity, Breitschwerdt Smutzler (2001) would
  produce too much O VI

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(LHB) Solution?

• Charge Exchange Reactions
• O8 H ? O7 H ?
• Cause of flaming comets

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(LHB) Solution?

• Charge Exchange Reactions
• Source of the ROSAT Long-Term Enhancements and
  consistent with background seen towards the moon.

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(LHB) Solution?

• Charge Exchange Reactions
• X-rays due to interaction of solar wind with
• material in Earths Magnetosphere and with the
  ISM flowing through the solar system
• Since the solar wind is time variable, so is the
  X-ray emission.

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(LHB) Solution?

• Time-variable lines due to solar wind (Snowden,
  Collier Kuntz 2004)

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Other Bubbles and Stuff

• Monogem Ring, Plucinsky et al (1996)
• nearby (300pc?) SNR log T6.34
• Eridion Bubble, Guo Burrows (1995)
• log T6.00-6.24
• Thus Bubbles are too soft to be seen with CXO
• Loop I Super-bubble
• log T6.5, Willingale et al (2005)
• Galactic Bulge
• log T6.6, Snowden et al (1997)

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Loop I Superbubble
Galactic Bulge
Eridion Bubble
Monogem
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Loop I Super-bubble

• By careful study of absorption, Snowden showed
  that the Loop I superbubble emission is in front
  of the emission from the Galactic bulge

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The Galactic Halo

• From Kuntz Snowden (2000)
• The halo has two thermal components
• 1. Soft patchy, log T6.05
• Galactic chimney effluvia?
• 2. Hard uniform, log T6.45
• Hydrostatic halo? Or WHIM/WHIGM?
• Had the right temperature and strength to be the
  Warm-Hot Intergalactic Medium

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• Maps of the North Galactic Pole

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The WarmHot Intergalactic Medium

• The WHIM contains the bulk of the baryons in the
  local universe

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The Galactic Halo

• The X-ray Quantum Calorimeter
• McCammon et al. (2002)
• energy range 0.05-1.0 keV
• energy resolution 5-12 eV
• exposure time 100.2 s
• effective area 0.33 cm2

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The Galactic Halo

• The XQC FOV

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The Galactic Halo
The XQC spectrum
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The Galactic Halo

• The XQC spectrum showed that
• Bulk of the hard component is due to O VII
• at zlt0.01
• At most 34 of emission is WHIM
• Depletions are required for OK spectral fits
• The XQC spectrum is consistent with the DXS
  spectrum.

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The Galactic Ridge

• (Seemingly) Diffuse Emission
• longitude 45, latitude 1
• scale height100pc
• Worral et al (1982) Warwick et al (1988)
• FeK emission ? thermal emission
• Problems
• 1. Point source contamination
• (not a problem, Ebisawa 2002)
• 2. Non-thermal components

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The Galactic Ridge

• Kaneda et al (1997) observed the Galactic Ridge
  towards the scutum arm with ASCA

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The Galactic Ridge

• The spectrum required two NEI components
• kT0.75 keV, kT7.0 keV
• (log T6.9, log T7.9)
• The hot gas is way too
• hot to be retained by
• the Galaxy

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The Galactic Ridge

• Valinia et al (2000)
• There is a significant non-thermal tail
• low energy cosmic rays can produce line spectrum
  that mimics a thermal spectrum
• LECR2 CIE components kT0.56, kT2.8
• Thus the problem of the really hot gas resolved.

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The Galactic Ridge

• Tanaka (2001)
• Some lines are too broad for bulk motions
• (Would be faster than sound speed.)
• Resolved with charge-exchange reactions?
• Dogiel et al (2004), Masai et al (2004)
• 2. Quasi-thermal population

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The Galactic Ridge

• The Galactic Ridge is one of the few components
  of the Galactic diffuse emission that emits
  within the Chandra bandpass and is interesting at
  imaging CCD spectral resolution.
• The papers listed on the previous panel suggest
  that this may be an exciting field of study.

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Chandra Studies of Diffuse ISM

• Difficulties
• Small FOV ? small number of photons
• Hard halo 0.018 counts/s/chip
• Soft halo 0.002 counts/s/chip
• Fills the FOV
• whats the instrumental background?
• Backgrounds may be time-variable!

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Chandra Studies of Diffuse ISM

• Markevitch et al (2003)
• Limited study of 4 LOS
• Line emission varies with position
• Emission is dominated by O VII

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Chandra Studies of Diffuse ISM

• Just because it is hard doesnt mean we arent
  still trying!

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Other Galaxies

• M101 (as an example)
• Kuntz et al (2003)
• Two thermal components, kT0.25,0.75
• Sources?
• Contamination by binaries? No!
• Binaries have PL spectra
• Contamination by unresolved stars?

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Other Galaxies

• Study of the diffuse X-ray emission in galaxies
  need not be restricted to the study of the Milky
  Way. In some ways it is easier to study the
  diffuse emission in other galaxies than in our
  own.
• Of course, there are different problems

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Other Galaxies

• M101 (as an example)
• Kuntz et al (2003)
• Two thermal components, kT0.25,0.75
• Soft due to super bubbles?
• Hard Galactic Ridge equivalent?
• Contamination by binaries? No!
• Binaries have PL spectra
• Contamination by unresolved stars?

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Other Galaxies
The Chandra spectrum of M101
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Other Galaxies

• Dashed lines show possible amount of stellar
  contamination.

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Chandra Studies of Diffuse ISM

• What about other galaxies?
• ? Bubbles (too soft for current telescopes)
• ? Super-bubbles (but not currently resolved)
• ? Galactic Ridge
• ? Amount of stellar contamination

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Things to Keep in Mind

• Galactic Foreground is spatially variable
• both in strength and spectral shape
• Can be important up to 2.0 keV
• Use the RASS to check for problems!
• Solar Wind Charge Exchange (SWCX) Emission may
  produce time variable lines.

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Things to Keep in Mind

• Below 1.5 keV Galactic emission dominates.
• Emission primarily thermal but
• Charge Exchange reactions may be imp.
• Depletion probably important

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