Special Issue "On the Origin (and Evolution) of Baryonic Galaxy Halos"

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (31 August 2017)

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Guest Editor
Dr. Duncan A. Forbes

Swinburne University of Technology, Centre for Astrophysics and Supercomputing, John St, Hawthorn VIC 3122, Australia
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Interests: extragalactic astronomy and cosmology; galaxies formation and evolution
Guest Editor
Prof. Dr. Ericson D. Lopez

Observatorio Astronómico de Quito, Escuela Politécnica Nacional, Interior del Parque La Alameda, Av. Gran Colombia s/n, Quito, Ecuador
Website | E-Mail
Interests: high-energy astrophysics; AGNs; blazars; relativistic jets; relativistic astrophysics; plasma astrophysics; cosmology

Special Issue Information

Dear Colleagues,

The stellar components of galaxy halos hold informational clues about the early history of galaxies and their subsequent evolution.

The halos of our own Milky Way galaxy and close neighbor M31 have been studied in some detail. Deep, wide and detailed observations of galaxy halos beyond the Local Group are becoming more ubiquitous. Simulations, which incorporate realistic baryonic physics in a cosmological context have also made significant progress in recent years in modeling galaxy halos. These simulations predict outer halo regions that differ strongly in their formation processes and properties from the well-studied inner regions of galaxies. Halos have long dynamical times and, as such, preserve the unique signatures of galaxy assembly.

This Special Issue of Galaxies is based on the contributions presented during the international symposium “On the Origin (and Evolution) of Baryonic Galaxy Halos” held in Galapagos Islands, Ecuador, between 13 and 17 March, 2017. Gathering the leading theorists and observers around the world, along with young scientists and students, the conference analyzed. The origin and evolution of baryonic halos, how to define the stellar halo of an elliptical galaxy, the stellar components of galaxy halos: Metallicity, age, kinematics, density, substructures in galaxy halos, in-situ vs. ex-situ formed stars, halo tracers, such as resolved stars, globular clusters, planetary nebulae, satellite galaxies and diffuse gas.

The timing of the conference is well matched to the availability of premier facilities, such as new wide-field imagers, new spectrographs, such as MUSE the VLT (and soon KCWI on Keck). The upcoming launch of JWST will provide another step-change in observational capabilities. Significant progress as also been made recently with realistic galaxy simulations, such as Illustris, EAGLE and Magneticum.

Dr. Duncan Forbes
Dr. Ericson Lopez
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Galaxies is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 350 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Published Papers (29 papers)

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Editorial

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Open AccessEditorial
A Conference on the Origin (and Evolution) of Baryonic Galaxy Halos
Received: 10 May 2017 / Revised: 10 May 2017 / Accepted: 10 May 2017 / Published: 17 May 2017
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Abstract
A conference was held in March 2017 in the Galapagos Islands on the topic of The Origin (and Evolution) of Baryonic Galaxy Halos. It attracted some 120 researchers from around the world. They presented 68 talks (nine of which were invited) and 30 [...] Read more.
A conference was held in March 2017 in the Galapagos Islands on the topic of The Origin (and Evolution) of Baryonic Galaxy Halos. It attracted some 120 researchers from around the world. They presented 68 talks (nine of which were invited) and 30 posters over five days. A novel element of the talk schedule was that participants were asked which talks they wanted to hear and the schedule was made up based on their votes and those of the Scientific Organizing Committee SOC . The final talk schedule had 34% of the talks given by women. An emphasis was given to discussion time directly after each talk. Combined with limited/no access to the internet, this resulted in high level of engagement and lively discussions. A prize was given to the poster voted the best by participants. A free afternoon included organized excursions to see the local scenery and wildlife of the Galapagos (e.g., the giant tortoises). Four public talks were given, in Spanish, for the local residents of the town. A post-conference survey was conducted, with most participants agreeing that the conference met their scientific needs and helped to initiate new research directions. Although it was challenging to organize such a large international meeting in such an isolated location as the Galapagos Islands (and much credit goes to the Local Organizing Committee LOC and staff of Quito Astronomical Observatory for their logistical efforts, organizing the meeting for over a year), it was very much a successful conference. We hope it will play a small part in further developing astronomy in Ecuador. Full article
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Research

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Open AccessFeature PaperArticle
Decoding Galactic Merger Histories
Received: 30 June 2017 / Revised: 30 November 2017 / Accepted: 1 December 2017 / Published: 8 December 2017
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Abstract
Galaxy mergers are expected to influence galaxy properties, yet measurements of individual merger histories are lacking. Models predict that merger histories can be measured using stellar halos and that these halos can be quantified using observations of resolved stars along their minor axis. [...] Read more.
Galaxy mergers are expected to influence galaxy properties, yet measurements of individual merger histories are lacking. Models predict that merger histories can be measured using stellar halos and that these halos can be quantified using observations of resolved stars along their minor axis. Such observations reveal that Milky Way-mass galaxies have a wide range of stellar halo properties and show a correlation between their stellar halo masses and metallicities. This correlation agrees with merger-driven models where stellar halos are formed by satellite galaxy disruption. In these models, the largest accreted satellite dominates the stellar halo properties. Consequently, the observed diversity in the stellar halos of Milky Way-mass galaxies implies a large range in the masses of their largest merger partners. In particular, the Milky Way’s low mass halo implies an unusually quiet merger history. We used these measurements to seek predicted correlations between the bulge and central black hole (BH) mass and the mass of the largest merger partner. We found no significant correlations: while some galaxies with large bulges and BHs have large stellar halos and thus experienced a major or minor merger, half have small stellar halos and never experienced a significant merger event. These results indicate that bulge and BH growth is not solely driven by merger-related processes. Full article
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Open AccessFeature PaperArticle
Hot Gaseous Halos in Early Type Galaxies
Received: 31 July 2017 / Revised: 26 September 2017 / Accepted: 27 September 2017 / Published: 4 October 2017
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Abstract
The hot gas in early type galaxies (ETGs) plays a crucial role in their formation and evolution. As the hot gas is often extended to the outskirts beyond the optical size, the large scale structural features identified by Chandra (including cavities, cold fronts, [...] Read more.
The hot gas in early type galaxies (ETGs) plays a crucial role in their formation and evolution. As the hot gas is often extended to the outskirts beyond the optical size, the large scale structural features identified by Chandra (including cavities, cold fronts, filaments, and tails) point to key evolutionary mechanisms, e.g., AGN feedback, merging history, accretion/stripping, as well as star formation and quenching. We systematically analyze the archival Chandra data of ETGs to study the hot ISM. Using uniformly derived data products with spatially resolved spectral information, we revisit the X-ray scaling relations of ETGs and address their implications by comparing them with those of groups/clusters and simulations. Full article
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Open AccessArticle
Upper Limits to Magnetic Fields in the Outskirts of Galaxies
Received: 31 July 2017 / Revised: 12 September 2017 / Accepted: 18 September 2017 / Published: 19 September 2017
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Abstract
Based on CO(2-1) public data, we study the monoxide oxygen gas excitation conditions and the magnetic field strength of four spiral galaxies. For the galaxy outskirts, we found kinetic temperatures in the range of ≲35–38 K, CO column densities ≲1015 [...] Read more.
Based on CO(2-1) public data, we study the monoxide oxygen gas excitation conditions and the magnetic field strength of four spiral galaxies. For the galaxy outskirts, we found kinetic temperatures in the range of ≲35–38 K, CO column densities ≲ 10 15 10 16 cm - 2 , and H 2 masses ≲ 4 × 10 6 6 × 10 8 M . An H 2 density ≲ 10 3 cm - 3 is suitable to explain the 2 σ upper limits of the CO(2-1) line intensity. We constrain the magnetic field strength for our sample of spiral galaxies and their outskirts by using their masses and H 2 densities to evaluate a simplified magneto-hydrodynamic equation. Our estimations provide values for the magnetic field strength on the order of ≲6–31 μ G. Full article
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Open AccessArticle
Dust Deficiency in the Interacting Galaxy NGC 3077
Received: 24 July 2017 / Revised: 12 September 2017 / Accepted: 13 September 2017 / Published: 15 September 2017
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Abstract
Using 70 μm observations taken with the PACS instrument of the Herschel space telescope, the dust content of the nearby and interacting spiral galaxy NGC 3077 has been compared with the dust content of the isolated galaxies such as NGC 2841, NGC [...] Read more.
Using 70 μ m observations taken with the PACS instrument of the Herschel space telescope, the dust content of the nearby and interacting spiral galaxy NGC 3077 has been compared with the dust content of the isolated galaxies such as NGC 2841, NGC 3184 and NGC 3351. The dust content has allowed us to derive dust-to-gas ratios for the four spiral galaxies of our sample. We find that NGC 2841, NGC 3184 and NGC 3351 have dust masses of 6.5–9.1 × 10 7 M , which are a factor of ∼10 higher than the value found for NGC 3077. This result shows that NGC 3077 is a dust deficient galaxy, as was expected, because this galaxy is affected by tidal interactions with its neighboring galaxies M81 and M82. NGC 3077 reveals a dust-to-gas ratio of 17.5%, much higher than the average ratio of 1.8% of the isolated galaxies, evidencing that NGC 3077 is also deficient in H 2 + HI gas. Therefore, it seems that, in this galaxy, gas has been stripped more efficiently than dust. Full article
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Open AccessArticle
The Outer Halos of Very Massive Galaxies: BCGs and their DSC in the Magneticum Simulations
Received: 1 August 2017 / Revised: 24 August 2017 / Accepted: 28 August 2017 / Published: 1 September 2017
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Abstract
Recent hydrodynamic cosmological simulations cover volumes up to Gpc3 and resolve halos across a wide range of masses and environments, from massive galaxy clusters down to normal galaxies, while following a large variety of physical processes (star-formation, chemical enrichment, AGN feedback) to [...] Read more.
Recent hydrodynamic cosmological simulations cover volumes up to Gpc 3 and resolve halos across a wide range of masses and environments, from massive galaxy clusters down to normal galaxies, while following a large variety of physical processes (star-formation, chemical enrichment, AGN feedback) to allow a self-consistent comparison to observations at multiple wavelengths. Using the Magneticum simulations, we investigate the buildup of the diffuse stellar component (DSC) around massive galaxies within group and cluster environments. The DSC in our simulations reproduces the spatial distribution of the observed intracluster light (ICL) as well as its kinematic properties remarkably well. For galaxy clusters and groups we find that, although the DSC in almost all cases shows a clear separation from the brightest cluster galaxy (BCG) with regard to its dynamic state, the radial stellar density distribution in many halos is often characterized by a single Sérsic profile, representing both the BCG component and the DSC, very much in agreement with current observational results. Interestingly, even in those halos that clearly show two components in both the dynamics and the spatial distribution of the stellar component, no correlation between them is evident. Full article
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Open AccessCommunication
Globular Clusters and the Halos of Dwarf Galaxies
Received: 25 July 2017 / Revised: 11 August 2017 / Accepted: 14 August 2017 / Published: 29 August 2017
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Abstract
Many dwarf galaxies have disproportionately rich globular cluster (GC) systems for their luminosities. Moreover, the GCs tend to be preferentially associated with the most metal-poor stellar populations in their parent galaxies, making them attractive tracers of the halos of dwarf (and larger) galaxies. [...] Read more.
Many dwarf galaxies have disproportionately rich globular cluster (GC) systems for their luminosities. Moreover, the GCs tend to be preferentially associated with the most metal-poor stellar populations in their parent galaxies, making them attractive tracers of the halos of dwarf (and larger) galaxies. In this contribution, I briefly discuss some constraints on cluster disruption obtained from studies of metal-poor GCs in dwarf galaxies. I then discuss our recent work on detailed abundance analysis from integrated-light spectroscopy of GCs in Local Group dwarf galaxies. Full article
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Open AccessArticle
Disk Heating, Galactoseismology, and the Formation of Stellar Halos
Received: 1 July 2017 / Revised: 13 August 2017 / Accepted: 14 August 2017 / Published: 26 August 2017
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Abstract
Deep photometric surveys of the Milky Way have revealed diffuse structures encircling our Galaxy far beyond the “classical” limits of the stellar disk. This paper reviews results from our own and other observational programs, which together suggest that, despite their extreme positions, the [...] Read more.
Deep photometric surveys of the Milky Way have revealed diffuse structures encircling our Galaxy far beyond the “classical” limits of the stellar disk. This paper reviews results from our own and other observational programs, which together suggest that, despite their extreme positions, the stars in these structures were formed in our Galactic disk. Mounting evidence from recent observations and simulations implies kinematic connections between several of these distinct structures. This suggests the existence of collective disk oscillations that can plausibly be traced all the way to asymmetries seen in the stellar velocity distribution around the Sun. There are multiple interesting implications of these findings: they promise new perspectives on the process of disk heating; they provide direct evidence for a stellar halo formation mechanism in addition to the accretion and disruption of satellite galaxies; and, they motivate searches of current and near-future surveys to trace these oscillations across the Galaxy. Such maps could be used as dynamical diagnostics in the emerging field of “Galactoseismology”, which promises to model the history of interactions between the Milky Way and its entourage of satellites, as well examine the density of our dark matter halo. As sensitivity to very low surface brightness features around external galaxies increases, many more examples of such disk oscillations will likely be identified. Statistical samples of such features not only encode detailed information about interaction rates and mergers, but also about long sought-after dark matter halo densities and shapes. Models for the Milky Way’s own Galactoseismic history will therefore serve as a critical foundation for studying the weak dynamical interactions of galaxies across the universe. Full article
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Open AccessArticle
On the Kinematics, Stability and Lifetime of Kinematically Distinct Cores: A Case Study
Received: 6 July 2017 / Revised: 11 August 2017 / Accepted: 11 August 2017 / Published: 17 August 2017
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Abstract
We present a case study of a early-type galaxy (ETG) hosting a kinematically distinct core (KDC) formed in a binary high resolution 1:1 spiral galaxy merger simulation. The runtime of the simulation is pushed up to 10Gyr to follow the complete evolution [...] Read more.
We present a case study of a early-type galaxy (ETG) hosting a kinematically distinct core (KDC) formed in a binary high resolution 1:1 spiral galaxy merger simulation. The runtime of the simulation is pushed up to 10 Gyr to follow the complete evolution of various physical properties. To investigate the origin of the KDC, the stellar component residing within the KDC is dissected, revealing that the rotational signal is purely generated by stars that belong to the KDC for at least 0 . 5 Gyr and are newly formed during the merging process. Following the orientation of the total stellar angular momentum of the KDC, we show that it performs a motion comparable to the precession of a gyroscope in a gravitational potential. We draw the conclusion that the motion of the KDC is a superposition of an intrinsic rotation and a global precession that gets gradually damped over cosmic time. Finally, the stability of the KDC over the complete runtime of the simulation is investigated by tracing the evolution of the widely used λ R parameter and the misalignment angle distribution. We find that the KDC is stable for about 3 Gyr after the merger and subsequently disperses completely on a timescale of ≈1.5 Gyr . Full article
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Open AccessCommunication
How Clumpy Star Formation Affects Globular Cluster Systems
Received: 7 June 2017 / Revised: 31 July 2017 / Accepted: 2 August 2017 / Published: 4 August 2017
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Abstract
There is now clear evidence the metallicities of globular clusters are not simple tracers of the elemental abundances in their protocluster clouds; some of the heavy elements were formed subsequently within the cluster itself. It is also manifestly clear that star formation is [...] Read more.
There is now clear evidence the metallicities of globular clusters are not simple tracers of the elemental abundances in their protocluster clouds; some of the heavy elements were formed subsequently within the cluster itself. It is also manifestly clear that star formation is a clumpy process. We present a brief overview of a theoretical model for how self-enrichment by supernova ejecta proceeds in a protocluster undergoing clumpy star formation, and show that it predicts internal abundance spreads in surprisingly good agreement with those in observed Milky Way clusters. Full article
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Open AccessArticle
Galaxies with Shells in the Illustris Simulation: Metallicity Signatures
Received: 1 July 2017 / Revised: 28 July 2017 / Accepted: 1 August 2017 / Published: 4 August 2017
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Abstract
Stellar shells are low surface brightness arcs of overdense stellar regions, extending to large galactocentric distances. In a companion study, we identified 39 shell galaxies in a sample of 220 massive ellipticals (M200crit>6×1012M [...] Read more.
Stellar shells are low surface brightness arcs of overdense stellar regions, extending to large galactocentric distances. In a companion study, we identified 39 shell galaxies in a sample of 220 massive ellipticals ( M 200 crit > 6 × 10 12 M ) from the Illustris cosmological simulation. We used stellar history catalogs to trace the history of each individual star particle inside the shell substructures, and we found that shells in high-mass galaxies form through mergers with massive satellites (stellar mass ratios μ stars 1 : 10 ). Using the same sample of shell galaxies, the current study extends the stellar history catalogs in order to investigate the metallicity of stellar shells around massive galaxies. Our results indicate that outer shells are often times more metal-rich than the surrounding stellar material in a galaxy’s halo. For a galaxy with two different satellites forming z = 0 shells, we find a significant difference in the metallicity of the shells produced by each progenitor. We also find that shell galaxies have higher mass-weighted logarithmic metallicities ([Z/H]) at 2– 4 R eff compared to galaxies without shells. Our results indicate that observations comparing the metallicities of stars in tidal features, such as shells, to the average metallicities in the stellar halo can provide information about the assembly histories of galaxies. Full article
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Open AccessArticle
A Photometric Study of Giant Ellipticals and Their Stellar Halos With VST
Received: 15 June 2017 / Revised: 19 July 2017 / Accepted: 20 July 2017 / Published: 26 July 2017
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Abstract
Observations of diffuse starlight in the outskirts of galaxies are thought to be a fundamental source of constraints on the cosmological context of galaxy assembly in the ΛCDM model. Such observations are not trivial because of the extreme faintness of such regions. [...] Read more.
Observations of diffuse starlight in the outskirts of galaxies are thought to be a fundamental source of constraints on the cosmological context of galaxy assembly in the Λ CDM model. Such observations are not trivial because of the extreme faintness of such regions. In this work, we investigated the photometric properties of six massive early-type galaxies (ETGs) in the VST Elliptical GAlaxies Survey (VEGAS) sample (NGC 1399, NGC 3923, NGC 4365, NGC 4472, NGC 5044, and NGC 5846) out to extremely low surface brightness levels with the goal of characterizing the global structure of their light profiles for comparison to state-of-the-art galaxy formation models. We carried out deep and detailed photometric mapping of our ETG sample taking advantage of deep imaging with VST/OmegaCAM in the g and i bands. By fitting the light profiles, and comparing the results to simulations of elliptical galaxy assembly, we have identified signatures of a transition between relaxed and unrelaxed accreted components and can constrain the balance between in situ and accreted stars. The very good agreement of our results with predictions from theoretical simulations demonstrates that the full VEGAS sample of 100 ETGs will allow us to use the distribution of diffuse light as a robust statistical probe of the hierarchical assembly of massive galaxies. Full article
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Open AccessArticle
Assembly Pathways and the Growth of Massive Early-Type Galaxies
Received: 20 April 2017 / Revised: 26 May 2017 / Accepted: 1 June 2017 / Published: 7 June 2017
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Abstract
Based on data from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, I present results on the assembly pathways, dark matter content and halo growth of massive early-type galaxies. Using galaxy starlight information we find that such galaxies had an early dissipative [...] Read more.
Based on data from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, I present results on the assembly pathways, dark matter content and halo growth of massive early-type galaxies. Using galaxy starlight information we find that such galaxies had an early dissipative phase followed by a second phase of halo growth from largely minor mergers (and in rare cases major mergers). Thus our result fits in well with the two-phase scenario of galaxy formation. We also used globular cluster radial velocities to measure the enclosed mass within 5 effective radii. The resulting dark matter fractions reveal a few galaxies with very low dark matter fractions that are not captured in the latest cosmological models. Multiple solutions are possible, but none yet is convincing. Translating dark matter fractions into epochs of halo assembly, we show that low mass galaxies tend to grow via gas-rich accretion, while high mass galaxies grow via gas-poor mergers. Full article
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Other

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Open AccessFeature PaperConference Report
Constraints on the Formation of M31’s Stellar Halo from the SPLASH Survey
Received: 16 August 2017 / Revised: 12 September 2017 / Accepted: 13 September 2017 / Published: 30 September 2017
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Abstract
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 [...] Read more.
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. Full article
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Open AccessConference Report
Revisiting the Globular Cluster Systems of NGC 3258 and NGC 3268
Received: 27 June 2017 / Revised: 18 August 2017 / Accepted: 22 August 2017 / Published: 31 August 2017
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Abstract
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, [...] Read more.
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. Full article
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Open AccessConference Report
The HI Distribution Observed toward a Halo Region of the Milky Way
Received: 18 July 2017 / Revised: 23 August 2017 / Accepted: 24 August 2017 / Published: 28 August 2017
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Abstract
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°×90° region of the Galaxy halo. The HI column densities in the range of 3–11 × 1020 [...] Read more.
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° × 90° region of the Galaxy halo. The HI column densities in the range of 3–11 × 10 20 cm 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. Full article
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Open AccessConference Report
Better Galactic Mass Models through Chemistry
Received: 29 July 2017 / Revised: 10 August 2017 / Accepted: 11 August 2017 / Published: 21 August 2017
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Abstract
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 [...] Read more.
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. Full article
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Open AccessConference Report
The Baryonic Halos of Isolated Elliptical Galaxies
Received: 23 June 2017 / Revised: 11 August 2017 / Accepted: 15 August 2017 / Published: 18 August 2017
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Abstract
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 [...] Read more.
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. Full article
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Open AccessConference Report
Resolving the Extended Stellar Halos of Nearby Galaxies: The Wide-Field PISCeS Survey
Received: 15 June 2017 / Revised: 11 August 2017 / Accepted: 14 August 2017 / Published: 17 August 2017
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Abstract
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, D4 Mpc) out to a galactocentric radius of 150 kpc. The survey to [...] Read more.
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 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 . Full article
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Open AccessLetter
Deep MOS Spectroscopy of NGC 1316 Globular Clusters
Received: 29 June 2017 / Revised: 8 August 2017 / Accepted: 9 August 2017 / Published: 15 August 2017
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Abstract
The giant elliptical galaxy NGC 1316 is the brightest galaxy in the Fornax cluster, and displays a number of morphological features that might be interpreted as an intermediate age merger remanent (∼3 Gyr). Based on the idea that globular clusters systems (GCS) constitute [...] Read more.
The giant elliptical galaxy NGC 1316 is the brightest galaxy in the Fornax cluster, and displays a number of morphological features that might be interpreted as an intermediate age merger remanent (∼3 Gyr). Based on the idea that globular clusters systems (GCS) constitute genuine tracers of the formation and evolution of their host galaxies, we conducted a spectroscopic study of approximately 40 globular clusters (GCs) candidates associated with this interesting galaxy. We determined ages, metallicities, and α -element abundances for each GC present in the sample, through the measurement of different Lick indices and their subsequent comparison with simple stellar populations models (SSPs). Full article
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Open AccessLetter
The Brazil–Argentina Gemini Group for the Study of Globular Cluster Systems (BAGGs GCs): FLAMINGOS-2 and GMOS Data for NGC 1395
Received: 1 July 2017 / Revised: 3 August 2017 / Accepted: 7 August 2017 / Published: 14 August 2017
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Abstract
In this letter, we present preliminary results of the analysis of Flamingos-2 and GMOS-S photometry of the globular cluster (GC) system of the elliptical galaxy NGC 1395. This is the first step of a long-term Brazilian–Argentinian collaboration for the study of GC systems [...] Read more.
In this letter, we present preliminary results of the analysis of Flamingos-2 and GMOS-S photometry of the globular cluster (GC) system of the elliptical galaxy NGC 1395. This is the first step of a long-term Brazilian–Argentinian collaboration for the study of GC systems in early-type galaxies. In the context of this collaboration, we obtained deep NIR photometric data in two different bands (J and K s), which were later combined with high quality optical Gemini + GMOS photometry previously obtained by the Argentinian team. This allowed us to obtain different color indices, less sensitive to the effect of horizontal branch (HB) stars for several hundreds of GC candidates, and to make an initial assessment of the presence or absence of multiple GC populations in colors in NGC 1395. Full article
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Open AccessLetter
One Piece at a Time—Adding to the Puzzle of S0 Formation
Received: 29 June 2017 / Revised: 1 August 2017 / Accepted: 3 August 2017 / Published: 7 August 2017
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Abstract
Understanding the origin of galaxies remains a topic of debate in the current astronomy. In this work, we have focused on lenticular (S0) galaxies located in low-density environments, using their associated globular cluster (GC) systems as a tool. Initially, we have started the [...] Read more.
Understanding the origin of galaxies remains a topic of debate in the current astronomy. In this work, we have focused on lenticular (S0) galaxies located in low-density environments, using their associated globular cluster (GC) systems as a tool. Initially, we have started the study of three S0 galaxies—NGC 2549, NGC 3414 and NGC 5838—using photometric data in several filters obtained with the GMOS camera mounted on the Gemini North telescope. The different GC systems, as well as their host galaxies, have shown particular features, such as multiple GC subpopulations and low-brightness substructures. These pieces of evidence show that the mentioned galaxies have suffered several merger/interaction events, even the accretion of satellite companions, probably causing their current morphologies. Full article
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Open AccessLetter
Distribution and Evolution of Metals in the Magneticum Simulations
Received: 30 June 2017 / Revised: 28 July 2017 / Accepted: 31 July 2017 / Published: 4 August 2017
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Abstract
Metals are ideal tracers of the baryonic cycle within halos. Their composition is a fossil record connecting the evolution of the various stellar components of galaxies to the interaction with the environment by in- and out-flows. The Magneticum simulations allow us to study [...] Read more.
Metals are ideal tracers of the baryonic cycle within halos. Their composition is a fossil record connecting the evolution of the various stellar components of galaxies to the interaction with the environment by in- and out-flows. The Magneticum simulations allow us to study halos across a large range of masses and environments, from massive galaxy clusters containing hundreds of galaxies, down to isolated field galaxies. They include a detailed treatment of the chemo-energetic feedback from the stellar component and its evolution, as well as feedback from the evolution of supermassive black holes. Following the detailed evolution of various metal species and their relative composition due to continuing enrichment of the IGM and ICM by SNIa, SNII and AGB winds of the evolving stellar population is revealed the complex interplay of local star-formation processes, mixing, global baryonic flows, secular galactic evolution and environmental processes. We present results from the Magneticum simulations on the chemical properties of simulated galaxies and galaxy clusters, carefully comparing them to observations. We show that the simulations already reach a very high level of realism within their complex descriptions of the chemo-energetic feedback, successfully reproducing a large number of observed properties and scaling relations. Our simulated galaxies clearly indicate that there are no strong secondary parameters (such as star-formation rates at a fixed redshift) driving the scatter in these scaling relations. The remaining differences clearly point to detailed physical processes, which have to be included in future simulations. Full article
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Open AccessConference Report
The “Building Blocks” of Stellar Halos
Received: 30 June 2017 / Revised: 27 July 2017 / Accepted: 28 July 2017 / Published: 2 August 2017
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Abstract
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 [...] Read more.
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. Full article
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Open AccessConference Report
The Globular Cluster System of the Galaxy NGC 6876
Received: 27 May 2017 / Revised: 19 July 2017 / Accepted: 21 July 2017 / Published: 25 July 2017
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Abstract
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 [...] Read more.
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. Full article
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Open AccessFeature PaperConference Report
The Extended Baryonic Halo of NGC 3923
Received: 1 July 2017 / Revised: 12 July 2017 / Accepted: 14 July 2017 / Published: 20 July 2017
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Abstract
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 [...] Read more.
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 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 3 × 10 13 M . 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). Full article
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Open AccessConference Report
Interstellar Reddening Effect on the Age Dating of Population II Stars
Received: 27 April 2017 / Revised: 15 June 2017 / Accepted: 18 June 2017 / Published: 22 June 2017
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Abstract
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 [...] Read more.
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. Full article
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Open AccessConference Report
The SLUGGS Survey: Understanding Lenticular Galaxy Formation via Extended Stellar Kinematics
Received: 4 May 2017 / Revised: 22 May 2017 / Accepted: 26 May 2017 / Published: 30 May 2017
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Abstract
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 [...] Read more.
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. Full article
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Open AccessConference Report
Dissecting Halo Components in IFU Data
Received: 20 April 2017 / Revised: 22 May 2017 / Accepted: 23 May 2017 / Published: 25 May 2017
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Abstract
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 [...] Read more.
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. Full article
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