A Galactic Molecular Cloud Clump Catalog from Hi-GAL Data: Method and Initial Results Comparison to BGPS

Abstract

As the precursors to stellar clusters, it is imperative that we understand the distribution and physical properties of dense molecular gas clouds and clumps. Such a study has been done with the ground-based Bolocam Galactic Plane Survey (BGPS). Now the Herschel infrared GALactic plane survey (Hi-GAL) allows us to do the same with higher quality data and complete coverage of the Galactic plane. We have made a pilot study comparing dense molecular gas clumps identified in the Hi-GAL and BGPS surveys, using six $2^\circ \times 2^\circ$ regions centered at Galactic longitudes of $\ell = 11^\circ$, 30$^\circ$, 41$^\circ$, 50$^\circ$, 202$^\circ$, and $217^\circ$. We adopted the BGPS methodology for identifying clumps and estimating distances, leading to 6198 clumps being identified in our substudy, with 995 of those having well-constrained distances. These objects were evenly distributed with Galactic longitude, a consequence of Hi-GAL being source confusion limited. These clumps range in mass from 10$^{-2}$ M$_\odot$ to 10$^{5}$ M$_\odot$, and have heliocentric distances of up to 16 kpc. When clumps found in both surveys are compared, we see that distances agree within 1 kpc and ratios of masses are of order unity. This serves as an external validation for BGPS and instills confidence as we move forward to cataloging the clumps from the entirety of Hi-GAL. In addition to the sources that were in common with BGPS, Hi-GAL found many additional sources, primarily due to the lack of atmospheric noise. We expect Hi-GAL to yield $2\times10^5$ clumps, with 20% having well-constrained distances, an order of magnitude above what was found in BGPS.