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

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the hot ism

The Hot ISM


The Henry Rowland Dept. of Physics

The Johns Hopkins University


what is the hot ism
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?
why study the hot ism
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

why study the hot ism1

Greyscale: Hα, Contours: X-ray

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


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


Chandra image of M101

GALEX image of M101

  • X-ray more associated with star-formation
introductory concepts
Introductory Concepts

The higher the energy, the further one can see!

historical background
Historical Background

Soft X-ray (<2 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?

wisconsin rocket flights
Wisconsin Rocket Flights

Large FOV (6 degrees)


Primarily thermal

Copernicus - O VI

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.

three models
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

local ism
Local ISM

HI in the solar neighborhood is deficient Knapp (1975)

local ism1
Local ISM

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

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.

local ism2
Local ISM

Juda (1991)

LB is empty!

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.


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

rosat shadows

0.25 keV


ROSAT Shadows

Left: map of column density, Right: X-rays,

There really is emission from outside the disk!

absorption can be your friend

MBM 12

Absorption Can Be Your Friend


Thus, by measuring the aborption due to a molecular cloud at a known distance, one can determine the amount of foreground emission.


0.25 keV

Since MBM 12 casts almost no shadow at ¼ keV, all of the local emission must be closer than the cloud.

0.75 keV

absorption can be your friend1
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.

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.
whole sky decomposition
Whole Sky Decomposition

The top panels are Snowden’s map of the Galactic halo emission towards the galactic poles.

whole sky decomposition1
Whole Sky Decomposition

Snowden’s image of the foreground (Local Hot Bubble ) emission from the ROSAT All-Sky Survey

Cross-sections of the Local Hot Bubble derived from the previous map.

Note: irregular, smaller in the Galactic plane than towards the poles.

the rosat all sky survey1
The ROSAT All-Sky Survey

0.75 keV

0.25 keV

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.
whole sky decomposition2
Whole Sky Decomposition

Map of the local Galactic disk

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, let’s take a more detailed look at some of its principal components.

local hot bubble lhb
Local Hot Bubble (LHB)


  • 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)
local hot bubble lhb1
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)

local hot bubble lhb2
Local Hot Bubble (LHB)

Sfeir et al’s map of the local cavity (thin lines)

Snowdens’s map of the LHB (thick lines)

The two are consistent.

local hot bubble lhb3
Local Hot Bubble (LHB)

The Pressure Problem:

Hot Gas

T~106 K, P/k~15000 cm-3 K

Partially Ionized Cloudlets within LHB

T~7500 K, P/k~1400-3600, N~1017-1018

Lallement, Jenkins (1992)

Total column < few×1018, Hutchinson (1998)

local hot bubble lhb4
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

dxs spectrum of lhb sanders
DXS Spectrum of LHB (Sanders)

Depleted models provide best fit, but not good

Line identification questionable for many lines

The Spectrum Problem (1)

Diffuse X-ray Spectrometer (DXS)

energy range: 0.15-0.31 keV

resolution: 4-14 eV

Sanders et al. (2001)

local hot bubble lhb5
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

local hot bubble lhb6
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.

local hot bubble lhb7
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

local hot bubble lhb8
Local Hot Bubble (LHB)

The UV Problem:

O VI emission, Shelton (2003)

EM is too small for B&S 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.

local hot bubble lhb9
Local Hot Bubble (LHB)


  • 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
lhb solution
(LHB) Solution?

Charge Exchange Reactions:

O+8 + H → O+7 + H++ ν

Cause of “flaming comets”

lhb solution1
(LHB) Solution?

Charge Exchange Reactions:

Source of the ROSAT “Long-Term Enhancements” and consistent with background seen towards the moon.

lhb solution2
(LHB) Solution?

Charge Exchange Reactions:

X-rays due to interaction of solar wind with

material in Earth’s Magnetosphere and with the ISM flowing through the solar system

Since the solar wind is time variable, so is the X-ray emission.

lhb solution3
(LHB) Solution?

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

other bubbles and stuff
Other Bubbles and Stuff

Monogem Ring, Plucinsky et al (1996)

nearby (300pc?) SNR log T~6.34

Eridion Bubble, Guo & Burrows (1995)

log T~6.00-6.24

Thus: Bubbles are too soft to be seen with CXO

Loop I Super-bubble

log T~6.5, Willingale et al (2005)

Galactic Bulge

log T~6.6, Snowden et al (1997)


0.75 keV

0.25 keV

Loop I Superbubble

Galactic Bulge

Eridion Bubble


loop i super bubble
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

the galactic halo
The Galactic Halo

From Kuntz & Snowden (2000)

The halo has two thermal components:

1. Soft & patchy, log T~6.05

Galactic chimney effluvia?

2. Hard & uniform, log T~6.45

Hydrostatic halo? Or WHIM/WHIGM?

Had the right temperature and strength to be the Warm-Hot Intergalactic Medium

the warmhot intergalactic medium
The WarmHot Intergalactic Medium

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

the galactic halo1
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

the galactic halo3
The Galactic Halo

The XQC spectrum

the galactic halo4
The Galactic Halo

The XQC spectrum showed that:

Bulk of the hard component is due to O VII

at z<0.01

At most 34% of emission is WHIM

Depletions are required for OK spectral fits

The XQC spectrum is consistent with the DXS spectrum.

the galactic ridge
The Galactic Ridge

(Seemingly) Diffuse Emission

longitude ±45, latitude ±1

scale height~100pc

Worral et al (1982) Warwick et al (1988)

FeK emission → thermal emission


1. Point source contamination

(not a problem, Ebisawa 2002)

2. Non-thermal components

the galactic ridge1
The Galactic Ridge

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

the galactic ridge2
The Galactic Ridge

The spectrum required two NEI components:

kT~0.75 keV, kT~7.0 keV

(log T~6.9, log T~7.9)

The hot gas is way too

hot to be retained by

the Galaxy

the galactic ridge3
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

LECR+2 CIE components: kT~0.56, kT~2.8

Thus the problem of the really hot gas resolved.

the galactic ridge4
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

the galactic ridge5
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.

chandra studies of diffuse ism
Chandra Studies of Diffuse ISM


Small FOV → small number of photons

Hard halo: 0.018 counts/s/chip

Soft halo: 0.002 counts/s/chip

Fills the FOV

what’s the instrumental background?

Backgrounds may be time-variable!

chandra studies of diffuse ism1
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

chandra studies of diffuse ism2
Chandra Studies of Diffuse ISM

Just because it is hard doesn’t mean we aren’t still trying!

other galaxies
Other Galaxies

M101 (as an example)

Kuntz et al (2003)

Two thermal components, kT~0.25,0.75


Contamination by binaries? No!

Binaries have PL spectra

Contamination by unresolved stars?

other galaxies1
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…

other galaxies2
Other Galaxies

M101 (as an example)

Kuntz et al (2003)

Two thermal components, kT~0.25,0.75

Soft: due to super bubbles?

Hard: Galactic Ridge equivalent?

Contamination by binaries? No!

Binaries have PL spectra

Contamination by unresolved stars?

other galaxies3
Other Galaxies

The Chandra spectrum of M101

other galaxies4
Other Galaxies

Dashed lines show possible amount of stellar contamination.

chandra studies of diffuse ism3
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

things to keep in mind
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.

things to keep in mind1
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