arXiv:astro-ph/0104227v1 12 Apr 2001
The Line-of-Sight Depth of Populous Clusters in the Small
Magellanic Cloud
Hugh H. Crowl
1
and Ata Sarajedini
2,3
Astronomy Department, Wesleyan University, Middletown, CT 06459
hugh@astro.yale.edu; ata@astro.ufl.edu
Andr´es E. Piatti
Observatorio Astro´omico, Laprida 854, 5000, C´ordoba, Argentina
andres@mail.oac.uncor.edu
Doug Geisler
Grupo de Astronomia, Departmento de Fisica, Universidad de Concepci´on, Casilla 160-C,
Concepci´on, Chile
doug@kukita.cfm.udec.cl
Eduardo Bica
Departamento de Astronomia, Instituto de Fisica, UFRGS, CP 15051, 91501-970, Porto
Alegre, RS, Brazil
bica@if.ufrgs.br
1
Current Address: Astronomy Department, Yale University, P. O. Box 208101, New
Haven, CT 06520
2
Guest User, Canadian Astronomy Data Centre, which is operated by the Dominion
Astrophysical Observatory for the National Research Council of Canada’s Herzberg Institute
of Astrophysics.
3
Current address: University of Florida, Department of Astronomy, Gainesville, FL 32611
– 2 –
Juan J. Clari´a
Observatorio Astro´omico, Laprida 854, 5000, C´ordoba, Argentina
claria@mail.oac.uncor.edu
and
Jo˜ao F. C. Santos, Jr.
Departamento de F isica, ICEx, Universidade Federal de Minas Gerais, CP 702, 30123-9 70
Belo Horizonte, MG, Brazil
jsantos@fisica.ufmg.br
Received
; accepted
– 3 –
ABSTRACT
We present an analysis of age, metal abundance, and positional data on pop-
ulous clusters in the Small Magellanic Cloud (SMC) with the ultimate aim of
determining the line-of- sight (LOS) depth of the SMC using these clusters as
proxies. Our dataset contains 12 objects and is limited to clusters with the high-
est quality data for which the ages and abundances are best known and can be
placed on an internally consistent scale. We have analyzed the variation of the
clusters’ properties with position on the sky and with line-of-sight depth. Based
on this a na lysis, we draw the following conclusions. 1) The observational data
indicates that the eastern side of the SMC (facing the LMC) contains younger
and more metal-rich clusters as compared with t he western side. This is not a
strong correlation because our dataset of clusters is necessarily limited, but it
is suggestive and warrants further study. 2) D epending on how t he reddening
is computed to our clusters, we find a mean distance modulus that ranges from
(m − M)
0
= 18.71 ± 0.06 to (m − M)
0
= 18.82 ± 0.05. 3) The intrinsic ±1-σ
line-of-sight depth of the SMC populous clusters in our study is between ∼ 6 kpc
and ∼ 12 kpc depending primarily on whether we adopt the Burstein & Heiles
reddenings or those from Schlegel et al. 4) Viewing the SMC as a triaxial galaxy
with the Declination, Right Ascension, a nd LOS depth as the three axes, we find
axial ratios of approximately 1:2:4. Taken together, these conclusions largely
agree with those of previous investigators and serve to underscore the utility of
populous star clusters as probes of the structure of the Small Mag ellanic Cloud.
Subject headings: Clusters: Globular, SMC
– 4 –
1. Introduction
The effect o f interactions/mergers on the star formation histories of galaxies has
become a field of intense activity (e.g. Carlson et al. 1998; Whitmore et al. 1999; Elmegreen
et al. 2000). While many investigators have studied distant g alaxy systems as a means
of addressing this question, the Milky Way / Large Magellanic / Small Magellanic Cloud
system provides a nearby example that is especially profitable scientifically (e.g. Gardiner
1999). This is because the relative proximity of these galaxies allows us to perform detailed
age and abundance studies of individual field a nd cluster stars. G iven the possibility that
the Milky Way and other spiral galaxies formed via the accretion/merger of dwarf satellite
galaxies (Searle & Zinn 1978; Cˆot´e et al. 2000), understanding the interplay between
dynamical interactions and star formation is important in unlocking t he secrets of galaxy
formation.
One o f the first pieces of evidence for the occurrence of dynamical interactions in the
Milky Way/LMC/SMC system was the discovery of the Magellanic Stream by Wannier &
Wrixon (1 972). Welch et al. (1987), Westerlund (1990), Bica & Schmitt (1995) and Bica
et al. (1999) discuss evidence for interactions between the Clouds based on the spatial
distributions of stellar po pulation tracers, such as the star clusters, associations, and
emission nebulae. The recent work of Kunkel et al. (2000) reviews the for matio n of the
Magellanic Stream and the interactions between the Large and Small Magellanic Clouds
in detail. There is a fair amount of evidence, both from simulations (Murai & Fujimoto
1980) and observations (Mathewson et al. 198 8; Hatzidimitriou et al. 19 89a; Gardiner &
Hawkins 199 1) that, as a result of these interactions, the SMC may extend beyond its tidal
radius. Some authors (Mathewson 1984; Mathewson et al. 1986) have even suggested t hat
the SMC has split into two components: the Mini Magellanic Cloud (MMC) and the SMC
Remnant. In a contrasting view, Welch et al. ( 1987) utilize infrared observa t io ns of cepheids
– 5 –
to assert that the spatial extent of the SMC is well within its tidal radius. One of the most
important pieces of evidence in determining the validity of these claims is t he overall line of
sight (LOS) depth of the Small Magellanic Cloud (SMC).
The most extensive series of papers dealing with the LOS depth of the SMC are those
by Hatzidimitriou et al. (19 89a), Hatzidimitriou et al. (1989b), Gardiner & Hawkins (1991),
and Gardiner & Hatzidimitriou (1992 ) . These authors studied the depth of the SMC using
the magnitude spread of the horizontal branch/red clump (HB/RC) stars among t he SMC
field population. Their photographic survey, augmented by CCD observations of select
fields, was quite extensive, covering 4 8.5 square degrees centered on the SMC. The basic
premise of t heir investigation was that the mag nitude of the HB/RC stars is mainly affected
by distance and photometric errors but minimally influenced by the physical properties of
the stars themselves, such as metallicity and age. Based on this assumption, they conclude
that, not only is the SMC significantly dispersed in the LOS direction, but that the amount
of dispersion varies with position on the sky. For example, the northeastern region located
more than 2 kpc from the optical center suffers the greatest LOS dispersion with an average
LOS depth of 17 kpc and a maximum of 23 kpc. In contrast, the analogous region located
in the southwest displays a depth that is ∼10 kpc shallower on average. In addition, most
of the areas to the north and northwest of the optical center display a depth of between 4
and 9 kpc.
In the years since these landmark studies, there has been some contr oversy about the
age and metallicity sensitivity of the red clump absolute magnitude. The reader is referred
to the papers by Paczy´nski & Stanek (1998), Udalski (1998), Cole (1998), Sarajedini
(1999), and Girar di & Salaris (2000) for some perspective on this question. The degree
of sensitivity could have a significant effect on the derived LOS depth of the SMC based
on the magnitude of the red clump. One way to explore the extent of this effect is to use
