Once in a blue stream: Detection of recent star formation in the NGC
  7241 stellar stream with MEGARA
  We study the striking case of a blue narrow stream with a possible globular
cluster-like progenitor around the Milky Way-size galaxy NGC 7241 and its
foreground dwarf companion. We present a follow-up spectroscopic study of this
stream based on data taken with the MEGARA instrument at the 10.4-m Gran
Telescopio Canarias using the integral field spectroscopy mode. Although our
data suggest that this compact object in the stream is actually a foreground
Milky Way halo star, we detect emission lines overlapping a less compact, bluer
and fainter blob of the stream that is clearly visible in both ultra-violet and
optical deep images. From its heliocentric systemic radial velocity derived
from the [OIII] 5007A lines (V_syst= 1548.58+/-1.80 km\s^-1) and new UV and
optical broad-band photometry, we conclude that this over-density could be the
actual core of the stream, with an absolute magnitude of Mg~ -10 and a g-r =
0.08+/- 0.11, consistent with a remnant of a low-mass dwarf satellite
undergoing a current episode of star formation. From the width of the stream,
we calculate that the progenitor mass is between 6.4 x 10^6 Mo -2.7 x 10^7 Mo,
which is typical of a dwarf galaxy. These estimates suggest that this is one of
the lowest mass streams detected so far beyond the Local Group. We find that
blue stellar streams containing star formation regions are commonly predicted
by high-resolution cosmological simulations of galaxies lighter than the Milky
Way. This scenario is consistent with the processes explaining the bursty star
formation history of some dwarf satellites, which are followed by a gas
depletion and a fast quenching once they enter within the virial radius of
their host galaxies. Thus, it is likely that the stream's progenitor is
suffering a star-formation burst comparable to those that have shaped the
star-formation history of several Local Group dwarfs in the last few Gigayears.
