Spitzer IRAC Low Surface Brightness Observations of the Virgo Cluster

TL;DR

This study uses Spitzer IRAC imaging to analyze low surface brightness features in the Virgo cluster, measuring the masses and ages of intracluster light plumes and the halo of M87, supporting gravitational formation mechanisms.

Contribution

First measurement of the mass of intracluster light plumes in Virgo, constraining their ages, masses, and supporting gravitational origin of intracluster light features.

Findings

Masses of intracluster light plumes range from 5.5 x 10^8 to 4.5 x 10^9 solar masses.

Plume ages are greater than 3 and 5 Gyr, respectively.

Outer halo of M87 shows a flat or blue optical color gradient, indicating older or lower metallicity stars.

Abstract

We present 3.6 and 4.5 micron Spitzer IRAC imaging over 0.77 square degrees at the Virgo cluster core for the purpose of understanding the formation mechanisms of the low surface brightness intracluster light features. Instrumental and astrophysical backgrounds that are hundreds of times higher than the signal were carefully characterized and removed. We examine both intracluster light plumes as well as the outer halo of the giant elliptical M87. For two intracluster light plumes, we use optical colors to constrain their ages to be greater than 3 & 5 Gyr, respectively. Upper limits on the IRAC fluxes constrain the upper limits to the masses, and optical detections constrain the lower limits to the masses. In this first measurement of mass of intracluster light plumes we find masses in the range of 5.5 x 10^8 - 4.5 x 10^9 and 2.1 x 10^8 - 1.5 x 10^9 solar masses for the two plumes for which we have coverage. Given their expected short lifetimes, and a constant production rate for these types of streams, integrated over Virgo's lifetime, they can account for the total ICL content of the cluster implying that we do not need to invoke ICL formation mechanisms other than gravitational mechanisms leading to bright plumes. We also examined the outer halo of the giant elliptical M87. The color profile from the inner to outer halo of M87 (160 Kpc) is consistent with either a flat or optically blue gradient, where a blue gradient could be due to younger or lower metallicity stars at larger radii. The similarity of the age predicted by both the infrared and optical colors (> few Gyr) indicates that the optical measurements are not strongly affected by dust extinction.