Kavli Affiliate: Erin Kara
| First 5 Authors: Minghao Yue, Anna-Christina Eilers, Tonima Tasnim Ananna, Christos Panagiotou, Erin Kara
| Summary:
Recent James Webb Space Telescope (JWST) observations have revealed a
population of compact extragalactic objects at $zgtrsim4$ with red
near-infrared colors, which have been dubbed as “Little Red Dots" (LRDs). The
spectroscopically-selected LRDs exhibit broad H$alpha$ emission lines, which
likely indicates that type-I active galactic nuclei (AGN) are harbored in the
galaxies’ dust-reddened cores. However, other mechanisms, like strong
outflowing winds, could also produce broad H$alpha$ emission lines, and thus,
the nature of LRDs is still under debate. We test the AGN hypothesis for LRDs
by stacking the archival {em Chandra} observations of 34
spectroscopically-selected LRDs. We obtain tentative detections in the soft
$(0.5-2text{ keV})$ and hard $(2-8text{ keV})$ X-ray bands with $2.9sigma$
and $3.2sigma$ significance, and with $4.1sigma$ significance when combining
the two bands. Nevertheless, we find that the soft (hard) band $3sigma$ upper
limit is $sim1$dex ($sim 0.3$dex) lower than the expected level from the
$L_text{X}-L_{text{H}alpha}$ relation for typical type-I AGNs. Our results
indicate that AGN activity is indeed likely present in LRDs, though these
objects have significantly different properties compared to previously
identified type-I AGNs, i.e., LRDs may have intrinsically weak X-ray emissions.
We find it difficult to explain the low $L_text{X}/L_{text{H}alpha}$ ratios
observed in LRDs solely by absorption. It is also unlikely that fast outflows
have major contributions to the broad H$alpha$ lines. Our findings indicate
that empirical relations (e.g., for black hole mass measurements) established
for typical type-I AGNs should be used with caution when analyzing the
properties of LRDs.
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