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

Abstract

The origin and configuration of the gas which emits broad lines in Type I active galactic nuclei is not established yet. The lack of small-scale structure in the broad emission-line profiles is consistent with a smooth gas flow, or a clumped flow with many small clouds. An attractive possibility for the origin of many small clouds is the atmospheres of bloated stars, an origin which also provides a natural mechanism for the cloud confinement. Earlier studies of the broad-line profiles have already put strong lower limits on the minimum number of such stars, but these limits are sensitive to the assumed width of the lines produced by each cloud. Here we revisit this problem using high-resolution Keck spectra of the Hα line in NGC 4395, which has the smallest known broad-line region (∼ 10 14 cm). Only a handful of the required bloated stars (each having r * ≈ 10 14 cm) could fit into the broad-line region of NGC 4395, yet the observed smoothness of the Hα line implies a lower limit of ∼ 10 4 − 10 5 on the number of discrete clouds. This rules out conclusively the bloated-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 to the size implied by photoionization models. This strongly suggests that gas in the 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 generates double-peaked emission lines only in a small fraction of active galactic nuclei.

Radiation pressure confinement – II. Application to the broad-line region in active galactic nuclei

“…However, we now know that the BLR dynamics is largely dominated by the gravity of the black hole, which indicates it must be composed of a large column gas, where gravity inevitably dominates radiation pressure. We also know that the BLR gas is not composed of small clouds (Arav et al 1998;Laor et al 2006), and is most likely a smooth flow, possibly formed by a failed disc wind (Czerny & Hryniewicz 2011). Thus, the earlier studies are not relevant for the BLR we know today.…”

Broadband Photometric Reverberation Mapping of NGC 4395

“…It has also been suggested that there is a non-negligible intermediate line region in NGC 4395 that could make up a significant fraction of the total Balmer line profiles, but does not vary on these short time scales (A. Laor, private communication). From a more recent, high resolution Keck spectrum (Laor et al 2006) the narrow lines with asymmetric blue wings are roughly FWHM = 70 km s −1 . Kraemer et al (1999).…”

Continuum and Spectral Line Radiation from a Random Clumpy Medium

“…Since a sphere is the ultimate isotropic shape while an elongated filament and a semi-infinite slab are at the other extreme end of anisotropy, these geometries can be expected to bracket the full range of K-factor variation that cloud shapes can induce. Note, in particular, that in studies of AGN, the slab geometry is invoked in all calculations of broad-line emission (see, e.g., Laor et al 2006) and in some models of torus IR emission (Nenkova et al 2008). The averages involve integrations over the cloud projected area for a sphere, orientation for a slab and both of them in the case of a filament; Appendix B gives the details.…”

“…Example continuum applications include IR dust emission from circumnuclear tori in active galactic nuclei (AGN; Nenkova et al 2002Nenkova et al , 2008, free-free absorption affecting supernova radio light curves and spectra (Weiler et al 2004), radio-millimeter wave thermal emission from single massive stars (Ignace & Churchwell 2004) and star-formation induced radio synchrotron emission accompanied by free-free absorption in galaxies (Lacki 2013). Spectral line applications include modeling the optical and UV spectra from AGN broad line regions (Laor et al 2006) and interstellar atomic and molecular lines (Martin et al 1984;Wall 2006Wall , 2007. A common approach to the analysis of emission from clumpy media, pioneered by Martin et al (1984), is to assume some geometry for the individual clouds and proceed by averaging over properties along the line of sight (LOS).…”