Modelling dust rings in early-type galaxies through a sequence of radiative transfer simulations and 2D image fitting

Abstract

A large fraction of early-type galaxies (ETGs) host prominent dust features, and central dust rings are arguably the most interesting among them. We present here 'Lord Of The Rings' (LOTR), a new methodology which allows to integrate the extinction by dust rings in a 2D fitting modelling of the surface brightness distribution. Our pipeline acts in two steps, first using the surface fitting software Galfit to determine the unabsorbed stellar emission, and then adopting the radiative transfer code Skirt to apply dust extinction. We apply our technique to NGC 4552 and NGC 4494, two nearby ETGs. We show that the extinction by a dust ring can mimic, in a surface brightness profile, a central point source (e.g. an unresolved nuclear stellar cluster or an active galactic nucleus; AGN) superimposed to a 'core' (i.e. a central flattening of the stellar light commonly observed in massive ETGs). We discuss how properly accounting for dust features is of paramount importance to derive correct fluxes especially for low luminosity AGNs (LLAGNs). We suggest that the geometries of dust features are strictly connected with how relaxed is the gravitational potential, i.e. with the evolutionary stage of the host galaxy. Additionally, we find hints that the dust mass contained in the ring relates to the AGN activity.

The Stellar Initial Mass Function and Population Properties of M89 from Optical and NIR Spectroscopy: Addressing Biases in Spectral Index Analysis

“…We speculate that this difference, though small, in the SP properties at the very center of the galaxy may be connected to the presence of the dust disk in the center of M89. Indeed, as measured in Bonfini et al (2018), the radial trend of the color F 475W − F 850LP shows a "red bump", a signature of the dust disk, within a radius of ∼ 0.4", similar to the scale of our inner radial bin (i.e. ∼ 0.3").…”

“…Lauer et al 2005;Richtler et al 2020). Since M89 exhibits shells and stellar streams (Schweizer & Seitzer 1992;Bonfini et al 2018), the presence of dust (and gas, that is observed at larger radii) can be a signature of an accreted small stellar component that can have different characteristics (although diluted by the bulk of stars of M89).…”

“…It is easily seen that all trends present a gradient which is clearly negative with the exception of Hδ and Hγ that have instead positive gradients. It can also be noticed that almost all indices (excluding: Ca4592, Fe4046, Fe4064, NaI8190sdss and TiO1) have slightly lower values in the very center of the galaxy (first to second radial bins) that can probably be connected with the presence of the inner dust disk of M89 (Lauer et al 2005;Bonfini et al 2018) (more comments follow in Section 4).…”

Re. I. Understanding galaxy sizes, associated luminosity densities, and the artificial division of the early-type galaxy population

“…z NGC 4552 (VCC 1632) is one of these high-n galaxies, with a reported major-axis Sérsic index equal to 9.22 (K09) and a (geometric mean)-axis Sérsic index equal to 7.6 (Ferrarese et al, 2006). Bonfini et al (2018) reassign NGC 4552 from a core-Sérsic galaxy type to a Sérsic galaxy given that its 'core' is due to dust and, after accounting for interloping/undigested galaxy flux, Sahu, Graham, Davis, 2019 measure Sérsic indices, from Spitzer 3.6 μm images, equal to 5.42 and 5.36 for the major-axis and (geometric mean)axis, respectively. Krajnović et al (2013) report a Sérsic index of 6.2 for this galaxy.…”

Expected intermediate-mass black holes in the Virgo cluster. I. Early-type galaxies

“…The B-band magnitudes were based on the HST/ACS g and z band galaxy magnitudes published by Ferrarese et al (2006), who had also reported on the presence of any (light profile)-flattened cores seen in these passbands. Following Bonfini et al (2018), who report on dust-dimmed cores as opposed to those with a central stellar deficit, we reassign NGC 4552's (VCC 1632's) designation from a core-Sérsic galaxy ) to a Sérsic galaxy given that its 'core' is apparently due to dust. Of the 8 Virgo cluster galaxies reported to have a flattened core by Ferrarese et al (2006), this galaxy had the faintest apparent B-band magnitude.…”