Detection of a Spatially Extended Stellar Population in M33: A Shallow Stellar Halo?
arxiv(2024)
摘要
We analyze the outer regions of M33, beyond 15 kpc in projected distance from
its center using Subaru/HSC multi-color imaging. We identify Red Giant Branch
(RGB) stars and Red Clump (RC) stars using the surface gravity sensitive
NB515 filter for the RGB sample, and a multi-color selection for both
samples. We construct the radial surface density profile of these RGB and RC
stars, and find that M33 has an extended stellar population with a shallow
power-law index of α > -3. This result represents a flatter profile than
the stellar halo which has been detected by the previous study focusing on the
central region, suggesting that M33 may have a double-structured halo
component, i.e. inner/outer halos or a very extended disk. Also, the slope of
this extended component is shallower than those typically found for halos in
large galaxies, implying intermediate-mass galaxies may have different
formation mechanisms (e.g., tidal interaction) from large spirals. We also
analyze the radial color profile of RC/RGB stars, and detect a radial gradient,
consistent with the presence of an old and/or metal-poor population in the
outer region of M33, thereby supporting our proposal that the stellar halo
extends beyond 15 kpc. Finally, we estimate that the surface brightness of this
extended component is μ_ V = 35.72 ± 0.08 mag arcsec^-2. If our
detected component is the stellar halo, this estimated value is consistent with
the detection limit of previous observations.
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