ATLAS. II. Extremely High Incidence of Balmer Line Absorption with Predominant Blueshifts in LRDs: Statistical Insights through Comparison with Type 1 AGNs

Abstract

We present the statistical properties of H$\alpha$ and H$\beta$ line absorption in little red dots (LRDs) at $z\simeq2.5$--7.2 using archival JWST/NIRSpec spectra from the DAWN JWST Archive and complementary NIRSpec/IFU observations. Among 40 LRDs with broad H$\alpha$ and [O~{\sc iii}] obtained with medium- or high-resolution gratings, 14 objects exhibit H$\alpha$ absorption. We find that the incidence of Balmer line absorption is $\sim35$\% ($=14/40$), significantly higher than that in SDSS low-$z$ type 1 AGNs ($\sim0.04$\%), demonstrating that Balmer line absorption occurs approximately 850 times more frequently in LRDs than in type 1 AGNs. We combine our 14 detections with 32 additional LRD Balmer absorbers from the literature, yielding a census of 46 absorbers. Their velocities span $\Delta v_\mathrm{abs}(\mathrm{H\alpha})\sim-430$ to $+140\ {\rm km\,s^{-1}}$, markedly narrower than the $-800$ to $+1600\ {\rm km\,s^{-1}}$ range of Balmer absorption in SDSS type~1 AGNs, for which our simulations confirm that the velocity difference is too large to be explained by detection incompleteness. The lower absolute absorber velocities in LRDs may partly reflect the shallower gravitational potential at their characteristic BLR radii. We also find that 38 of the 46 absorbers (83\%) are blueshifted, with only eight redshifted, indicating that most of Balmer absorbers are moving outward. An analytic model with radiation pressure suggests that most absorbers with $N_{\rm H}\gtrsim10^{24}\ {\rm cm^{-2}}$ remains gravitationally bound. The smaller number of redshifted (i.e., infalling) absorbers may indicate that outbound absorbers lose density: some return to the BLR, whereas others undergo stronger radiative acceleration and escape.