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 $L^{*}$ 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 $\sigma_0=271\pm28$ km s$^{-1}$. Metallicity gradients are throughout negative with comparatively flat slopes of $\nabla_{[\rm{Fe/H}]}=- 0.19\pm0.08$ while age gradients are found to be insignificant ($\nabla_{\rm{age}}=0.00\pm0.05$). 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 ($\nabla_{Z}=-0.5$) 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 führen 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 häufig sind und als die Vorläufer der hellsten elliptischen Galaxien im Zentrum massereicher Haufen gelten könnten. Durch ihre unspektakuläre Erscheinung im Optischen konnten bis heute jedoch nur wenige fossile Gruppen identifiziert werden, weswegen erst wenig über ihre Entstehung und Entwicklung bekannt ist. In dieser Arbeit wurden ergänzend zu früheren 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 durchgeführt, um die stellaren Populationen und kinematischen Eigenschaften der zentralen elliptischen Galaxien von 6 fossilen Kandidaten zu untersuchen. Alter und Metallizität 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 Metallizitätsgradienten, sowie die räumliche Variation der Absorptionslinienstärken in diesen Galaxien. Zusätzlich 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 Metallizität von Gruppenmitgliedern, sowie die Dynamik des Systems zu analysieren. Die beobachteten zentralen elliptischen Galaxien am WHT zeigen unter anderem negative Metallizitätsgradienten. Des Weiteren liegen diese Objekte auf der Faber-Jackson Relation und auf der Fundamentalebene von Galaxien frühen morphologischen Typs, was ein Entstehungsszenario ähnlich 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 ältesten 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 außerdem ausgeprägte symmetrische Schalen entlang der Hauptachse der Galaxie, ein Indiz für einen kürzlichen 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 = −18, 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 $u, g$ and $i$-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 $0.1\!<\!r_e\!<\!2.8$ kpc and average effective surface brightness values $22.0\!<\!\mu_i\!<\!28.0$ mag arcsec$^{-2}$. 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 $M_i\!=\!-8.0$\,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 $>\!75\%$ at luminosities brighter than $M_i\!\simeq\!-15.0$ mag to $0\%$ at luminosities fainter than $M_i\!\simeq\!-10.0$ mag. The two-point correlation function analysis of the NGFS dwarf sample shows an excess on length scales below $\sim\!100$ 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=

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

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 ($C=B<A$), we find that the data favor the oblate-triaxial models ($C<B\lesssim A$) 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