Kavli Affiliate: Huawei Zhang
| First 5 Authors: Hao Wu, Hao Wu, , ,
| Summary:
The proper motions (PMs) of M31 and M33 are key to understanding the Local
Group’s dynamical evolution. However, measurement discrepancies between Gaia
blue and red samples, regarding whether the transverse velocity is remarkable,
introduce significant ambiguity. In this work, we remeasure the systemic PMs of
M31 and M33 using massive supergiant stars from Gaia Data Release 3. Clean disk
tracers are selected via color-color diagrams, with foreground contaminants
removed through kinematic and astrometric cuts. We identify the discrepancy in
M31’s blue and red samples as arising from systematic differences between
Gaia’s 5-parameter (5p) and 6-parameter (6p) astrometric solutions. The 6p
solution, applied to sources lacking accurate color information, relies on a
pseudo-color approximation, leading to lower precision and larger
uncertainties. Two key limitations of the 6p solution are: 1) degraded
astrometric accuracy for very red sources (GBP – GRP > 2.6); 2) significant PM
zero-point offsets. In our sample, red sources are dominated by the 6p
solution, while blue sources include a substantial fraction of 5p sources; this
mismatch drives the observed discrepancy. By excluding extreme red sources and
calibrating PM zero-points separately for 5p and 6p sources using background
quasars, we reduce the discrepancy, bringing blue and red measurements into
agreement within 1 sigma. We ultimately report the most robust Gaia-based PMs
using high-quality 5p sources. For M31, we obtain (mu_alpha*,
mu_delta)_M31 = (45.9 +/- 8.1, -20.5 +/- 6.6) muas/yr, consistent
with, but more precise than, the HST result. For M33, we find (mu_alpha*,
mu_delta)_M33 = (45.3 +/- 9.7, 26.3 +/- 7.3) muas/yr, agreeing with
VLBA measurement within 1.5 sigma. These results support a first infall
scenario for M33.
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