Galaxies, Volume 5, Issue 3 (September 2017) – 28 articles

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 ≲ $4\times {10}^6$ – $6\times {10}^8$ M ${}_{\odot }$ . An H ${}_2$ density ≲ ${10}^3$ cm ${}^{-3}$ is suitable to explain the 2 $\sigma$ 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 $\mathsf{\mu }$ G. Full article

We combine three-dimensional general-relativistic numerical models of hot, magnetized Advection Dominated Accretion Flows around a supermassive black hole and the corresponding outflows from them with a general relativistic polarized radiative transfer model to produce synthetic radio images and spectra of jet outflows. We apply the model to the underluminous core of M87 galaxy. The assumptions and results of the calculations are discussed in context of millimeter observations of the M87 jet launching zone. Our ab initio polarized emission and rotation measure models allow us to address the constrains on the mass accretion rate onto the M87 supermassive black hole. Full article

Using 70 $\mathsf{\mu }$ 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 ${}_{\odot }$ , 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

We report on recent results froma target-of-opportunity program to obtain spectropolarimetry observations with the Southern African Large Telescope (SALT) on flaring gamma-ray blazars. SALT spectropolarimetry and contemporaneous multi-wavelength spectral energy distribution (SED) data are being modelled self-consistently with a leptonic single-zone model. Such modeling provides an accurate estimate of the degree of order of the magnetic field in the emission region and the thermal contributions (from the host galaxy and the accretion disk) to the SED, thus putting strong constraints on the physical parameters of the gamma-ray emitting region. For the specific case of the $\gamma$ -ray blazar 4C+01.02, we demonstrate that the combined SED and spectropolarimetry modeling constrains the mass of the central black hole in this blazar to $M_{\mathrm{BH}}\sim {10}^9{M}_{\odot }$ . Full article

Recently the so-called mimetic gravity approach has been used to obtain corrections to the Friedmann equation of General Relativity similar to the ones present in loop quantum cosmology. In this paper, we propose an alternative way to derive this modified Friedmann equation via the so-called non-polynomial gravity approach, which consists of adding geometric non-polynomial higher derivative terms to Hilbert–Einstein action, which are nonetheless polynomials and lead to a second-order differential equation in Friedmann–Lemaître–Robertson–Walker space-times. Our explicit action turns out to be a realization of the Helling proposal of effective action with an infinite number of terms. The model is also investigated in the presence of a non-vanishing cosmological constant, and a new exact bounce solution is found and studied. Full article

We investigate the plasmoid knot formation in stratified relativistic jet by means of relativistic magneto-hydrodynamics simulations. Indeed, astrophysical jets in active galactic nuclei (AGN) seem to be transversely stratified, with a fast inner jet and a slower outer jet. It is likely that the launching mechanism for each component is different. On the other hand, the steady and moving knots’ properties are observed along these jets. With the proposed model, we were able to link the different types of observed knot in various radio loud AGN with specific stratified jet characteristics. We showed that the increase energy flux at the outer edge of the jet induces a steady knot near the core and a moving knot at a greater distance. Full article

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

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

Contemporary projects to measure anisotropies in the cosmic microwave background (CMB) are now detecting hundreds to thousands of extragalactic radio sources, most of them blazars. As a member of a group of CMB scientists involved in the construction of catalogues of such sources and their analysis, I wish to point out the potential value of CMB surveys to studies of AGN jets and their polarization. Current CMB projects, for instance, reach mJy sensitivity, offer wide sky coverage, are “blind” and generally of uniform sensitivity across the sky (hence useful statistically), make essentially simultaneous multi-frequency observations at frequencies from 30 to 857 GHz, routinely offer repeated observations of sources with interesting cadences and now generally provide polarization measurements. The aim here is not to analyze in any depth the AGN science already derived from such projects, but rather to heighten awareness of their promise for the AGN community. Full article

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

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 ${}^{20}$ 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 $\sigma$ 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

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

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

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

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 $10\mathrm{Gyr}$ 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 $0.5\mathrm{Gyr}$ 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 ${\lambda }_R$ parameter and the misalignment angle distribution. We find that the KDC is stable for about $3\mathrm{Gyr}$ after the merger and subsequently disperses completely on a timescale of ≈1.5 $\mathrm{Gyr}$ . Full article

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 $M_V<-8$ . Full article

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 $\alpha$ -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

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 $Ks$), 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

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

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

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

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 ( ${\mathrm{M}}_{200\mathrm{crit}}>6\times {10}^{12}{\mathrm{M}}_{\odot }$ ) 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 ${\mu }_{\mathrm{stars}}\gtrsim 1:10$ ). 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– $4{\mathrm{R}}_{\mathrm{eff}}$ 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

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

Understanding magnetic field strength and morphology is very important for studying astrophysical jets. Polarization signatures have been a standard way to probe the jet magnetic field. Radio and optical polarization monitoring programs have been very successful in studying the space- and time-dependent jet polarization behaviors. A new era is now arriving with high-energy polarimetry. X-ray and $\gamma$ -ray polarimetry can probe the most active jet regions with the most efficient particle acceleration. This new opportunity will make a strong impact on our current understanding of jet systems. This paper summarizes the scientific potential and current model predictions for X-ray and $\gamma$ -ray polarization of astrophysical jets. In particular, we discuss the advantages of using high-energy polarimetry to constrain several important problems in the jet physics, including the jet radiation mechanisms, particle acceleration mechanisms, and jet kinetic and magnetic energy composition. Here we take blazars as a study case, but the general approach can be similarly applied to other astrophysical jets. We conclude that by comparing combined magnetohydrodynamics (MHD), particle transport, and polarization-dependent radiation transfer simulations with multi-wavelength time-dependent radiation and polarization observations, we will obtain the strongest constraints and the best knowledge of jet physics. Full article

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 $\Lambda$ 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 $\sim 100$ 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

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 $g^{\prime }$ and $i^{\prime }$ bands, with the purpose of disentangling the evolutionary history of the galaxy on the basis of its characteristics. Full article

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 $\sim 37R_e$ , making this one of the most extended cluster systems studied. The total number of clusters implies a halo mass of ${\mathcal{M}}_h\sim 3\times {10}^{13}$ M ${}_{\odot }$ . 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

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 $\mathrm{E}(\mathrm{B}-\mathrm{V})$ 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