Offset of M54 from the Sagittarius Dwarf Spheroidal Galaxy
arxiv(2024)
摘要
We present results from simultaneous modeling of 2D (projected along the line
of sight) position, proper motion and line-of-sight velocity for Gaia-
and APOGEE-observed stars near the centre of the Sagittarius (Sgr) dwarf
spheroidal galaxy. We use a mixture model that allows for independent
sub-populations contributed by the Sgr galaxy, its nuclear star cluster M54,
and the Milky Way foreground. We find an offset of 0.295± 0.029 degrees
between the inferred centroids of Sgr and M54, corresponding to a (projected)
physical separation of 0.135± 0.013 kpc. The detected offset might
plausibly be driven by unmodelled asymmetry in Sgr's stellar configuration;
however, standard criteria for model selection favour our symmetric model over
an alternative that allows for bilateral asymmetry. We infer an offset between
the proper motion centres of Sgr and M54 of
[Δμ_αcosδ,Δμ_δ]=[4.9, -19.7] ± [6.8, 6.2]
μas yr^-1, with magnitude similar to the covariance expected due to
spatially-correlated systematic error. We infer an offset of 4.1± 1.2 km
s^-1 in line-of-sight velocity. Using inferred values for the systemic
positions and motions of Sgr and M54 as initial conditions, we calculate the
recent orbital history of a simplified Sgr/M54 system, which we demonstrate to
be sensitive to any line-of-sight distance offset between M54 and Sgr, and to
the distribution of dark matter within Sgr.
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