A Spitzer search for cold dust within globular clusters

TL;DR

This study used Spitzer observations to search for dust within globular clusters, finding minimal evidence of intracluster dust and suggesting efficient dust removal or destruction mechanisms.

Contribution

First direct infrared search for intracluster dust in globular clusters using Spitzer, setting upper limits and discussing dust removal processes.

Findings

Most clusters lack significant 70 micron emission.

Dust mass upper limits are below expected production levels.

Background galaxies account for some point sources.

Abstract

Globular cluster stars evolving off the main sequence are known to lose mass, and it is expected that some of the lost material should remain within the cluster as an intracluster medium (ICM). Most attempts to detect such an ICM have been unsuccessful. The Multiband Imaging Photometer for Spitzer on the Spitzer Space Telescope was used to observe eight Galactic globular clusters in an attempt to detect the thermal emission from ICM dust. Most clusters do not have significant detections at 70 microns; one cluster, NGC 6341, has tentative evidence for the presence of dust, but 90 micron observations do not confirm the detection. Individual 70 micron point sources which appear in several of the cluster images are likely to be background galaxies. The inferred dust mass and upper limits are < 4e-4 solar masses, well below expectations for cluster dust production from mass loss in red and asymptotic giant branch stars. This implies that either globular cluster dust production is less efficient, or that ICM removal or dust destruction is more efficient, than previously believed. We explore several possibilities for ICM removal and conclude that present data do not yet permit us to distinguish between them.