Keck high-resolution spectroscopy of Mrk 335: constraints on the number of emitting clouds in the broad-line region

Abstract

We present high-resolution (∼6 km s⁻¹), high signal-to-noise ratio (−400 at Hα line centre) spectroscopy of Mrk 335. Cross-correlation (CC) analysis of the data yields a lower limit of ∼ 3 × 10⁶ for the number of emitting clouds in the broad-line region (BLR) of this object. This limit is applicable for clouds with T=2 × 10⁴ K and an optical depth of ∼ 10⁴ in their Hα line. The result is obtained from the absence of a CC signal in the data and from extensive Monte Carlo simulations that show the minimum number of clouds necessary in order to prevent the creation of a detectable CC signal. The simulations can be used to test any BLR model which is based on a contribution from discrete sources provided that the individual emission profile of the sources and their distribution are given. Current BLR models based on stellar atmospheres of bloated stars can be ruled out, unless the linewidth of an individual star exceeds 100 km s⁻¹. The lower limit on the number of emitting clouds also provides constraints on traditional photoionization models for a system of clouds.