The properties of the global hot gas on scales comparable to the size of the Galaxy remained largely unknown until recently. Before Chandra, we did have various broad-band X-ray emission surveys of the sky such as the one made by ROSAT in the 0.1-2.4 keV range, which is sensitive to the hot gas [9]. But such a survey alone cannot be used to directly determine the origin of the X-ray emission, which carries little distance information. The interpretation of the emission depends sensitively on the assumed cool (X-ray-absorbing) and hot gas distributions. Spectroscopic information on the X-ray emission has been obtained from rocket experiments [10] and more recently from Suzaku observations, but only for a number of sample regions (e.g., [11]). There are also large uncertainties in the contributions from faint discrete sources and other irrelevant phenomena such as solar wind charge exchange (SWCX) to the emission.
 |
Figure 1. Relative line strength
(ionization fraction times oscillation strength) of the
K |
A breakthrough for the study of the global hot gas has been made
from the use of the X-ray absorption line spectroscopy (e.g.,
[12,
13,
14,
15,
16,
17,
18,
19,
20]).
While absorption line spectroscopy is commonly used in optical and UV
studies of the interstellar medium (ISM), this technique became feasible in
the soft X-ray regime only with grating spectra from Chandra and
XMM-Newton. Unlike the emission, which is sensitive
to the density structure, absorption lines produced by ions such as O
VII, O VIII, and Ne IX (Fig. 1)
directly probe their column densities,
which are proportional to the mass of the hot gas.
The relative strengths of such absorption lines give direct diagnostics
of the thermal, chemical and/or kinetic properties of the hot gas.
Although the absorption lines are rarely resolved in the spectra (with
a resolution of ~ 400-500 km s-1 FWHM), the velocity
dispersion of the hot gas can be derived from the relative line
saturation of different transitions of same ion species (e.g.,
O VII K
vs.
K
). Because
the K-shell transitions of carbon through iron are all in the X-ray
regime, the same technique can be used to study the ISM in essentially
all phases (cold, warm, and hot) and forms (atomic, molecular, and dust
grain;
[16]).
Furthermore, the measurements are insensitive to the photo-electric
absorption by the cool ISM (kT
104 K)
and to the SWCX. Therefore, the X-ray absorption line spectroscopy
allows us to probe the global ISM unbiasedly along a sight line.
The effectiveness of the technique can be further enhanced
when multiple sight lines are analyzed jointly (e.g.,
[18])
and/or emission data are included (e.g.,
[19]).
We can then infer differential properties of hot gas between sight lines
of different depths or directions and/or estimate the pathlength and
density of the hot gas. The application of this X-ray tomography, though
only to a very limited number of sight lines so far, has led
to the first characterization of the global hot gas:
10 kpc away
from the Galactic disk/bulge
[19]
and ~ 1 × 1015 cm-2 for
50 kpc
[20].
This low column density of the hot CGM or the local hot intragroup
medium is consistent with the lack of evidence for enhanced X-ray line
absorptions along sight lines near the major axis of the Local Group
[23].
This basic characterization demonstrates the potential of using the X-ray tomography to greatly advance our understanding of the global hot gas in and around the Galaxy.
