We present deep spectroscopic observations of a Lyman break galaxy (LBG) candidate (hereafter MACS1149-JD) at z ∼ 9.5 with the Hubble Space Telescope (HST) WFC3/IR grisms. The grism observations were taken at four distinct position angles, totaling 34 orbits with the G141 grism, although only 19 of the orbits are relatively uncontaminated along the trace of MACS1149-JD. We fit a three-parameter (z, F160W mag, and Lyα equivalent width [EW]) LBG template to the three least contaminated grism position angles using a Markov chain Monte Carlo approach. The grism data alone are best fit with a redshift of (68% confidence), in good agreement with our photometric estimate of (68% confidence). Our analysis rules out Lyα emission from MACS1149-JD above a 3σ EW of 21 Å, consistent with a highly neutral IGM. We explore a scenario where the red Spitzer/IRAC [3.6]-[4.5] color of the galaxy previously pointed out in the literature is due to strong rest-frame optical emission lines from a very young stellar population rather than a 4000 Å break. We find that while this can provide an explanation for the observed IRAC color, it requires a lower redshift (z ≲ 9.1), which is less preferred by the HST imaging data. The grism data are consistent with both scenarios, indicating that the red IRAC color can still be explained by a 4000 Å break, characteristic of a relatively evolved stellar population. In this interpretation, the photometry indicates that a Myr stellar population is already present in this galaxy only ∼500 Myr after the big bang.
Bibliographical noteFunding Information:
A.H. and this work were supported by NASA (National Aeronautics and Space Administration) Headquarters under the NASA Earth and Space Science Fellowship Program, grant ASTRO14F-0007. Support for the Grism Lens-Amplified Survey from Space (GLASS) (HST-GO-13459) was provided by NASA through a grant from the Space Telescope Science Institute (STScI). Support for this work was also provided by NASA through an award issued by JPL/Caltech (for SURFS UP project) and by HST/STScI HST-AR-13235, HST-AR-14280, and HST-GO-13177. This work utilizes gravitational lensing models produced by PIs M. Bradač, J. Richard, P. Natarajan & J. P. Kneib, K. Sharon, L. Williams, C. Keeton & G. Bernstein & J. M. Diego, and M. Oguri. This lens modeling was partially funded by the HST Frontier Fields program conducted by STScI. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The lens models were obtained from the Mikulski Archive for Space Telescopes (MAST). We wish to thank the anonymous referee for constructive feedback.
- galaxies: high-redshift
- gravitational lensing: strong
- techniques: spectroscopic