Observing supermassive dark stars with James Webb Space Telescope

Abstract

We study the capability of the James Webb Space Telescope ( JWST ) to detect supermassive dark stars (SMDSs). If the first stars are powered by dark matter (DM) heating in triaxial DM haloes, they may grow to be very large (>10 6 M ⊙ ) and very bright (>10 9 L ⊙ ). These SMDSs would be visible in deep imaging with JWST and even Hubble Space Telescope ( HST ). We use sensitivity limits from previous HST surveys to place bounds on the numbers of SMDSs that may be detected in future JWST imaging surveys. We showed that SMDS in the mass range 10 6 –10 7 M ⊙ are bright enough to be detected in all the wavelength bands of the NIRCam on JWST (but not in the less sensitive MIRI camera at higher wavelengths). If SMDSs exist at z ∼ 10, 12 and 14, they will be detectable as J ‐, H ‐ or K ‐band dropouts, respectively. With a total survey area of 150 arcmin 2 (assuming a multiyear deep parallel survey with JWST ), we find that typically the number of 10 6 M ⊙ SMDSs found as H ‐ or K ‐band dropouts is ∼10 5 f SMDS , where the fraction of early DM haloes hosting DS is likely to be small, f SMDS ≪ 1. If the SMDS survive down to z = 10 where HST bounds apply, then the observable number of SMDSs as H ‐ or K ‐band dropouts with JWST is ∼1–30. While individual SMDS are bright enough to be detected by JWST , standard Population III stars (without DM annihilation) are not, and would only be detected in first galaxies with total stellar masses of 10 6 –10 8 M ⊙ . Differentiating first galaxies at z > 10 from SMDSs would be possible with spectroscopy: the SMDS (which are too cool produce significant nebular emission) will have only absorption lines, while the galaxies are likely to produce emission lines as well. Of particular interest would be the He ii emission lines at m as well as Hα lines which would be signatures of early galaxies rather than SMDSs. The detection of SMDSs with JWST would not only provide alternative evidence for weakly interacting massive particles, but also provide a possible pathway for the formation of massive (10 4 –10 6 M ⊙ ) seeds for the formation of supermassive black holes that power quasi‐stellar objects at z = 6.