Is the Broad‐Line Region Clumped or Smooth? Constraints from the Hα Profile in NGC 4395, the Least Luminous Seyfert 1 Galaxy

The origin and configuration of the gas which emits broad lines in Type Iactive galactic nuclei is not established yet. The lack of small-scalestructure in the broad emission-line profiles is consistent with a smooth gasflow, or a clumped flow with many small clouds. An attractive possibility forthe origin of many small clouds is the atmospheres of bloated stars, an originwhich also provides a natural mechanism for the cloud confinement. Earlierstudies of the broad-line profiles have already put strong lower limits on theminimum number of such stars, but these limits are sensitive to the assumedwidth of the lines produced by each cloud. Here we revisit this problem usinghigh-resolution Keck spectra of the H alpha line in NGC 4395, which has thesmallest known broad-line region (~10^14 cm). Only a handful of the requiredbloated stars (each having r~10^14 cm) could fit into the broad-line region ofNGC 4395, yet the observed smoothness of the H alpha line implies a lower limitof ~10^4-10^5 on the number of discrete clouds. This rules out conclusively thebloated-stars scenario, regardless of any plausible line-broadening mechanisms.The upper limit on the size of the clouds is ~10^12 cm, which is comparable tothe size implied by photoionization models. This strongly suggests that gas inthe broad-line region is structured as a smooth rather than a clumped flow,most likely in a rotationally dominated thick disk-like configuration. However,it remains to be clarified why such a smooth, gravity-dominated flow generatesdouble-peaked emission lines only in a small fraction of active galacticnuclei.Comment: 12 pages, including 3 figures, accepted for publication in The Astrophysical Journa