36 research outputs found
Age and metallicity gradients in fossil ellipticals
Fossil galaxy groups are speculated to be old and highly evolved systems of
galaxies that formed early in the universe and had enough time to deplete their
galaxies through successive mergers of member galaxies, building up one
massive central elliptical, but retaining the group X-ray halo. Considering
that fossils are the remnants of mergers in ordinary groups, the merger history
of the progenitor group is expected to be imprinted in the fossil central
galaxy (FCG). We present for the first time radial gradients of single-stellar
population (SSP) ages and metallicites in a sample of FCGs to constrain their
formation scenario. Our sample comprises some of the most massive galaxies in
the universe exhibiting an average central velocity dispersion of
km s. Metallicity gradients are throughout negative
with comparatively flat slopes of while
age gradients are found to be insignificant ().
All FCGs lie on the fundamental plane, suggesting that they are virialised
systems. We find that gradient strengths and central metallicities are similar
to those found in cluster ellipticals of similar mass. The comparatively flat
metallicity gradients with respect to those predicted by monolithic collapse
() suggest that fossils are indeed the result of multiple
major mergers. Hence we conclude that fossils are not 'failed groups' that
formed with a top heavy luminosity function. The low scatter of gradient slopes
suggests a similar merging history for all galaxies in our sample.Comment: 14 pages, 12 Figures, accepted for publication in A&
On the nature of fossil galaxy groups
Galaxienwechselwirkungen und die Verschmelzung von Galaxien treten vorwiegend in losen Galaxiengruppen auf. Numerische Simulationen sowie die Beobachtungen im optischen und im Röntgenbereich der letzten Jahre haben gezeigt, dass dieser galaktische Kannibalismus zur Entstehung sogenannter fossiler Gruppen fĂŒhren kann. Die dynamische Reibung als wesentlicher
physikalischer Prozess fördert die Verschmelzung einzelner Galaxien in gewöhnlichen losen Galaxiengruppen, wobei eine zentrale, massereiche elliptische Galaxie das Endprodukt darstellt.
Die Untersuchungen der letzten Jahre legen nahe, dass fossile Galaxiengruppen sehr hĂ€ufig sind und als die VorlĂ€ufer der hellsten elliptischen Galaxien im Zentrum massereicher Haufen gelten könnten. Durch ihre unspektakulĂ€re Erscheinung im Optischen konnten bis heute jedoch nur wenige fossile Gruppen identifiziert werden, weswegen erst wenig ĂŒber ihre Entstehung und Entwicklung bekannt ist.
In dieser Arbeit wurden ergĂ€nzend zu frĂŒheren Studien, die SDSS und RASS Durchmusterungen systematisch durchsucht, um neue fossile Galaxiengruppen, basierend auf deren optischen
Auswahlkriterien, zu finden. Diese Suche resultierte in 34 neuen, zuvor noch nicht klassifizierten Kandidaten. Es wurden Beobachtungen mit dem Spektrographen ISIS am 4.2 m
William Herschel Telescope (WHT) auf La Palma durchgefĂŒhrt, um die stellaren Populationen und kinematischen Eigenschaften der zentralen elliptischen Galaxien von 6 fossilen Kandidaten
zu untersuchen. Alter und MetallizitÀt wurden mittels der Analyse von Lick/IDS indices und dem Fitten von single stellar population (SSP) Modellen an die beobachteten Galaxienspektren
bestimmt. Dies lieferte Alters- und MetallizitÀtsgradienten, sowie die rÀumliche Variation der AbsorptionslinienstÀrken in diesen Galaxien. ZusÀtzlich konnte mit dem multiobject Spektrographen VIMOS am ESO VLT die Galaxienpopulation des fossilen Kandidaten RX J1548.9+0851 untersucht werden, um Gruppenmitgliedschaften, die optische Leuchtkraftfunktion,
Alter und MetallizitÀt von Gruppenmitgliedern, sowie die Dynamik des Systems zu analysieren.
Die beobachteten zentralen elliptischen Galaxien am WHT zeigen unter anderem negative MetallizitĂ€tsgradienten. Des Weiteren liegen diese Objekte auf der Faber-Jackson Relation und auf der Fundamentalebene von Galaxien frĂŒhen morphologischen Typs, was ein Entstehungsszenario Ă€hnlich dem von gewöhnlichen elliptischen Galaxien nahelegt. Gruppenmitglieder des fossilen Kandidaten RX J1548.9+0851 zeigen eine klare Korrelation zwischen SSP Alter, Radialgeschwindigkeiten, sowie der Lage der Galaxien innerhalb der Gruppe. Die Ă€ltesten Mitglieder der Gruppe sind entlang einer filamentartigen Struktur, welche mit der Orientierung der Hauptachse der zentralen elliptischen Galaxie korreliert, verteilt. Da diese Objekte auch den Kern der beobachteten Radialgeschwindigkeitsverteilung bilden, ist es naheligend, dass diese Galaxien zuerst entstanden sind und ein virialisiertes subsample darstellen, welches die Orientierung des Dunkle-Materie-Halos der Gruppe skizziert. Die zentrale elliptische Galaxie zeigt auĂerdem ausgeprĂ€gte symmetrische Schalen entlang der Hauptachse der Galaxie, ein Indiz fĂŒr einen kĂŒrzlichen minor merger.Poor groups of galaxies are known to be the sites where galaxy-galaxy interactions and galaxy mergers dominate. Numerical simulations as well as optical and X-ray observations over the
last few years have shown that this galactic cannibalism can lead to what is called a fossil group. Dynamical friction as the driving process leads to the coalescence of individual galaxies
in ordinary poor groups, leaving behind nothing more than a central, massive elliptical galaxy, supposed to contain the merger history of the whole group. Recent studies suggest that fossil
groups are very abundant and could be the progenitors of the brightest cluster galaxies (BCGs) in the centers of rich galaxy clusters. However, because of their unremarkable appearance in
the optical, only a few fossils have been identified up to now, leaving their process of formation and evolution unclear.
Complementary to previous research, the SDSS and RASS surveys have been cross-correlated to identify new fossil structures based on the optical selection criteria for fossils as defined in the literature. This query resulted in 34 newly etected and previously unidentified fossil group candidates. Observations with ISIS at the 4.2m William Herschel Telescope on La Palma have been carried out to study the stellar populations and kinematics of the central ellipticals of six fossil candidates. Ages and metallicities have been determined by analysing Lick indices and fitting single stellar population (SSP) models to the observed galaxy spectra resulting in age, metallicity, and line strength gradients for these galaxies. Complementary, multi-object spectroscopy with ESO VLT VIMOS has been carried out for the fossil candidate RX J1548.9+0851 to determine group memberships, study the optical luminosity function down to MR = â18, investigate SSP ages and metallicities of member galaxies, and study the luminosity-weighted dynamics of the system to shed new light on the formation and evolution of fossils.
Amongst others, results have revealed negative metallicity and abundance gradients for all fossil central ellipticals observed at the WHT. The galaxies fall on the Faber-Jackson relation and
fundamental plane of early-type galaxies suggesting a similar formation scenario compared to ordinary ellipticals. Group members of the fossil candidate RX J1548.9+0851 show a clear
correlation between SSP ages, radial velocities, and the galaxy location within the group. The sample of old member galaxies is distributed along an elongated structure within the system,
correlated with the orientation of the major axis of the central elliptical. As they lie at the center of the radial velocity distribution, this suggests that these galaxies formed first, building up a virialized subsample tracing the dark matter halo of the aggregate. The morphology of the central elliptical reveals pronounced symmetric shells along the galaxy major axis indicative of a recent minor merger
Unveiling a Rich System of Faint Dwarf Galaxies in the Next Generation Fornax Survey
We report the discovery of 158 previously undetected dwarf galaxies in the
Fornax cluster central regions using a deep coadded and -band image
obtained with the DECam wide-field camera mounted on the 4-meter Blanco
telescope at the Cerro Tololo Interamerican Observatory as part of the {\it
Next Generation Fornax Survey} (NGFS). The new dwarf galaxies have
quasi-exponential light profiles, effective radii kpc and
average effective surface brightness values mag
arcsec. We confirm the existence of ultra-diffuse galaxies (UDGs) in the
Fornax core regions that resemble counterparts recently discovered in the Virgo
and Coma galaxy clusters.~We also find extremely low surface brightness NGFS
dwarfs, which are several magnitudes fainter than the classical UDGs. The
faintest dwarf candidate in our NGFS sample has an absolute magnitude of
\,mag. The nucleation fraction of the NGFS dwarf galaxy sample
appears to decrease as a function of their total luminosity, reaching from a
nucleation fraction of at luminosities brighter than
mag to at luminosities fainter than
mag. The two-point correlation function analysis of the
NGFS dwarf sample shows an excess on length scales below kpc,
pointing to the clustering of dwarf galaxies in the Fornax cluster core.Comment: 6 pages, 3 figures. Accepted for publication in The Astrophysical
Journal Letters. Download the high-resolution version of the paper from the
following link: https://www.dropbox.com/s/xb9vz8s29wlzjgf/ms.pdf?dl=
Intrinsic Morphology of Ultra-diffuse Galaxies
With the published data of apparent axis ratios for 1109 ultra-diffuse
galaxies (UDGs) located in 17 low-redshift (z~ 0.020 - 0.063) galaxy clusters
and 84 UDGs in 2 intermediate-redshift (z~ 0.308 - 0.348) clusters, we take
advantage of a Markov Chain Monte Carlo approach and assume a ubiquitous
triaxial model to investigate the intrinsic morphologies of UDGs. In contrast
to the conclusion of Burkert (2017), i.e., the underlying shapes of UDGs are
purely prolate (), we find that the data favor the oblate-triaxial
models () over the nearly prolate ones. We also find that the
intrinsic morphologies of UDGs are relevant to their stellar
masses/luminosities, environments, and redshifts. First, for the low-redshift
UDGs in the same environment, the more-luminous ones are always thicker than
the less-luminous counterparts, possibly due to the more voilent internal
supernovae feedback or external tidal interactions for the progenitors of the
more-luminous UDGs. The UDG thickness dependence on luminosity is distinct from
that of the typical quiescent dwarf ellipticals (dEs) and dwarf spheroidals
(dSphs) in the local clusters and groups, but resembles that of massive
galaxies; in this sense, UDGs may not be simply treated as an extension of the
dE/dSph class with similar evolutionary histories. Second, for the low-redshift
UDGs within the same luminosity range, the ones with smaller cluster-centric
distances are more puffed-up, probably attributed to tidal interactions.
Finally, the intermediate-redshift cluster UDGs are more flattened, which
plausibly suggests a `disky' origin for high-redshift, initial UDGs.Comment: Accepted for publication in ApJ; new versio
The Next Generation Virgo Cluster Survey - Infrared (NGVS-IR): I. A new Near-UV/Optical/Near-IR Globular Cluster selection tool
The NGVS-IR project (Next Generation Virgo Survey - Infrared) is a contiguous
near-infrared imaging survey of the Virgo cluster of galaxies. It complements
the optical wide-field survey of Virgo (NGVS). The current state of NGVS-IR
consists of Ks-band imaging of 4 deg^2 centered on M87, and J and Ks-band
imaging of 16 deg^2 covering the region between M49 and M87. In this paper, we
present the observations of the central 4 deg^2 centered on Virgo's core
region. The data were acquired with WIRCam on the Canada-France-Hawaii
Telescope and the total integration time was 41 hours distributed in 34
contiguous tiles. A survey-specific strategy was designed to account for
extended galaxies while still measuring accurate sky brightness within the
survey area. The average 5\sigma limiting magnitude is Ks=24.4 AB mag and the
50% completeness limit is Ks=23.75 AB mag for point source detections, when
using only images with better than 0.7" seeing (median seeing 0.54"). Star
clusters are marginally resolved in these image stacks, and Virgo galaxies with
\mu_Ks=24.4 AB mag arcsec^-2 are detected. Combining the Ks data with optical
and ultraviolet data, we build the uiK color-color diagram which allows a very
clean color-based selection of globular clusters in Virgo. This diagnostic plot
will provide reliable globular cluster candidates for spectroscopic follow-up
campaigns needed to continue the exploration of Virgo's photometric and
kinematic sub-structures, and will help the design of future searches for
globular clusters in extragalactic systems. Equipped with this powerful new
tool, future NGVS-IR investigations based on the uiK diagram will address the
mapping and analysis of extended structures and compact stellar systems in and
around Virgo galaxies.Comment: 23 pages, 18 figures. Accepted for publication in ApJ