One of the overarching priorities for the future of astrophysics is to reach into previously unexplored realms of the Universe. Sensitive X-ray observations are instrumental for a great of frontier investigations. Achieving a panchromatic view of the quickly emerging population of galaxies and supermassive black holes at z ~10. Tracing formation of galaxies and their assembly into large-scale structures starting from the earliest possible epochs. Observing baryons and large-scale physical processes outside of the very densest regions in the local Universe. All of this and much more can be achieved if we have an X-ray observatory significantly more sensitive than Chandra. Angular resolution and sensitivity go hand-in-hand for X-ray telescopes. Chandra-scale angular resolution (1″ or better) is essential in building more powerful, higher throughput observatories to avoid source confusion and remain photon-limited rather than background-limited.

Recent technological developments support the feasibility of building an X-ray observatory in the 2020s that will have angular resolution comparable to Chandra accompanied by 30 to 100-fold increase in sensitivity in the 0.2–10 keV energy band. We call this mission concept SMART-X, the Square Meter Arcsecond-Resolution Telescope for X-rays. SMART-X will provide an incredibly powerful capability for a wide variety of cutting-edge science where X-ray information is indispensable: supernova remnants, accretion onto black holes and neutron stars, atmospheres of active stars, quasar winds, etc. Here are just three exciting illustrations of what SMART-X will make possible:

  • Observations of the first generation supermassive black holes are key for understanding their progenitors and the relation between SMBHs and host galaxies, as well as solving the puzzle of 109 M⊙ black hole “monster” formation by z=6–7. The lower-z experience indicates that a large fraction (perhaps, >50%) of SMBHs in high-z galaxies will be highly obscured in the optical and near-IR bands, limiting what even JWST can accomplish in this area. The envisioned X-ray observatory described below will detect 2–100 keV X-rays (rest frame, mostly unaffected by absorption) from z=10 SMBHs with masses as low as a few × 104 M⊙, thus providing a unique view into very early stages of the SMBH growth.

  • Objects at a mass scale of >> 1 galaxy are expected to first form at redshift ~ 6. The next generation X-ray observatory should spatially resolve the emission and measure the temperature and mass of hot gas in these protoclusters, even if their central galaxies host luminous quasars. Thus, we will be able to “write” a complete history of the largest bound structures in the Universe and understand the role of SMBHs in their evolution.

  • HST/COS detection of low-z OVI absorption lines confirms the presence of 105.5 K gas in the intergalactic medium and/or halos of ~ 0.3L* galaxies. However, this technique is insensitive to the higher-T IGM expected at higher densities and more massive galaxy halos. Deep Chandra exposures detect the T >107 K gas near the virial radius (r200) of galaxy clusters where the density is ~150 times higher than the cosmic mean. The next step should be mapping in emission the intergalactic medium at lower temperatures (>106 K) and overdensities (≈ a few × 10’s). For the first time, these observations will bring to light ~ 15% of all baryons in the local Universe and reveal structures ranging from denser parts of the filamentary Cosmic Web to the “hot-mode” IGM accretion onto massive galaxies.

Cluster Simulation

X-ray brightness in the 0.5–2 keV band for a simulated 30×30 Mpc region [1]. The dotted circle shows the r200 radius for the most massive cluster in this box (M ≈ 1015 M⊙). Chandra has detected X-rays to this radius in the deepest, ~ 106 s exposures. The large-scale white contour locates the X-ray brightness level, ~1/30 that at r200, accessible with SMART-X

Supermassive Black Hole Models

Models of eary supermassive black hole growth (from [1]) with the SMART-X sensitivity overplotted (assuming that 10% of the flux from Eddington-limited black holes is emitted in the X-ray band). The inset shows statistics of the AGNs detected at z<1 in the Bootes survey [2]. IRAC selection relies heavily on the 8μm band, redshifted to 90μm for z=10 objects.

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