We present the HST ACS G800L grism spectroscopy observation of the faint active galactic nuclei (AGN) candidates in the COSMOS field at redshift of 6 selected by the point-source morphology and the photometry drop-off at 8000\AA. Among the sample of 7 objects, only one is detected by multiple bands, and has similar shape of spectral energy distribution as the so-called ``little red dots'' JWST selected AGN candidates, but our object is 3 magnitude brighter than the JWST sample. We draw the upper limit of the AGN luminosity function Φ=1.1×10−7Mpc3 mag−1 for MUV=−21 at redshift of 6. The rest of the sample shows inconsistent flux density when comparing magnitudes of HST ACS F814W to the Subaru i-band and z-band magnitudes combined. The HST ACS G800L grism observation shows that this inconsistency cannot be created from an emission line. Therefore, we speculate that these objects are transients with the light curve decay timescale at most 6 years in observed frame.
@article{arxiv.2504.07196,
title = {Joint Survey Processing. III. Compact Oddballs in the COSMOS Field -- Little Red Dots and Transients},
author = {Yu-Heng Lin and Andreas L. Faisst and Ranga-Ram Chary and Anton M. Koekemoer and Joseph Masiero and Daniel Masters and Vihang Mehta and Harry I. Teplitz and Gregory L. Walth and John R. Weaver},
journal= {arXiv preprint arXiv:2504.07196},
year = {2025}
}