Exploring the Damped Lyman-α Clouds with AXAF

The High Energy Transmission Grating (HETG) Spectrometer on the Advanced X-ray Astrophysics Facility (AXAF) (scheduled for launch in August, 1998) will provide a new tool for the study of absorption in the X-ray spectra of high redshift quasars due to the material along the line of sight. In this paper we try to explore the possibility of using AXAF HETG to detect resonance absorption lines from the Damped Lyman-α (DLA) clouds. 1 The simulated spectrum AXAF HETG 1 is designed to provide high resolution spectroscopy up to E/∆E∼ 1000 (for point source) between 0.4 keV and 10 keV. If the DLA systems contain a sufficient column density of highly ionized metals, the high spectral resolution of the HETG will permit detection of resonance lines. For specificity, we consider Q2223-052, a high redshift quasar at z = 1.404 which has a DLA cloud at z = 0.484. X-ray observations with Einstein [1] show its flux to be 9.5 × 10ergs scm between 0.16-3.5 keV. This implies a count rate of 0.36 counts/sec with AXAF MEG in the first order. Given the observation time and the instrument resolving power we can calculate the minimum detectable equivalent width of an absorption feature and the required ion column density. In Table 1 N1 is the required column density. Here τ is the optical depth at the line center, and the velocity dispersion b = 200km/sec. All energies are in the observer frame. We assume the spectrum of the high-z quasar has the form of a power law + Galactic absorption + an assumed resonance line from the DLA system due to Si xiii at 1.26keV in the observer frame, then fit it with a model not containing the absorption line. In the χ plot we can clearly find the line. Table 1. Two Resonance Lines in AXAF Band ion E(keV) EW(eV) τ N1(×10 ) N2(×10 ) Mg XI 0.91 1.4 3 6 9.8 Si XIII 1.26 1.2 1.2 3.2 4.6 See web page http://asc.harvard.edu/ Figure 1: Q2223-052, the residual χ plot 2 Can we detect DLA systems? Table 1 gives the minimum column density which could be detected the by AXAF HETG. Could a DLA cloud gives such a column density? To answer this question, we have simulated the ionization structure using the CLOUDY package [2]. We assume the DLA cloud is photonionizated by a power law continuum radiation with a mean intensity of Jν at 1 Rydberg,NHI = 10 cm, nH = 10cm. The temperature and metallicity are fixed at T = 2 × 10K and [Z/H ] = −0.5 [3]. The value of Jν is very uncertain, especially at low redshift . Our calculation shows that only when Jν ≥ 5× 10 ergs scmHzsr are ion column densities high enough to produce resonance lines which could be detected by AXAF HETG (see Fig.1). The results presented in the sixth column of Table 2 is calculated with Jν = 5 × 10 . Haardt & Madau [4] give a value of Jν = 10 −22 at z = 0.5 when taking into account QSOs as the source of ionization. However, some other photonionization sources, such as the young stars inside the DLA clouds and nearby quasars, may significantly increase the intensity of ionization photon flux. On the other hand, there may also exist other sources of ionization (such as collisional ionization) or other ionized material co-located with DLA clouds, which would then simply be markers of higher density regions in an ionized intergalactic medium. Acknowledgements. We are grateful to the MIT/ASC team for their assistance with MARX and XSPEC 10.0 and for helpful discussions. This work was supported in part by NASA contract, NAS 8.38249.