Anomalously Steep Reddening Law in Quasars: An Exceptional Example Observed in IRAS14026+4341

TL;DR

This study characterizes an exceptionally steep reddening law in the quasar IRAS 14026+4341, revealing unique dust grain size distributions and suggesting possible dust destruction or formation mechanisms in quasar environments.

Contribution

It provides a detailed analysis of an anomalously steep reddening curve in a quasar, proposing a dust model with a specific grain size distribution and exploring potential dust formation and destruction processes.

Findings

Reddening curve rises steeply below 3000 Å

Dust model with silicate grains truncated at 70 nm fits observations

IRAS 14026+4341 is a weak emission-line quasar with heavy reddening

Abstract

A fraction of the heavily reddened quasars require a reddening curve which is even steeper than that of the Small Magellanic Cloud. In this paper, we thoroughly characterize the anomalously steep reddening law in quasars, via an exceptional example observed in IRAS 14026+4341. By comparing the observed spectrum to the quasar composite spectrum, we derive a reddening curve in the rest-frame wavelength range of 1200 {\AA}--10000 {\AA}. It is featured with a steep rise at wavelengths shorter than 3000 {\AA}, but no significant reddening at longer wavelengths. The absence of dust reddening in optical continuum is confirmed by the normal broad-line Balmer decrement (the H$\alpha$/H$\beta$ ratio) in IRAS 14026+4341. The anomalous reddening curve can be satisfactorily reproduced by a dust model containing silicate grains in a power-law size distribution, $dn(a)/da \propto a^{-1.4}$, truncated at a maximum size $a_{max}=70 {\rm nm}$. The unusual size distribution may be caused by the destruction of large "stardust" grains by quasar activities or a different dust formation mechanism (i.e., the in situ formation of dust grains in quasar outflows). It is also possible that the analogies of the dust grains observed toward the Galactic center is responsible for the steep reddening curve. In addition, we find that IRAS 14026+4341 is a weak emission-line quasar (i.e., PHL 1811 analogies) with heavy dust reddening and blueshifted broad absorption lines.