Follow-on Science Instrument

Contract NAS8-01129


Monthly Status Report No. 005

July 2002

HETG Science Theme: The Inter-Galactic Medium (IGM)


Prepared in accordance with DR 972MA-002

DPD #972


Prepared for

National Aeronautics and Space Administration

Marshall Space Flight Center, Alabama 35812


Center for Space Research; Massachusetts Institute of Technology; Cambridge, MA 02139



Inter-Galactic Medium Research Progress



Image generated by Aladdin Ghostscript 

CREATOR: XV Version 3.10a  Rev: 12/29/94 
(PNG patch 1.2)  Quality = 100, Smoothing = 20The standard cosmological theory (these days!) predicts that huge amounts of hot gas should exist in the vast space between galaxies in our local universe. This hot gas has been hidden from view since the time galaxies formed because at its high temperature it does not emit substantial visible light. It is thought that this gas forms part of a gigantic system, or web, of hot gas and dark matter that defines the cosmic landscape. The hot gas part of this system could contain more material than all the stars in the universe. Cosmological hydrodynamic simulations have been carried out to model the formation of structure in the universe as predicted by this theory.



At left is the image from such a simulation which shows the cosmic web of warm/hot gas as the light-blue filamentary structures. The denser regions in yellow and red are where clusters and groups of galaxies are located and are the only regions we’d see in optical light.


The image represents about 60 million light years along each side which is equivalent to a red shift of z ~ 0.004. Objects observed with Chandra for our IGM studies are much further away than this with z generally greater than 0.1 and as high as 2.

Basically, to detect and measure this warm/hot (0.1 to 10 million degrees) intergalactic medium (WHIM), we select X-ray bright background sources such as distant galaxies/AGN that can be detected by Chandra. When photons pass through places where this hot gas is located, they are absorbed by the highly ionized, "heavy metals" (C, N, O, Si, Fe) in the hot gas and an X-ray absorption feature or “dip” will be produced in the spectrum of the background source. Based on the detected absorption signatures, we can derive the physical properties of the absorbing gas. A key parameter to measure is the Doppler shift of the absorption dips which measures the location of the gas along the line-of-sight.

In this illustrative diagram an X-ray source is shown emitting three different wavelengths of X-rays, coded by the red, yellow, and blue waves. In going through the filament of IGM gas the yellow wavelengths are absorbed and do not reach Chandra. The spectrum in the inset shows actual data where absorption “dips” are visible due to Oxygen in the IGM.

This image is taken from the Chandra Press release at:



Summary of IGM GTO Observations and Activities

In the HETG GTO program we've specifically included some bright distant AGN sources to probe for warm/hot gas in the intergalactic medium, in addition some AGN used as calibration targets are also useful in IGM studies. Relevant targets are tabulated here:





Lines seen?




No line, set upper limit


PKS 2155-304


OVII, OVIII at z=0; OVIII at z=0.055


Mkn 421


In progress


PKS 2149-306


No line, set upper limit


3C 273


OVII at z=0


H 1821+643


Fe emission lines at E=6.4 and 6.9 keV






3C 279


In progress


1ES 1028+511


In progress


1H 0414+009


recently observed, data will arrive soon






1H 1426+428


to be observed in Cycle 4 (starting about Dec ’02)


HETGS data have been (or will be) taken on all of these targets and in addition for the "Cal" targets LETG data are often available as well, using the LETG improves sensitivity to redshifted Oxygen lines (O VII, OVIII.)


Our science activities to study the IGM are not limited to Chandra observations. We are also making use of the capabilities of the XMM-Newton observatory, working on AO1 data received from two targets: PG 1407+265 and PKS 2135-14, Guest Investigators: T. Fang and C.R. Canizares. We have also talked with the PI of the NASA SMEX mission SPIDR (Spectroscopy and Photometry of the IGM's Diffuse Radiation) and are working on creating an OVI emission map relevant to SPIDR from our simulations. (The SPIDR website is: http://www.bu.edu/spidr/noflash/overview.html .)




High Redshift Quasars: Q[S5]0836+710 and PKS 2149-306

(Published by T. Fang, H.L. Marshall, G.L. Bryan, and C.R. Canizares, ApJ, 555, 356)

We obtained the first high-resolution X-ray spectra of two high-redshift quasars, Q0836+710 and PKS 2149-306, obtained with the Chandra High Energy Transmission Grating Spectrometer (HETGS). The primary goal of this observation is to use the high spectral resolving power of the HETGS to detect X-ray absorption produced by a hot intergalactic medium. The continuum of both quasars can be fit by absorbed power laws. Excess continuum absorption above the Galactic value is found in Q0836+710, as evidenced in ASCA and ROSAT observations. No significant emission or absorption features are detected in either source at a +/-3 sigma level. Based on the detection limits we constrain the properties of possible emitters and absorbers. Absorbers with a column density higher than 8e16 /cm^2 for OVIII or 5e16 /cm^2 for SiXIV would have been detected. We propose a method to constrain the cosmological parameters (namely Omega_0 and Omega_b) via the X-ray forest theory, but current data do not give significant constraints. We also find that combined with the constraints from the distortion of the CMB spectrum, the X-ray Gunn-Peterson test can marginally constrain a uniform, enriched IGM.


HEG spectra of Q[S5]0836+710 (top panel) and PKS 2149-306 (bottom panel). The solid line and dotted line in each panel are the observed and fitted spectrum, respectively. The spectral model is an absorbed power-law.

H 1821+643: Setting Limits on IGM Temperature

(Published by T. Fang, D.S. Davis, J.C. Lee, H.L. Marshall, G.L. Bryan, and C.R. Canizares, ApJ, 565, 86)

H 1821+643 is one of the most luminous quasars (m_v = 14.1) at low redshift (z = 0.297). It was discovered as a serendipitous X-ray source detected with the Einstein Observatory and has been studied extensively. At a moderate redshift, the sight line toward H 1821+643 traverses a distance of nearly 1.5 Gpc. Numerous absorption systems (such as H, C, N, O, Si, etc.) have been discovered in this sight line in optical and UV bands. Especially in recent observations, the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer have detected a number of OVI absorption lines which are not clearly associated with any galactic system.


We searched for absorption features from a warm/hot component of the IGM along the line of sight to this quasar and no absorption features were detected at or above the 3-sigma level. Based on the lack of OVII/OVIII absorption lines and by assuming collisional ionization equilibrium, we constrain the gas temperature of a typical OVI absorber to be between 0.1 to 1 million K, which is consistent with the results from hydrodynamic simulations of the intergalactic medium.




At left the zeroth-order image of H~1821+643, with North to the top and East to the left. The star K1-16 is indicated. From the contours it appears that there is extended (cluster) emission around the central source.


An iron emission line attributed to H1821+643 is clearly resolved at 6.43 keV (rest frame), with an equivalent width of 100 eV. Although we cannot rule out contributions to the line from a putative torus, the diskline model provides an acceptable fit to this iron line. We also detect a weak emission feature at 6.9 keV (rest frame). We suggest that both lines could originate in an accretion disk comprised of a highly ionized optically thin atmosphere sitting atop a mostly neutral disk.



The simultaneous fit to MEG (red) and HEG (black) spectra between 2 and 7 keV. Two Gaussian components are added to fit the emission features around 5 and 5.3 keV.



As revealed in the zeroth order image there is also extended emission from the cluster of galaxies which surrounds H1821+643. We have been able to separate the moderate CCD X-ray spectrum of the surrounding cluster from the central quasar and find that this is a hot cluster with a temperature of ~ 10 keV and a metal abundance of 0.3 Z_sun. We also independently obtain the redshift of the cluster, which is consistent with the optical results. We estimate that the cluster makes negligible contributions to the 6.9 keV iron K line flux identified above.


PKS 2155-304 : Clear detection!

(Published by T. Fang, H.L. Marshall, J.C. Lee, D.S. Davis, and C.R. Canizares, ApJ Letters, 572, 127)

We report the first detection of an OVIII Ly-alpha absorption line associated with an over-dense region in the intergalactic medium (IGM) along the line of sight towards PKS 2155-304 with the Chandra LETG-ACIS. The absorption line is detected at a 4.5-sigma level with cz ~ 16,600 km/s. At the same velocity Shull et al. detected a small group of spiral galaxies (with an overdensity of 100 and low metalicity HI Ly-alpha clouds. We constrain the intragroup gas that gives rise to the OVIII Ly-alpha line to a baryon density in the range 1e-5 < n_b < 7.5e-5 cm^-3 and a temperature of 4 to 5 million K, assuming 0.1 solar abundance. These estimates are in accordance with those of the warm/hot intergalactic medium (WHIM) that is predicted from hydrodynamic simulations. Extrapolating from this single detection implies a large fraction of the ``missing baryons'' are probed by the OVIII absorber.


The Chandra LETG-ACIS spectrum of PKS~2155-304 between 18 and 22 A, the red solid line is the fitted continuum plus three Gaussian absorption dips, the ion species are labeled in blue. The absorption feature at 20.02 A has been identified as OVIII Ly-alpha absorption from the IGM.








Our detection and upper limits can be compared with theory on a plot of cumulative probability of OVIII absorption per unit redshift vs. column density. Three data points are: PKS 2155, H1821, Q[S5]0836. We find that the detection is consistent with predictions from cosmological simulations (soild curve).








Inter-Galactic Medium Plans and Further Work



- Complete analysis of the three sources from AO3: 3C 279, 1H0414, 1ES1028; publish detections or limits to lines from O VII and O VIII.


- Analyze new Chandra cycle 3 and cycle 4 GTO observations.

- Continue analyzing data from 3C 273 and Mkn 421 to search for weak absorption features from the IGM.


- Continue analysis of XMM-Newton GO data on PG 1407+265 and PKS 2135-14.


- Collaborate with F. Nicastro on Chandra/XMM TOO observations of Seyfert I Galaxies and blazers in outburst.


- Perform cosmological simulations on the non-equilibrium evolution of IGM ionization structure.