The X-ray absorption line spectroscopy can also be used to probe
the hot CGM around other galaxies. Yao et al. have recently
studied the hot CGM along the sight lines toward luminous AGNs
[24].
As an example, Fig. 2
shows a Chandra grating spectrum of PKS 2155-304.
No absorption line is apparent at the
redshifts of the three groups of galaxies with the
impact distances smaller than 500 kpc of the
sight line (in particular, the group at the highest redshift contains
two galaxies brighter than L*). The same
is true when the spectrum is folded according to the redshifts
of the individual groups. To increase the counting statistics further,
Yao et al. have stacked spectra from the Chandra grating
observations of eight AGNs with such intervening galaxies or groups.
No significant absorption is detected for any of these
individual systems or in the final stacked spectrum
with a total equivalent exposure of about 10 Ms!
Upper limits to the mean column densities of various ion species per galaxy
or group are then estimated. In particular, they find that
NOVII 6
× 1014 cm-2,
consistent with the constraints for the CGM around our Galaxy.
They have estimated the total mass contained in the CGM as
MCGM
0.6 × (0.5 /
fOVII) ×
(0.3A
/ A) × (R / 500 kpc)2 × 1011
M
,
where fOVII, A, and R are the ionization
fraction of O VII, metal abundance, and the radius of the hot CGM,
respectively. This is in contrast to the expected baryon mass
2 ×
1011
M
for
the halo of a Milky Way-type galaxy or a typical galaxy group
[25].
Thus the bulk of the CGM unlikely resides
in such a chemically enriched warm-hot phase
at temperatures ranging from 105.5-106.5 K
(Fig. 1),
which our X-ray absorption line spectroscopy is sensitive to.
This conclusion has strong implications for understanding the
accumulated effect of the stellar and AGN feedback on the galactic
ecosystem (see the discussion section).
![]() |
Figure 2. Part of the Chandra grating spectrum of PKS 2155-304 with a total exposure of 1 Ms. Various absorption lines (marked in blue) are detected and are produced by hot gas in and around the Galaxy (zero redshift). The red-shaded regions enclose spectral ranges of the same lines for three red-shifted groups of galaxies located within the 500 kpc impact distance of the AGN sight line. |
To study the effect of ongoing stellar and AGN feedback, one can map out diffuse X-ray emission from hot gas in and around nearby galaxies of various masses and star formation rates. Much attention has been paid to the feedback in starburst and massive elliptical galaxies, which are relatively bright in diffuse X-ray emission. Chandra observations have shown convincingly that the AGN feedback is important in shaping the morphology and thermal evolution of hot gas in massive elliptical galaxies, particularly those at centers of galaxy groups and clusters ([26] and references therein). The asymmetry in the global diffuse X-ray morphology is correlated with radio and X-ray luminosities of AGNs in elliptical galaxies, even in rather X-ray-faint ones [27]. This calls into question the hydrostatic assumption commonly used in order to infer the gravitational mass distribution in such galaxies. Nevertheless, the hydrostatic assumption may hold approximately for hot gas around the central supermassive black holes (SMBHs), if they are in a sufficiently quiescent state. The SMBH masses may then be measured from spatially resolved X-ray spectroscopy of the hot gas. Humphrey et al. have made such mass measurements for four SMBHs with Chandra data [28]. Three of them already have mass determinations from the kinematics of either stars or a central gas disk. It is encouraging to find a good agreement between the measurements using the different methods. From this agreement, they further infer that no more than ~ 10%-20% of the ISM pressure around the SMBHs should be nonthermal.
The feedback in nuclear starburst galaxies is manifested in the so-called
galactic superwinds driven by the mechanical energy injection from fast
stellar winds and supernovae (SNe) of massive stars (e.g.,
[29,
30,
31]).
The observed soft X-ray emission from a superwind typically has an
elongated morphology along the minor axis of such a galaxy and
is correlated well with extraplanar
H-emitting
features. This indicates that the detected hot gas arises primarily from
the interaction between the superwind and cool gas. The superwind
itself, believed to be very hot and low in density, is much difficult to
detect. From a detailed comparison between Chandra data and
hydrodynamic simulations, Strickland & Heckman infer that the
superwind of M82 has a mean temperature of 3-8 ×
107 K and a mass outflowing rate of ~ 2
M
yr-1
[31].
Such energetic superwinds with little radiative energy loss must
have profound effects on the large-scale CGM (e.g.,
[30]).
Recent X-ray observations have further shown the importance of the
feedback in understanding even "normal" intermediate-mass galaxies
(similar to the Milky Way and M31; e.g.,
[29,
30,
32,
33,
34,
35,
36,
37,
38,
39,
40]).
Chandra, in particular, has unambiguously detected diffuse hot gas
in and around normal disk galaxies. The total X-ray luminosity of the gas
is well correlated with the star formation rate for such galaxies. The
diffuse soft X-ray emission is shown to be strongly enhanced in recent
star forming regions or spiral arms within an individual galaxy viewed
face-on and is only slightly more diffuse than
H emission (e.g.,
[33,
34]).
This narrow appearance of spiral arms in X-ray
conflicts the expectation from population synthesis models:
the mechanical energy output rate from SNe should be nearly constant over
a time period that is up to 10 times longer than the lifetime of
massive ionizing stars. This means that SNe in inter-arm regions, where
ISM density is generally low, must produce less soft X-ray emission than
those in the arms. When disk galaxies are observed in an inclined angle
(e.g., Fig. 3),
the soft X-ray emission tends to appear as plumes,
most likely representing blown-out hot gas heated in
recent massive star forming regions and galactic spheroids. The cleanest
perspectives of the extraplanar hot gas are obtained from the observations
of edge-on disk galaxies such as NGC 891
[29]
and NGC 5775
[38].
The observed diffuse X-ray emission typically does
not extend significantly more than a few kpc away from the galactic
disks. A claimed detection of the
emission around the edge-on spiral NGC 5746 on larger scales was later
proved to be due to an instrumental artifact
[41].
Complementary observations from XMM-Newton and Suzaku give
consistent results and provide improved spectral information on the hot
gas (e.g.,
[35,
36,
40,
42,
43]).
The morphology of the X-ray emission as well as
its correlation with the star formation rate clearly shows that the
extraplanar hot gas is primarily heated by the stellar feedback.
In fact, the cooling of the hot gas accounts for only a small fraction
of the expected energy input from massive stars alone (typically no more
than a few %).
![]() |
Figure 3. Chandra diffuse 0.5-1.5 keV intensity contours, overlaid on a Spitzer mid-IR image of the Sb galaxy NGC 2841. The one-sided morphology of the diffuse X-ray emission apparently represents outflows of hot gas from the tilted galactic disk with its northeast edge closer to us. The emission from the back side is largely absorbed by the cool gas in the disk. The crosses mark the positions of excised discrete X-ray sources. |
The best example of mapping out hot gas in a galactic stellar bulge (or spheroid) is the discovery of an apparent hot gas outflow from the M31 bulge [37, 39]. This outflow with a 0.5-2 keV luminosity of ~ 2 × 1038 erg s-1 is driven apparently by the feedback from evolved stars in form of stellar mass loss and (Type) Ia SNe, because there is no evidence for an AGN or recent massive star formation in the bulge. The bipolar morphology of the truly diffuse soft X-ray emission further indicates that the outflow is probably influenced by the presence of strong vertical magnetic field, same as that observed in the central region of our Galaxy. The spectrum of the emission can be characterized by an optically-thin thermal plasma with a temperature of ~ 3 × 106 K [37, 39, 44]. Similar analyses have been carried out so far only for a couple of other early-type galaxies (NGC 3379, [45]; NGC 5866, [46]). The faint stellar contribution, which has a significantly different spectral shape from LMXBs', still needs to be carefully accounted for in the analysis of other galaxies. In any case, it is clear that the diffuse X-ray luminosity accounts for at most a few % of the energy input from Ia SNe alone in a normal early-type galaxy. Where does the missing feedback go?
Another remarkable puzzle about the diffuse hot gas in elliptical
galaxies is the apparent low metal abundances. As inferred from X-ray
spectral fits, the abundances are typically sub-solar for low- and
intermediate-mass galaxies with log(Lx)
41 to about solar
for more massive ones (e.g.,
[47,
48]),
substantially less than what are expected from the Ia SN enrichment (see
the discussion section). Furthermore, a number of galaxies show a
significant iron abundance drop toward their central regions, where the
stellar feedback should be the strongest. Examples of this abundance drop
include M87
[49],
NGC 4472, NGC 5846
[50],
and NGC 5044
[51].
The drop and the low abundance, if intrinsic,
would then indicate that only a small portion of metals produced
by Ia SNe is observed; the rest is either expelled or in a
state that the present X-ray data are not sensitive to.
In summary, the following are the key discoveries made with Chandra observations of diffuse hot gas in and around normal galaxies:
While the above results are generally consistent with those found for our own Galaxy, the studies of nearby galaxies also show intriguing sub-structures in the diffuse hot gas distributions (e.g., the apparent positive radial temperature and metal abundance gradients in elliptical galaxies).