The UV 2175{\AA} Attenuation Bump and its Correlation with PAH Emission
  at z~2
  The UV bump is a broad absorption feature centered at 2175{\AA} that is seen
in the attenuation/extinction curve of some galaxies, but its origin is not
well known. Here, we use a sample of 86 star-forming galaxies at z=1.7-2.7 with
deep rest-frame UV spectroscopy from the MUSE HUDF Survey to study the
connection between the strength of the observed UV 2175{\AA} bump and the
Spitzer/MIPS 24 micron photometry, which at the redshift range of our sample
probes mid-IR polycyclic aromatic hydrocarbon (PAH) emission at ~6-8 micron.
The sample has robust spectroscopic redshifts and consists of typical
main-sequence galaxies with a wide range in stellar mass (log(Mstar/Msun) ~
8.5-10.7) and star formation rates (SFRs; SFR ~ 1-100 Msun/yr). Galaxies with
MIPS detections have strong UV bumps, except for those with light-weighted ages
younger than ~100-200 Myr. We find that the UV bump amplitude does not change
with SFR at fixed stellar mass but increases with mass at fixed SFR. The UV
bump amplitude and the PAH strength (defined as mid-IR emission normalized by
SFR) are highly correlated and both also correlate strongly with stellar mass.
We interpret these correlations as the result of the mass-metallicity
relationship, such that at low metallicities PAH emission is weak due to a
lower abundance of PAH molecules. The weak or complete absence of the 2175{\AA}
bump feature on top of the underlying smooth attenuation curve at low
mass/metallicities is then expected if the PAH carriers are the main source of
the additional UV absorption.
