Extragalactic Globular Clusters

By: JR August 29, 2018 0

This image shows NGC 121, a globular cluster in the constellation of Tucana (The Toucan). Globular clusters are big balls of old stars that orbit the centres of their galaxies like satellites — the Milky Way, for example, has around 150. NGC 121 belongs to one of our neighbouring galaxies, the Small Magellanic Cloud (SMC). It was discovered in 1835 by English astronomer John Herschel, and in recent years it has been studied in detail by astronomers wishing to learn more about how stars form and evolve. Stars do not live forever — they develop differently depending on their original mass. In many clusters, all the stars seem to have formed at the same time, although in others we see distinct populations of stars that are different ages. By studying old stellar populations in globular clusters, astronomers can effectively use them as tracers for the stellar population of their host galaxies. With an object like NGC 121, which lies close to the Milky Way, Hubble is able to resolve individual stars and get a very detailed insight. NGC 121 is around 10 billion years old, making it the oldest cluster in its galaxy; all of the SMC's other globular clusters are 8 billion years old or younger. However, NGC 121 is still several billions of years younger than its counterparts in the Milky Way and in other nearby galaxies like the Large Magellanic Cloud. The reason for this age gap is not completely clear, but it could indicate that cluster formation was initially delayed for some reason in the SMC, or that NGC 121 is the sole survivor of an older group of star clusters. This image was taken using Hubble’s Advanced Camera for Surveys (ACS). A version of this image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Stefano Campani.

Look here for all you need to know to go above and beyond (our galaxy) to find some extraordinary extragalactic globular clusters!

The globular cluster NGC 121 shimmers in the southern constellation of Tucana (The Toucan).
*ESA / Hubble / NASA / Stefano Campani*

Off to hunt extragalactic globular clusters? Below you'll find an abundance of resources compiled by Contributing Editor Steve Gottlieb to help you plan and execute your observing sessions. We included an abbreviated data table with Gottlieb's article on extragalatic globular clusters that ran in the November 2018 issue of Sky & Telescope. Here you'll find a more exhaustive table; it includes a listing of all Local Group galaxies that contain globulars. The table is also available for download as an Excel file. For those of you who are feeling truly inspired, consider downloading Gottlieb's data on 75 brightest globulars in M31 as well.

Census of Local Group Globular Clusters
Galaxy	Type	# GC	Globular Cluster	Mag(v)	RA	Dec.	Note

Milky Way	SBbc	160	NGC 5139	3.7	13 26.8	-47 29	G

M31	Sb	>500	G1	13.8	00 32.8	+39 35	A
			PAndAS-53/54	15.5	01 18.0	+39 15
			Martin-GC1	15.5	00 50.7	+32 55

M33	Scd	~50	U49	16.3	01 33.8	+30 48	A

LMC	Irr	15	NGC 1835	10.2	05 05.1	-69 24	G
			NGC 1916	10.4	05 18.6	-69 24
			Reticulum	14.7	04 36.2	-58 51

M110 (NGC 205)	dE	11	G73	14.9	00 40.9	+41 41	A

NGC 147	dE	10	Hodge III	16.8	00 33.3	+48 27	A

NGC 185	dE	8	Hodge V	16.7	00 39.2	+48 23	A

NGC 6822	dIrr	8	SC 7	14.8	19 46.0	-14 33	L
			Hubble VII	16.3	19 44.9	-14 49
			SC 6	15.4	19 45.6	-14 41

Fornax Dwarf	dSph	5	NGC 1049	12.6	02 39.8	-34 15	G
			Fornax-2	14.1	02 38.7	-34 49
			Fornax-4	13.6	02 40.1	-34 32
			Fornax-5	13.6	02 42.4	-34 06

Sgr Dwarf	dSph	4	M54	7.7	18 55.1	-30 29	G

SMC	dIrr	1	NGC 121	11.2	00 26.8	-71 32	G

WLM	dIrr	1	WLM-1	16.1	00 01.9	-15 27	G


GCs in Nov 2018 S&T article are in bold
First row of each galaxy displays its brightest globular
Code for Note column
G = Milky Way satellite
A = M31 satellite
L = isolated member of the Local Group

Web Pages

Steve Gottlieb's personal website contains a wealth of observing data. Be sure and check it out! You'll find the following pages particular relevant to globular clusters: