• Scientific Justification
Research in the Local Group has seen a true revolution in the past 10 years, largely
driven by wide-field ground based surveys and HST’s ability to study individual
systems in great detail. HST’s contributions to this field include accurate proper
motion (PM) measurements of dwarf satellites (e.g., Piatek 2005, 2006, 2007, etc).
One of HST’s most remarkable findings is that the Large and Small Magellanic
Clouds (LMC and SMC, respectively) are moving much faster (by ~80 km/s) than
previously believed (Kallivayalil et al 2006a,b; hereafter K1 and K2). These high
speeds imply that the Magellanic Clouds (MCs) may be on their first infall towards
the Milky Way (MW) (Besla et al 2007) and dramatically alters the canonical
paradigm in which the MCs are considered long term companions of the MW,
traveling in a quasi-periodic orbit that slowly decays over time (Gardiner & Noguchi
1996). Although the K1 and K2 measurements have been verified by independent
groups (e.g. Piatek et al. 2008; Vieira et al. 2010), they remain a controversial result
(e.g. Bekki 2011 and references therein). HST is the most accurate astrometric
telescope created to date, making it a logical instrument for high precision PM
measurements of a multitude of astronomical objects; however, the controversy
surrounding the measurements for the MCs questions the validity of this approach.
This consequently raises concerns for more ambitious projects, such as determining
the PM of ultra-faint dwarfs (P.I. Piatek). Verifying the HST PM measurements of
the MCs is thus critical to solidifying HST’s ability to measure PMs with high
precision. Ultimately this means verifying that the properties of the Magellanic
system (morphology, kinematics, star formation history) can be explained on a highspeed orbit wherein the MCs have not made multiple passages about the MW.
Besla et al 2010 (hereafter B10) have shown that the gas morphology of the
Magellanic system and the existence of the Magellanic Stream, a stream of HI gas
that trails behind the MCs over ~150 degrees across the sky, can be explained without
relying on a previous pericentric passage about the MW (see Figure 1).
Figure 1: Hammer-Aitoff
projection of the B10
simulated Magellanic
Stream in Galactic
coordinates. The gas
distribution of the
simulated Magellanic
system is plotted over a
MW panorama (courtesy of
Axel Mellinger). The
Milky Way’s disk plane is
located at b=0. The past
orbit of the LMC(SMC) is
indicated by the thick
solid(dashed) white line.
The simulated stream spans
In this model the Magellanic Stream and the bridge of gas that150º
connects
thesky,
LMC and
across the
SMC are best described as a classical Toomre & Toomre (1972)
tidalthebridge
and tail.
through
south galactic
pole, as observed.
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These features are created by the action of LMC tides on the SMC as the SMC travels
in an eccentric orbit about the LMC. The simulated spatial and kinematics properties
of the resulting gaseous tidal features match those observed for the Magellanic
Stream. However, the model presented in B10 is not unique – the solution for the
orbit of the SMC about the LMC is degenerate. This degeneracy exists partly because
of the large error bars on the current SMC PMs (K2). The error space of the PM
measurements will be reduced by upcoming analyses of 3rd and 4th epoch PM data
obtained by P.I. Kallivayalil (e.g. Kallivayalil et al. 2009). However, it is likely that
there will still be multiple solutions owing to uncertainties in the LMC:SMC mass
ratio and the eccentricity of the SMC’s orbit about LMC. We must therefore turn to
other methods to break these degeneracies.
We propose to constrain the orbit and interaction history of the MCs by studying in
detail the simulated star formation histories (SFHs) and kinematics of the stellar
and gaseous components of the LMC and SMC from a variety of degenerate orbital
solutions that reproduce the observed properties of the Magellanic Stream. These
simulated properties will be compared with archival data of the MCs to narrow
down the parameter space and test whether the HST PMs can yield solutions
consistent with the observed kinematic properties and SFHs of the MCs.
Owing to their proximity, the LMC and SMC have been observed with unprecedented
detail in multiple wavebands and the kinematic properties of their stellar and gaseous
components have been mapped across their extent (e.g. van der Marel et al 2001,
2002, Kim et al 1998, 2003, Olsen & Massey 2007, Stanimirovic et al. 2004, Harris
& Zaritsky et al. 2006, Staveley-Smith et al. 2003). The MCs have also been resolved
into individual stars, allowing color magnitude diagrams to be created to trace their
SFHs over time (e.g. Gallagher et al. 1996, Smecker-Hane et al. 2002, Noel et al.
2009, Harris & Zaritsky 2009, Sabbi et al. 2009). The SFHs of the MCs and their
kinematic properties are all intimately linked with the orbital and interaction
histories of the MCs: close passages between the MCs may result in bursts of star
formation and strong tidal perturbations may induce distinct kinematic signatures in
the stellar and gaseous components.
The kinematic properties of the gas and stellar components of the SMC are poorly
understood theoretically. There is a strong disconnect between its gaseous and stellar
components: the gas displays a pronounced velocity gradient across the SMC’s extent
(+/- 60 km/s; Stanimirovic et al. 2004) whereas no pronounced velocity gradient has
been observed in the stellar component (Harris & Zaritsky 2006). This is likely
related to the fact that the older stellar distribution is ellipsoidal, whereas the newly
formed stars and gas have a much more irregular distribution. The LMC also displays
unusual kinematics: the kinematic centers of its gaseous and stellar components are
not spatially coincident (van der Marel et al. 2002). The reason for these kinematic
anomalies in both galaxies have not been explained by any theoretical model to date
and will be a focus of this proposed study. It is possible that repeated encounters with
between the LMC and SMC may explain these kinematic offsets.
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The star formation histories of the MCs are not observed to be quiescent – the LMC
and SMC both show evidence for bursts of star formation at characteristic epochs in
time (Da Costa 1997, Smecker-Hane 2002, Harris & Zaritsky 2009); however, the
trigger for these bursts is unknown. Zaritsky & Harris (2004) claim that some peaks
of star formation in both galaxies are coincident in time, suggesting that these bursts
may be triggered in the MCs by tidal encounters with the MW. This scenario is
incompatible with the new PMs, which do not allow for previous passages about the
MW on such short timescales (even in more massive MW models). Instead, in the
B10 model, such bursts may be related to close passages between the MCs
themselves.
The MCs are the closest and most well studied pair of interacting dwarf galaxies. As
such, a detailed analysis of their kinematic evolution and star formation histories has
broader applicability to HST Cosmic Origins science than just to the verification of
the PMs and interpretation of archival data on the MCs. Dwarf galaxies are the most
numerous types of galaxies and, in the canonical Λ Cold Dark Matter (ΛCDM)
paradigm, they are the building blocks of larger galaxies. Owing to the self-similar
nature of cold dark matter structure, low-mass galaxies are also expected to undergo a
series of mergers over their lifetimes. Indeed, groups of small galaxies are observed
in isolation (e.g. Tully et al. 2006). However, the role of interactions between dwarf
galaxies – e.g. tidal stripping, torquing and triggered star formation – to their
morphological evolution has been poorly studied both theoretically and
observationally. The Magellanic system provides evidence that dwarf galaxies do
interact with one another and can be accreted as interacting systems by larger hosts.
By conducting an in depth study of the interactions between the MCs over time, we
will be able to draw conclusions about the role of dwarf-dwarf interactions on the
SFHs and kinematic evolution of dwarf galaxies more generally.
In particular, multi-episodic starbursts (repeating short periods of unsustainably high
star formation rates that exceed the average past rate by a factor of 2-3) are observed
in dwarf galaxies in our local volume (e.g. Heckman et al. 1998, Da Costa 1997).
However, it is unclear whether starbursts in dwarfs are triggered by interactions or
whether they represent a normal mode of star formation operating in low-metallicity,
low-mass systems (Sargent & Searle 1970, McQuinn et al. 2010). Moreover, the
properties of the burst (gas consumption, efficiency and duration) are also uncertain
in these environments (Marlowe et al. 1999). The interaction history of the MCs is a
well-defined orbital problem, allowing an accurate study of the efficiency of tidally
triggered star formation in dwarf galaxy pairs that can be directly compared to
archival HST data for SFHs of the MCs (e.g. Smecker-Hane et al. 2002) and data for
starbursting dwarfs in the local volume (e.g. Lee et al. 2009).
Furthermore, the morphological transition of the SMC from a well defined disk
galaxy to an irregular galaxy after repeated interactions with the LMC can also be
compared with observational studies of the morphological evolution of dwarf galaxies
in various environments, such as nearby galaxy groups (Cote et al. 2010) and clusters
(e.g. ACS Virgo and Coma cluster surveys; Cote et al. 2004, Carter et al. 2002).
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