29 research outputs found
Merger relics of cluster galaxies
Context. Sheen and collaborators recently found that a surprisingly large
portion (38%) of massive early-type galaxies in heavy clusters show strong
merger-related disturbed features. This contradicts the general understanding
that massive clusters are hostile environments for galaxy mergers. Considering
the significance of mergers in galaxy evolution, it is important to understand
this. Aims. We aim to present a theoretical foundation that explains galaxy
mergers in massive clusters. Methods. We used the N-body simulation technique
to perform a cosmological-volume simulation and derive dark-halo merger trees.
Then, we used the semi-analytic modeling technique to populate each halo with
galaxies. We ran hydrodynamic simulations of galaxy mergers to estimate the
lifetime of merger features for the imaging condition used by Sheen and
collaborators. We applied this merger feature lifetime to our semi-analytic
models. Finally, we counted the massive early-type galaxies in heavy model
clusters that would show strong merger features. Results. While there still are
substantial uncertainties, our preliminary results are remarkably close to the
observed fraction of galaxies with merger features. Key ingredients for the
success are twofold: firstly, the subhalo motion in dark haloes has been
accurately traced, and, second, the lifetime of merger features has been
properly estimated. As a result, merger features are expected to last very long
in cluster environments. Many massive early-type galaxies in heavy clusters
therefore show merger features not because they experience mergers in the
current clusters in situ, but because they still carry their merger features
from their previous halo environments. Conclusions. Investigating the merger
relics of cluster galaxies is potentially important, because it uniquely allows
us to backtrack the halo merger history.Comment: 4 pages, 3 figures, accepted for publication in A&A Research Not
Discovery of ram-pressure stripped gas around an elliptical galaxy in Abell 2670
Studies of cluster galaxies are increasingly finding galaxies with
spectacular one-sided tails of gas and young stars, suggestive of intense
ram-pressure stripping. These so-called "jellyfish" galaxies typically have
late-type morphology. In this paper, we present MUSE observations of an
elliptical galaxy in Abell 2670 with long tails of material visible in the
optical spectra, as well as blobs with tadpole-like morphology. The spectra in
the central part of the galaxy reveals a stellar component as well as ionized
gas. The stellar component does not have significant rotation, while the
ionized gas defines a clear star-forming gas disk. We argue, based on deep
optical images of the galaxy, that the gas was most likely acquired during a
past wet merger. It is possible that the star-forming blobs are also remnants
of the merger. In addition, the direction and kinematics of the one-sided
ionized tails, combined with the tadpole morphology of the star-forming blobs,
strongly suggests that the system is undergoing ram pressure from the
intracluster medium. In summary, this paper presents the discovery of a
post-merger elliptical galaxy undergoing ram pressure stripping.Comment: 6 pages, 5 figures, accepted for publication in ApJ
BUDHIES II:A phase-space view of H I gas stripping and star formation quenching in cluster galaxies
We investigate the effect of ram-pressure from the intracluster medium on the stripping of H I gas in galaxies in a massive, relaxed, X-ray bright, galaxy cluster at z = 0.2 from the Blind Ultra Deep H I Environmental Survey (BUDHIES). We use cosmological simulations, and velocity versus position phase-space diagrams to infer the orbital histories of the cluster galaxies. In particular, we embed a simple analytical description of ram-pressure stripping in the simulations to identify the regions in phase-space where galaxies are more likely to have been sufficiently stripped of their H I gas to fall below the detection limit of our survey. We find a striking agreement between the model predictions and the observed location of H I-detected and non-detected blue (late-type) galaxies in phase-space, strongly implying that ram-pressure plays a key role in the gas removal from galaxies, and that this can happen during their first infall into the cluster. However, we also find a significant number of gas-poor, red (early-type) galaxies in the infall region of the cluster that cannot easily be explained with our model of ram-pressure stripping alone. We discuss different possible additional mechanisms that could be at play, including the pre-processing of galaxies in their previous environment. Our results are strengthened by the distribution of galaxy colours (optical and UV) in phase-space, that suggests that after a (gas-rich) field galaxy falls into the cluster, it will lose its gas via ram-pressure stripping, and as it settles into the cluster, its star formation will decay until it is completely quenched. Finally, this work demonstrates the utility of phase-space diagrams to analyse the physical processes driving the evolution of cluster galaxies, in particular H I gas stripping
Post-merger Signatures of Red-sequence Galaxies in Rich Abell Clusters at
We have investigated the post-merger signatures of red-sequence galaxies in
rich Abell clusters at 0.1: A119, A2670, A3330 and A389. Deep
images in u', g', r' and medium-resolution galaxy spectra were taken using
MOSAIC 2 CCD and Hydra MOS mounted on a Blanco 4-m telescope at CTIO.
Post-merger features are identified by visual inspection based on asymmetric
disturbed features, faint structures, discontinuous halo structures, rings and
dust lanes. We found that ~ 25% of bright (M_r < -20) cluster red-sequence
galaxies show post-merger signatures in four clusters consistently. Most (~
71%) of the featured galaxies were found to be bulge-dominated, and for the
subsample of bulge-dominated red-sequence galaxies, the post-merger fraction
rises to ~ 38%. We also found that roughly 4% of bulge-dominated red-sequence
galaxies interact (on-going merger). A total of 42% (38% post-merger, 4%
on-going merger) of galaxies show merger-related features. Compared to a field
galaxy study with a similar limiting magnitude (van Dokkum 2005), our cluster
study presents a similar post-merger fraction but a markedly lower on-going
merger fraction. The merger fraction derived is surprisingly high for the high
density of our clusters, where the fast internal motions of galaxies are
thought to play a negative role in galaxy mergers. The fraction of post-merger
and on-going merger galaxies can be explained as follows. Most of the
post-merger galaxies may have carried over their merger features from their
previous halo environment, whereas interacting galaxies interact in the current
cluster in situ. According to our semi-analytic calculation, massive cluster
haloes may very well have experienced tens of halo mergers over the last 4-5
Gyr; post-merger features last that long, allowing these features to be
detected in our clusters today. (Abridged)Comment: 16 pages, 15 figures, 7 tables, accepted for publication in ApJ
A Spectro-photometric Search for Galaxy Clusters in SDSS
Recent large-scale galaxy spectroscopic surveys, such as the Sloan Digital
Sky Survey (SDSS), enable us to execute a systematic, relatively-unbiased
search for galaxy clusters. Such surveys make it possible to measure the 3-d
distribution of galaxies but are hampered by the incompleteness problem due to
fiber collisions. In this study we aim to develop a density measuring technique
that alleviates the problem and derives densities more accurately by adding
additional cluster member galaxies that follow optical color-magnitude
relations for the given redshift. The new density measured with both
spectroscopic and photometric data shows a good agreement with apparent
information on cluster images and is supported by follow-up observations. By
adopting this new method, a total of 924 galaxy clusters are found
from the SDSS DR5 database in the redshift range , of which 212 are
new. Local maximum-density galaxies successfully represent cluster centers. We
provide the cluster catalogue including a number of cluster parameters.Comment: Web-page address has been revised and minor corrections are don
Formation of S0s in extreme environments I: clues from kinematics and stellar populations
Despite numerous efforts, it is still unclear whether lenticular galaxies (S0s) evolve from spirals whose star formation was suppressed, or formed trough mergers or disk instabilities. In this paper we present a pilot study of 21 S0 galaxies in extreme environments (field and cluster), and compare their spatially-resolved kinematics and global stellar populations. Our aim is to identify whether there are different mechanisms that form S0s in different environments. Our results show that the kinematics of S0 galaxies in field and cluster are, indeed, different. Lenticulars in the cluster are more rotationally supported, suggesting that they are formed through processes that involve the rapid consumption or removal of gas (e.g. starvation, ram pressure stripping). In contrast, S0s in the field are more pressure supported, suggesting that minor mergers served mostly to shape their kinematic properties. These results are independent of total mass, luminosity, or disk-to-bulge ratio. On the other hand, the mass-weighted age, metallicity, and star formation time-scale of the galaxies correlate more with mass than with environment, in agreement with known relations from previous work such as the one between mass and metallicity. Overall, our results re-enforce the idea that there are multiple mechanisms that produce S0s, and that both mass and environment play key roles. A larger sample is highly desirable to confirm or refute the results and the interpretation of this pilot study