Kavli Affiliate: Eric Peng
| First 5 Authors: Jeremy Fensch, Pierre-Alain Duc, Sungsoon Lim, Éric Emsellem, Michal Bílek
| Summary:
Stellar shells around galaxies could provide precious insights into their
assembly history. However, their formation mechanism remains poorly empirically
constrained, in particular the type of galaxy collisions at their origin. We
present MUSE@VLT data of the most prominent outer shell of NGC 474, to
constrain its formation history. The stellar shell spectrum is clearly
detected, with a signal-to-noise ratio of around 65 pix$^{-1}$. We use a full
spectral fitting method to determine the line-of-sight velocity and the age and
metallicity of the shell and associated point-like sources within the MUSE
field of view. We detect six GC candidates and eight PN candidates which are
all kinematically associated to the stellar shell. We show that the shell has
an intermediate metallicity, [M/H] = $-0.83^{+0.12}_{-0.12}$ and a possible
$alpha$-enrichment, [$alpha$/Fe] ~ 0.3. Assuming the material of the shell
comes from a lower mass companion, and that the latter had no initial
metallicity gradient, such a stellar metallicity would constrain the mass of
the progenitor to be around 7.4 x 10^8 M$_odot$, implying a merger mass ratio
of about 1:100. However our census of PNs and earlier photometry of the shell
would suggest a much higher ratio, around 1:20. Given the uncertainties, this
difference is significant only at the ~1sigma level. We discuss the
characteristics of the progenitor, in particular whether the progenitor could
also be composed of stars from the low metallicity outskirts from a more
massive galaxy. Ultimately, the presented data does not allow us to put a firm
constraint on the progenitor mass. We show that at least two globular cluster
candidates possibly associated with the shell are quite young, with ages below
1.5~Gyr. We also note the presence of a young (~1Gyr) stellar population in the
center of NGC 474. The two may have resulted from the same event.
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