Galaxy Clusters
Clusters embody an environment that makes the
evolutionary paths of galaxies a complex mixture of internal and external
processes.
One of the projects on clusters I am deeply involved in is STAGES
(Gray et al. 2009), a
study of the A901/902 cluster system at z~0.17, and
I am leading several spectroscopic follow-ups with VLT/VIMOS.
A class of galaxies we are particularly interested in are "red spirals" -
disk galaxies with a similar
amount of dust as normal spirals, but a much lower specific star formation
rate. Red spirals show a very regular stellar morphology, but
often disturbed gas kinematics, much more frequently than normal, blue
spirals
(Bösch, Böhm, et al. 2013a).
This hints towards ram-pressure, an interaction between the hot
intracluster medium (ICM) and the interstellar medium, which in the extreme
case can remove the entire gas reservoir of a galaxy (see figure).
Fig.1:
Morphological asymmetry Amorph (derived from HST images) versus gas kinematical asymmetry Akin (determined from ionized gas emission lines). Red spirals frequently show regular morphologies (low Amorph) coinciding with disturbed gas kinematics (high Akin). This indicates a stronger impact of ram-pressure in red spirals than in blue, normal ones.
Our analysis of the luminosity-rotation velocity (Tully-Fisher) relation
yielded further evidence that red spirals are in the process of being quenched
(Bösch, Böhm, et al. 2013b).
Red spirals are probably an intermediate stage in the
transformation of field spirals into cluster lenticulars, but further
observations are required to complete the puzzle.
Disk galaxies
without any
detectable star formation are preferentially found at small cluster-centric
radii, but can occur at larger distances from the central regions of
merging
clusters, in which ICM shock fronts can accelerate the quenching process
(Pranger, Böhm, et al. 2013).
Merging clusters are special also in other respects: in the core region
of a post-merger, we observed an unusually large fraction of galaxies with
disturbed morphologies
(Pranger, Böhm, et al. 2014).
As confirmed by numerical simulations, relative galaxy velocities can be
reduced during an ongoing cluster merger via dynamical friction, resulting
in frequent interactions between the cluster members. This explains
the large fraction of morphologically disturbed galaxies
(ibid.).
I am also a team member of the OMEGA survey lead by A. Aragon-Salamanca
(Nottingham). Its backbone are 90 hours of spatially resolved Hα
imaging of the A901/902 cluster with the Gran Telescopio Canarias, reaching a
star formation rate (SFR) detection limit of 0.1 M⊙/yr
for hundreds of cluster members.
This high sensitivity allowed to show that the specific SFR of cluster disks
extends to lower values than what is found in the field
(Rodriguez del Pino et al. 2017).
A phase-space analysis revealed that red spirals occur as frequently in
virialized cluster regions as in non-virialized ones
(Weinzirl et al. 2017).
This indicates a long time scale for the morphological transformation
to be completed.