# Presentation: 168669

## ISM X-ray Astrophysics

- Randall K. Smith
- Chandra X-ray Center

## Introduction

- The Interstellar Medium (X-rated)
-  Overview
-  Phases of the ISM
-  X-ray studies of the Hot ISM
-  X-ray studies of the Warm/Cold ISM
-  X-ray studies of  Dust Grains

## Overview

-  Constituents
- Gas: modern ISM has 90% H, 10% He by number
- Dust: refractory metals
- Cosmic Rays: relativistic e-, protons, heavy nuclei
- Magnetic Fields: interact with CR, ionized gas
-  Mass
- Milky Way has 10% of baryons in gas
- Low surface brightness galaxies can have 90%

## ISM: Phases

- Gas in the ISM has a number of phases:
- Cold Neutral Medium:    T ~ 100 K, n~100-104 cm-3
- Warm Neutral Medium:  T ~ 1000 K,  n ~ 1  cm-3
- Warm Ionized Medium:  T ~ 10,000 K, n ~ 0.1 cm-3
- Hot Interstellar Medium: T ~ 106 K, n ~ 0.01 cm-3
- Unsurprisingly, only the hot ISM emits any X-rays, and even these are easily absorbed since they are (mostly) soft.

## ISM: Phases

- Model surface density and temperature maps of the inner ISM

- Wada & Norman 1999

## ISM: Phases

- Vertical Distribution
-  Cold molecular gas has 100 pc scale height
-  HI has composite distribution (~150, 500 pc)
-  Reynolds layer of diffuse ionized gas (~1.5 kpc)
-  Hot halo extending into local IGM (~*few* kpc)

## ISM: Phases

- Boulares & Cox 1990

## ISM: Phases

- CO distribution in Galaxy

- Dame, Hartmann, & Thaddeus 2001

## ISM: Phases

- 21 cm emission

- Lyα abs.

- Vertical scale height of Halo HI layer

- Measurement of halo HI done by comparing Ly  absorption against high-Z stars to 21 cm emission (Lockman, Hobbs, Shull 1986)

- Need to watch for stellar contamination, radio beam sidelobes, varying spin temperatures.

## ISM: Phases

- Vertical scale height of Halo HI layer

- Lockman, Hobbs, Shull 1986

## ISM: Phases

- Vertical scale height of Main HI layer

- Lockman, Hobbs, Shull 1986

-  Overall density distribution (Dickey & Lockman 1990) at radii 4-8 kpc
-  “Lockman layer”
-  Disk flares substantially beyond solar circle.

## ISM: Phases

- Warm ionized gas in halo

- Dispersion measures and distances of pulsars in globular clusters show scale height of 1.5 kpc (Reynolds 1989). Revision using all pulsars by Taylor & Cordes (1993), Cordes & Lazio (2002 astro-ph)

-  Diffuse warm ionized gas extends to higher than 1 kpc, seen in H (Reynolds 1985)
- “Reynolds layer”,
** ****W**arm **I**onized **M**edium, or
-  **D**iffuse **I**onized **G**as

## ISM: Phases

- ROSAT made an all-sky survey in soft X-rays (0.1-2.2 keV); these results, after removing point sources, are from Snowden *et al*. 1997:

## ISM: Phases

- Interstellar Pressure

- Thermal pressures are very low:
*P**T** ~10**3*kB* = 1.4 *x *10**-13* erg cm-3.
-    (Perhaps reaches *3000*kB in plane)
- Magnetic pressures with *B=3-6****G *reach
*P**B** ~ 0.4-1.4 *x* 10**-12 *erg cm-3.
- Cosmic rays also exert a pressure:
-  *P**CR* ~ *0.8-1.6 *x* 10**-12 *erg cm-3.
- Turbulent motions of up to 20 km/s contribute:
*  **P**turb**~10**-12** *erg cm-3.
- Boulares & Cox (1990) show that total weight may require as much as *5 *x* 10**-12 *erg cm-3* * to support.

## ISM: Local

- Interestingly, we do not appear to be in a “normal” region of the Galaxy.  Partial proof of this may be seen this evening:

- There are frequently stars visible in the night sky

- If we lived in or near a molecular cloud, all of much of the night sky would be dark to visible light.  In fact, we can even see (from orbit) quite a few sources in the extreme ultraviolet (EUV) when a single “normal ISM” cloud would completely absorb them.
- Clearly, nearby space is not filled with dense (n > 1 cm-3) gas.  What is it filled with?

## ISM: Local

- Besides absorption studies of nearby (D ~ 100 pc) stars (used to quantify how little gas there is in our neighborhood), we can also see the material that fills our locale, in soft X-rays:

## ISM: Local

- Based on this evidence, it is believed that we live inside a “Local (Hot) Bubble” with average radius 100 pc, which happens to be right next to another bubble, Loop I.  The Local Bubble is filled with hot (T ~ 106 K), diffuse (n ~ 0.01 cm-3) gas, and radiates primarily below 0.25 keV.

- Diagram of  LB from Cox & Reynolds (1990)

## ISM: Absorption

- All the phases of the ISM can be studied using absorption
- spectroscopy.  Simply find a bright (ideally continuum) source, and
- look for absorption features:

- LMXB X0614+091 observed with Chandra LETG by Paerels *et al*. 2001

## ISM: Absorption

- McLaughlin & Kirby 1998

## ISM: Absorption

- Of course, one must also worry about calibration:

## ISM: Absorption

- However, good results are available:

## ISM: Absorption

- Clear limits can be placed on CO absorption:

## ISM: Dust Grains

- The counterpart to absorption studies is normally emission (*e.g.* radio 21 cm/H).  However, there is very little ISM gas with temperatures higher than 106 K, so neither Chandra nor XMM/Newton is much use.
- Surprisingly, however, X-rays can also probe IS dust grains.

- When an X-ray interacts with a dense cloud of electrons (such as are found in a dust grain), the electrons may vibrate coherently, scattering the X-ray slightly.

## ISM: Dust Grains

- The dust scattering cross section is

- Of course, we don’t observe single dust grains; we must integrate over a distribution of dust grains, and along the light of sight to a bright source.

- So what does this mean?

## ISM: Dust Grains

- So the observed surface brightness at position  from the source is:

## ISM: Dust Grains

## ISM: Dust Grains

- In order to properly measure the halo, the spectrum must be measured:

## ISM: Dust Grains

- In addition, we need to know the PSF of the telescope, as this must be subtracted to get the actual scattered halo:

## ISM: Dust Grains

- So here are some results from the LMXB GX13+1, at 3 different energies.  The only free parameter is NH.

- Smith, Edgar, & Shafer (2001)

## ISM: Dust Grains

- Integrating the total surface brightness (relative to the source flux) gives a result proportional to E-2.  The constant term can be easily related to any dust model.

## ISM: Dust Grains

- What does the future hold?  With sufficient energy resolution and effective area, it will be possible to diagnose dust abundances directly:

## Conclusions

- Studying the ISM in X-rays is a relatively new field.
-  Detailed absorption studies can only be done with high-resolution telescopes.  However, since X-rays penetrate all the way to the Galactic center, they open a new window on ISM studies.
-  It is possible (albeit very difficult) to study the IGM as well with deep observations.
-  Emission from the ISM in X-rays is dominated by very soft X-rays, mostly local.
-  The study of IS dust grains, especially the largest dust grains, can be done in a unique way with X-rays.