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On the Age of Galactic Bulge CSPNe: Too Young and Complicated?

Galaxies 2020, 8(2), 51; https://doi.org/10.3390/galaxies8020051

We present preliminary results of our study of a small sample of planetary nebulae in the Galactic Bulge for which high-angular resolution Hubble Space Telescope imaging is available. From this and from archival spectroscopy, we were able to calculate temperatures and luminosities for their central stars. These were then correlated to up-to-date evolutionary tracks found in the literature to help us estimate stellar masses and therefore ages for the central stars. Our current analysis indicates that our sample appears to represent a somewhat mixed population of planetary nebulae central stars, while at least one of the nebulae might have been formed by a more massive progenitor (i.e.,

M_{ZAMS} \sim 4

_{⊙}

).
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On the Connection of Radio and γ-Ray Emission in Blazars

Galaxies 2019, 7(1), 3; https://doi.org/10.3390/galaxies7010003

Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the

γ

-ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the

γ

-ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework,

γ

-ray photons are naturally produced at small distances (e.g.,

10^{- 3}

pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g., 1 pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between

γ

-rays and radio during flares.
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X-ray and Radio Observations of the Radio Relic Galaxy Clusters 1RXS J0603.3+4214 and RXC J1053.7+5453

Galaxies 2019, 7(1), 2; https://doi.org/10.3390/galaxies7010002

We study two galaxy clusters with radio relics, 1RXS J0603.3+4214 and RXC J1053.7+5453, through X-ray and radio observations. Radio relics are diffuse non-thermal radio sources found in outskirts of galaxy clusters. Because of their shape and location, they are thought to be related to cluster merger shocks. The galaxy cluster 1RXS J0603.3+4214 has a well-known linear-shape “toothbrush” radio relic. We investigate the temperature structure across the relic to constrain the Mach number of the associated shock. The results are compared with radio spectral results, which suggest that a simple diffusive shock acceleration model does not hold for this relic. The RXC J1053.7+5453 harbors a standard arc-like relic. We also get the Mach number from the temperature profile. In addition, we found an edge-like structure in the X-ray image between the X-ray peak and relic. We investigate the density and temperature profiles across the edge and found that the structure is likely relevant to not a shock but a contact discontinuity.
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Thermal Sunyaev–Zel’dovich Effect in the IGM due to Primordial Magnetic Fields

Galaxies 2018, 6(4), 143; https://doi.org/10.3390/galaxies6040143

In the present universe, magnetic fields exist with various strengths and on various scales. One possible origin of these cosmic magnetic fields is the primordial magnetic fields (PMFs) generated in the early universe. PMFs are considered to contribute to matter density evolution via Lorentz force and the thermal history of intergalactic medium (IGM) gas due to ambipolar diffusion. Therefore, information about PMFs should be included in the temperature anisotropy of the Cosmic Microwave Background through the thermal Sunyaev–Zel’dovich (tSZ) effect in IGM. In this article, given an initial power spectrum of PMFs, we show the spatial fluctuation of mass density and temperature of the IGM and tSZ angular power spectrum created by the PMFs. Finally, we find that the tSZ angular power spectrum induced by PMFs becomes significant on small scales, even with PMFs below the observational upper limit. Therefore, we conclude that the measurement of tSZ anisotropy on small scales will provide the most stringent constraint on PMFs.
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Faraday Tomography Tutorial

Galaxies 2018, 6(4), 140; https://doi.org/10.3390/galaxies6040140

The capabilities of wide-band polarization datasets that are now becoming available from precursors/pathfinders to the Square Kilometre Array (SKA), and eventually from the SKA itself, make it possible to use the Faraday tomography technique to facilitate the study of cosmic magnetism. While many programs enabling Faraday tomography have been developed by various authors and it is now becoming easier to apply the required techniques, the interpretation of the results is not straightforward. This is not only because of the lack of a one-to-one relation between the Faraday depth and the physical depth, and observational artifacts such as instrumental polarization, but also because the choice of the method that is used and its settings can be reflected in the results. Thus, it is essential to understand how the various methods enabling Faraday tomography are suited for the efficient application of the technique. In the workshop “The Power of Faraday Tomography”, we organized a Faraday tomography tutorial to help the participants understand the required tools. In this article, we summarize the basics of the techniques, and provide an overview of the tutorial.
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Magnetizing the Cosmic Web during Reionization

Galaxies 2018, 6(4), 124; https://doi.org/10.3390/galaxies6040124

Increasing evidence suggests that cosmological sheets, filaments, and voids may be substantially magnetized today. The origin of magnetic fields in the intergalactic medium (IGM) is, however, currently uncertain. It seems well known that non-standard extensions to the physics of the standard model can provide mechanisms susceptible of magnetizing the universe at large. Perhaps less well known is the fact that standard, classical physics of matter–radiation interactions actually possesses the same potential. We discuss a magnetogenesis mechanism based on the exchange of momentum between hard photons and electrons in an inhomogeneous IGM. Operating in the neighborhood of ionizing sources during the epoch of reionization, this mechanism is capable of generating magnetic seeds of relevant strengths over scales comparable to the distance between ionizing sources. In addition, summing up the contributions of all ionizing sources and taking into account the distribution of gas inhomogeneities, we show that this mechanism leaves the IGM, at the end of reionization, with a level of magnetization that might account, when amplification mechanisms take over, for the magnetic fields strengths in the current cosmic web.
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Challenges and Techniques for Simulating Line Emission

Galaxies 2018, 6(4), 100; https://doi.org/10.3390/galaxies6040100

Modeling emission lines from the millimeter to the UV and producing synthetic spectra is crucial for a good understanding of observations, yet it is an art filled with hazards. This is the proceedings of “Walking the Line”, a 3-day conference held in 2018 that brought together scientists working on different aspects of emission line simulations, in order to share knowledge and discuss the methodology. Emission lines across the spectrum from the millimeter to the UV were discussed, with most of the focus on the interstellar medium, but also some topics on the circumgalactic medium. The most important quality of a useful model is a good synergy with observations and experiments. Challenges in simulating line emission are identified, some of which are already being worked upon, and others that must be addressed in the future for models to agree with observations. Recent advances in several areas aiming at achieving that synergy are summarized here, from micro-physical to galactic and circum-galactic scale.
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Searching for Axion-Like Particles with X-ray Polarimeters

Galaxies 2018, 6(2), 45; https://doi.org/10.3390/galaxies6020045

X-ray telescopes are an exceptional tool for searching for new fundamental physics. In particular, X-ray observations have already placed world-leading bounds on the interaction between photons and axion-like particles (ALPs). ALPs are hypothetical new ultra-light particles motivated by string theory models. They can also act as dark matter and dark energy, and provide a solution to the strong CP problem. In a background magnetic field, ALPs and photons may interconvert. This leads to energy dependent modulations in both the flux and polarisation of the spectra of point sources shining through large magnetic fields. The next generation of polarising X-ray telescopes will offer new detection possibilities for ALPs. Here we present techniques and projected bounds for searching for ALPs with X-ray polarimetry. We demonstrate that upcoming X-ray polarimetry missions have the potential to place world-leading bounds on ALPs.
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On the Spectrum and Polarization of Magnetar Flare Emission

Galaxies 2018, 6(1), 35; https://doi.org/10.3390/galaxies6010035

Bursts and flares are among the distinctive observational manifestations of magnetars, isolated neutron stars endowed with an ultra-strong magnetic field (

B \approx 10^{14}

–

10^{15}

G). It is believed that these events arise in a hot electron-positron plasma, injected in the magnetosphere, due to a magnetic field instability, which remains trapped within the closed magnetic field lines (the “trapped-fireball” model). We have developed a simple radiative transfer model to simulate magnetar flare emission in the case of a steady trapped fireball. After dividing the fireball surface in a number of plane-parallel slabs, the local spectral and polarization properties are obtained integrating the radiative transfer equations for the two normal modes. We assume that magnetic Thomson scattering is the dominant source of opacity, and neglect contributions from second-order radiative processes, although the presence of double-Compton scattering is accounted for in establishing local thermal equilibrium in the fireball atmospheric layers. The spectra we obtained in the 1–100 keV energy range are in broad agreement with those of available observations. The large degree of polarization (≳80%) predicted by our model should be easily detectable by new-generation X-ray polarimeters, like IXPE, XIPE and eXTP, allowing one to confirm the model predictions.
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A Way Out of the Bubble Trouble?—Upon Reconstructing the Origin of the Local Bubble and Loop I via Radioisotopic Signatures on Earth

Galaxies 2018, 6(1), 26; https://doi.org/10.3390/galaxies6010026

Deep-sea archives all over the world show an enhanced concentration of the radionuclide ⁶⁰Fe, isolated in layers dating from about 2.2 Myr ago. Since this comparatively long-lived isotope is not naturally produced on Earth, such an enhancement can only be attributed to extraterrestrial sources, particularly one or several nearby supernovae in the recent past. It has been speculated that these supernovae might have been involved in the formation of the Local Superbubble, our Galactic habitat. Here, we summarize our efforts in giving a quantitative evidence for this scenario. Besides analytical calculations, we present results from high-resolution hydrodynamical simulations of the Local Superbubble and its presumptive neighbor Loop I in different environments, including a self-consistently evolved supernova-driven interstellar medium. For the superbubble modeling, the time sequence and locations of the generating core-collapse supernova explosions are taken into account, which are derived from the mass spectrum of the perished members of certain, carefully preselected stellar moving groups. The release and turbulent mixing of ⁶⁰Fe is followed via passive scalars, where the yields of the decaying radioisotope were adjusted according to recent stellar evolution calculations. The models are able to reproduce both the timing and the intensity of the ⁶⁰Fe excess observed with rather high precision. We close with a discussion of recent developments and give future perspectives.
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Galactic Structures from Gravitational Radii

Galaxies 2018, 6(1), 22; https://doi.org/10.3390/galaxies6010022

We demonstrate that the existence of a Noether symmetry in

f (R)

theories of gravity gives rise to an additional gravitational radius, besides the standard Schwarzschild one, determining the dynamics at galactic scales. By this feature, it is possible to explain the baryonic Tully-Fisher relation and the rotation curve of gas-rich galaxies without the dark matter hypothesis. Furthermore, under the same standard, the Fundamental Plane of elliptical galaxies can be addressed.
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The Third Quantization: To Tunnel or Not to Tunnel?

Galaxies 2018, 6(1), 19; https://doi.org/10.3390/galaxies6010019

Within the framework of the third quantization, we consider the possibility that an initially recollapsing baby universe can enter a stage of near de Sitter inflation by tunnelling through a Euclidean wormhole that connects the recollapsing and inflationary geometries. We present the solutions for the evolution of the scale factor in the Lorentzian and Euclidean regions as well as the probability that the baby universe indeed crosses the wormhole when it reaches its maximum size.
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Quantum-Gravitational Effects on Primordial Power Spectra in Slow-Roll Inflationary Models

Galaxies 2018, 6(1), 6; https://doi.org/10.3390/galaxies6010006

We review the computation of the power spectra of inflationary gauge-invariant perturbations in the context of canonical quantum gravity for generic slow-roll models. A semiclassical approximation, based on an expansion in inverse powers of the Planck mass, is applied to the complete Wheeler–DeWitt equation describing a perturbed inflationary universe. This expansion leads to a hierarchy of equations at consecutive orders of the approximation and allows us to write down a corrected Schrödinger equation that encodes information about quantum-gravitational effects. The analytical dependence of the correction to the power spectrum on the wavenumber is obtained. Nonetheless, some numerical work is needed in order to obtain its precise value. Finally, it is shown that the correction turns out to be positive, which leads to an enhancement of the power spectrum especially prominent for large scales. We will also discuss whether this correction leads to a measurable effect in the cosmic microwave background anisotropies.
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The Mimetic Born-Infeld Gravity: The Primordial Cosmos and Spherically Symmetric Solutions

Galaxies 2017, 5(4), 87; https://doi.org/10.3390/galaxies5040087

The Eddington-inspired-Born-Infeld (EiBI) model is reformulated within the mimetic approach. In the presence of a mimetic field, the model contains non-trivial vacuum solutions. We study a realistic primordial vacuum universe and we prove the existence of regular solutions. Besides, the linear instabilities in the EiBI model are found to be avoidable for some bouncing solutions. For a vacuum, static and spherically symmetric geometry, a new branch of solutions in which the black hole singularity that is replaced with a lightlike singularity is found.
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Constraints on the Formation of M31’s Stellar Halo from the SPLASH Survey

Galaxies 2017, 5(4), 59; https://doi.org/10.3390/galaxies5040059

The SPLASH (Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo) Survey has observed fields throughout M31’s stellar halo, dwarf satellites, and stellar disk. The observations and derived measurements have either been compared to predictions from simulations of stellar halo formation or modeled directly in order to derive inferences about the formation and evolution of M31’s stellar halo. We summarize some of the major results from the SPLASH survey and the resulting implications for our understanding of the build-up of M31’s stellar halo.
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Revisiting the Globular Cluster Systems of NGC 3258 and NGC 3268

Galaxies 2017, 5(3), 48; https://doi.org/10.3390/galaxies5030048

We present a photometric study of NGC 3258 and NGC 3268 globular cluster systems (GCSs) with a wider spatial coverage than previous works. This allowed us to determine the extension of both GCSs, and obtain new values for their populations. In both galaxies, we found the presence of radial colour gradients in the peak of the blue globular clusters. The characteristics of both GCSs point to a large evolutionary history with a substantial accretion of satellite galaxies.
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The HI Distribution Observed toward a Halo Region of the Milky Way

Galaxies 2017, 5(3), 45; https://doi.org/10.3390/galaxies5030045

We use observations of the neutral atomic hydrogen (HI) 21-cm emission line to study the spatial distribution of the HI gas in a 80°

\times

90° region of the Galaxy halo. The HI column densities in the range of 3–11 × 10

^{20}

^{- 2}

have been estimated for some of the studied regions. In our map—obtained with a spectral sensitivity of ∼2 K—we do not detect any HI 21-cm emission line above 2

σ

at Galactic latitudes higher than ∼46°. This report summarizes our contribution presented at the conference on the origin and evolution of barionic Galaxy halos.
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Better Galactic Mass Models through Chemistry

Galaxies 2017, 5(3), 43; https://doi.org/10.3390/galaxies5030043

With the upcoming release of the Gaia catalog and the many multiplexed spectroscopic surveys on the horizon, we are rapidly moving into a new data-driven era in the study of the Milky Way’s stellar halo. When combined, these data sets will give us a many-dimensional view of stars in accreted structures in the halo that includes both dynamical information about their orbits and chemical information about their formation histories. Using simulated data from the state-of-the-art Latte simulations of Milky-Way-like galaxies, which include hydrodynamics, feedback, and chemical evolution in a cosmological setting, we demonstrate that while dynamical information alone can be used to constrain models of the Galactic mass distribution in the halo, including the extra dimensions provided by chemical abundances can improve these constraints as well as assist in untangling different accreted components.
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The Baryonic Halos of Isolated Elliptical Galaxies

Galaxies 2017, 5(3), 42; https://doi.org/10.3390/galaxies5030042

Without the interference of a number of events, galaxies may suffer in crowded environments (e.g., stripping, harassment, strangulation); isolated elliptical galaxies provide a control sample for the study of galaxy formation. We present the study of a sample of isolated ellipticals using imaging from a variety of telescopes, focusing on their globular cluster systems as tracers of their stellar halos. Our main findings are: (a) GC color bimodality is common even in the most isolated systems; (b) the specific frequency of GCs is fairly constant with galaxy mass, without showing an increase towards high-mass systems like in the case of cluster ellipticals; (c) on the other hand, the red fraction of GCs follows the same inverted V shape trend with mass as seen in cluster ellipticals; and (d) the stellar halos show low Sérsic indices which are consistent with a major merger origin.
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Resolving the Extended Stellar Halos of Nearby Galaxies: The Wide-Field PISCeS Survey

Galaxies 2017, 5(3), 40; https://doi.org/10.3390/galaxies5030040

The wide-field Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) investigates the resolved stellar halos of two nearby galaxies (the spiral NGC 253 and the elliptical Centaurus A,

D \sim 4

Mpc) out to a galactocentric radius of 150 kpc. The survey to date has led to the discovery of 11 confirmed faint satellites and stunning streams/substructures in two environments substantially different from the Local Group; i.e., the loose Sculptor group of galaxies and the Centaurus A group dominated by an elliptical. The newly discovered satellites and substructures, with surface brightness limits as low as ∼32 mag/arcsec

^{2}

, are then followed-up with HST imaging and Keck/VLT spectroscopy to investigate their stellar populations. The PISCeS discoveries clearly testify the past and ongoing accretion processes shaping the halos of these nearby galaxies, and provide the first census of their satellite systems down to an unprecedented

M_{V} < - 8

.
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The “Building Blocks” of Stellar Halos

Galaxies 2017, 5(3), 33; https://doi.org/10.3390/galaxies5030033

The stellar halos of galaxies encode their accretion histories. In particular, the median metallicity of a halo is determined primarily by the mass of the most massive accreted object. We use hydrodynamical cosmological simulations from the apostle project to study the connection between the stellar mass, the metallicity distribution, and the stellar age distribution of a halo and the identity of its most massive progenitor. We find that the stellar populations in an accreted halo typically resemble the old stellar populations in a present-day dwarf galaxy with a stellar mass ∼0.2–0.5 dex greater than that of the stellar halo. This suggests that had they not been accreted, the primary progenitors of stellar halos would have evolved to resemble typical nearby dwarf irregulars.
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The Globular Cluster System of the Galaxy NGC 6876

Galaxies 2017, 5(3), 30; https://doi.org/10.3390/galaxies5030030

We present preliminary results of the deep photometric study of the elliptical galaxy NGC 6876, located at the center of the Pavo group, and its globular cluster system. We use images obtained with the GMOS camera mounted on the Gemini South telescope, in the

g^{'}

and

i^{'}

bands, with the purpose of disentangling the evolutionary history of the galaxy on the basis of its characteristics.
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The Extended Baryonic Halo of NGC 3923

Galaxies 2017, 5(3), 29; https://doi.org/10.3390/galaxies5030029

Galaxy halos and their globular cluster systems build up over time by the accretion of small satellites. We can learn about this process in detail by observing systems with ongoing accretion events and comparing the data with simulations. Elliptical shell galaxies are systems that are thought to be due to ongoing or recent minor mergers. We present preliminary results of an investigation of the baryonic halo—light profile, globular clusters, and shells/streams—of the shell galaxy NGC 3923 from deep Dark Energy Camera (DECam) g and i-band imaging. We present the 2D and radial distributions of the globular cluster candidates out to a projected radius of about 185 kpc, or

\sim 37 R_{e}

, making this one of the most extended cluster systems studied. The total number of clusters implies a halo mass of

M_{h} \sim 3 \times 10^{13}

_{⊙}

. Previous studies had identified between 22 and 42 shells, making NGC 3923 the system with the largest number of shells. We identify 23 strong shells and 11 that are uncertain. Future work will measure the halo mass and mass profile from the radial distributions of the shell, N-body models, and line-of-sight velocity distribution (LOSVD) measurements of the shells using the Multi Unit Spectroscopic Explorer (MUSE).
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Interstellar Reddening Effect on the Age Dating of Population II Stars

Galaxies 2017, 5(3), 28; https://doi.org/10.3390/galaxies5030028

The age measurement of the stellar halo component of the Galaxy is based mainly on the comparison of the main sequence turn-off luminosity of the globular cluster (GC) stars with theoretical isochrones. The standard procedure includes a vertical shift, in order to account for the distance and extinction to the cluster, and a horizontal one, to compensate the reddening. However, the photometry is typically performed with broad-band filters where the shape of the stellar spectra introduces a shift of the effective wavelength response of the system, dependent on the effective temperature (or color index) of the star. The result is an increasing distortion—actually a rotation and a progressive compression with the temperature—of the color-magnitude diagrams relatively to the standard unreddened isochrones, with increasing reddening. This effect is usually negligible for reddening

E (B - V)

on the order of or smaller than 0.15, but it can be quite relevant at larger extinction values. While the ratio of the absorption to the reddening is widely discussed in the literature, the importance of the latter effect is often overlooked. In this contribution, we present isochron simulations and discuss the expected effects on age dating of high-reddening globular clusters.
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The SLUGGS Survey: Understanding Lenticular Galaxy Formation via Extended Stellar Kinematics

Galaxies 2017, 5(2), 26; https://doi.org/10.3390/galaxies5020026

We present the latest published and preliminary results from the SLUGGS Survey discussing the formation of lenticular galaxies through analysis of their kinematics. These include a comparison of the measured stellar spin of low-mass lenticular galaxies to the spin of remnant galaxies formed by binary merger simulations to assess whether a merger is a likely formation mechanism for these galaxies. We determine that while a portion of lenticular galaxies have properties consistent with these remnants, others are not, indicating that they are likely “faded spirals”. We also present a modified version of the spin–ellipticity diagram, which utilises radial tracks to be able to identify galaxies with intermediate-scale discs. Such galaxies often have conflicting morphological classifications, depending on whether photometric or kinematic measurements are used. Finally, we present preliminary results on the total mass density profile slopes of lenticular galaxies to assess trends as lower stellar masses are probed.
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Dissecting Halo Components in IFU Data

Galaxies 2017, 5(2), 24; https://doi.org/10.3390/galaxies5020024

While most astronomers are now familiar with tools to decompose images into multiple components such as disks, bulges, and halos, the equivalent techniques for spectral data cubes are still in their infancy. This is unfortunate, as integral field unit (IFU) spectral surveys are now producing a mass of data in this format, which we are ill-prepared to analyze effectively. We have therefore been developing new tools to separate out components using this full spectral data. The results of such analyses will prove invaluable in determining not only whether such decompositions have an astrophysical significance, but, where they do, also in determining the relationship between the various elements of a galaxy. Application to a pilot study of IFU data from the cD galaxy NGC 3311 confirms that the technique can separate the stellar halo from the underlying galaxy in such systems, and indicates that, in this case, the halo is older and more metal poor than the galaxy, consistent with it forming from the cannibalism of smaller satellite galaxies. The success of the method bodes well for its application to studying the larger samples of cD galaxies that IFU surveys are currently producing.
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