Galaxies Research Articles

Spectroscopy and Photometry of the Least Massive Type II Globular Clusters: NGC 1261 and NGC 6934*

Abstract Recent work has revealed two classes of globular clusters (GCs), dubbed Type I and Type II. Type II GCs are characterized by both a blue and a red red giant branch composed of stars with different metallicities, often coupled with distinct abundances in the slow neutron-capture elements (s-elements). Here we continue the chemical tagging of Type II GCs by adding the two least massive clusters of this class, NGC 1261 and NGC 6934. Based on both spectroscopy and photometry, we find red stars in NGC 1261 to be slightly enhanced in [Fe/H] by ∼0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ∼0.2 dex. Neither NGC 1261 nor NGC 6934 show internal variations in the s-elements, which suggests a GC mass threshold for the occurrence of s-process enrichment. We found a significant correlation between the additional Fe locked in the red stars of Type II GCs and the present-day mass of the cluster. Nevertheless, most Type II GCs retained a small fraction of Fe produced by SNe II, lower than the 2%; NGC 6273, M54, and ω Centauri are remarkable exceptions. In the Appendix, we infer for the first time chemical abundances of lanthanum, assumed as representative of the s-elements, in M54, the GC located in the nucleus of the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in [La/Fe] by 0.10 ± 0.06 dex, in contrast with the large [La/Fe] spread of most Type II GCs. We suggest that different processes are responsible for the enrichment in iron and s-elements in Type II GCs.

The AGN Fraction in Dwarf Galaxies from eROSITA: First Results and Future Prospects

Abstract Determining the fraction of nearby dwarf galaxies hosting massive black holes (BHs) can inform our understanding of the origin of “seed” BHs at high redshift. Here we search for signatures of accreting massive BHs in a sample of dwarf galaxies (M ⋆ ≤ 3 × 109 M ⊙, z ≤ 0.15) selected from the NASA-Sloan Atlas (NSA) using X-ray observations from the eROSITA Final Equatorial Depth Survey (eFEDS). On average, our search is sensitive to active galactic nuclei (AGNs) in dwarf galaxies that are accreting at ≳1% of their Eddington luminosity. Of the ∼28,000 X-ray sources in eFEDS and the 495 dwarf galaxies in the NSA within the eFEDS footprint, we find six galaxies hosting possible active massive BHs. If the X-ray sources are indeed associated with the dwarf galaxies, the X-ray emission is above that expected from star formation, with X-ray source luminosities of L 0.5–8 keV ∼ 1039–40 erg s−1. Additionally, after accounting for chance alignments of background AGNs with dwarf galaxies, we estimate there are between zero and nine real associations between dwarf galaxies and X-ray sources in the eFEDS field at the 95% confidence level. From this we find an upper limit on the eFEDS-detected dwarf galaxy AGN fraction of ≤1.8%, which is broadly consistent with similar studies at other wavelengths. We extrapolate these findings from the eFEDS sky coverage to the planned eROSITA All-Sky Survey and estimate that upon completion, the all-sky survey could yield as many as ∼1350 AGN candidates in dwarf galaxies at low redshift.

LeMMINGs – IV. The X-ray properties of a statistically complete sample of the nuclei in active and inactive galaxies from the Palomar sample

ABSTRACT All 280 of the statistically complete Palomar sample of nearby (<120 Mpc) galaxies δ > 20° have been observed at 1.5 GHz as part of the LeMMINGs e-MERLIN legacy survey. Here, we present Chandra X-ray observations of the nuclei of 213 of these galaxies, including a statistically complete sub-set of 113 galaxies in the declination range 40° <δ < 65°. We observed galaxies of all optical spectral types, including ‘active’ galaxies [e.g. low-ionization nuclear emission line regions (LINERs) and Seyferts] and ‘inactive’ galaxies like ${\rm H\, \small {II}}$ galaxies and absorption line galaxies (ALG). The X-ray flux limit of our survey is 1.65 × 10−14 erg s−1 cm−2 (0.3−10 keV). We detect X-ray emission coincident within 2 arcsec of the nucleus in 150/213 galaxies, including 13/14 Seyferts, 68/77 LINERs, 13/22 ALGs and 56/100 ${\rm H\, \small {II}}$ galaxies, but cannot completely rule out contamination from non-AGN processes in sources with nuclear luminosities $\lesssim 10^{39}$ erg s−1. We construct an X-ray Luminosity function (XLF) and find that the local galaxy XLF, when including all active galactic nucleus (AGN) types, can be represented as a single power law of slope −0.54 ± 0.06. The Eddington ratio of the Seyferts is usually 2−4 decades higher than that of the LINERs, ALGs, and ${\rm H\, \small {II}}$ galaxies, which are mostly detected with Eddington ratios $\lesssim 10^{-3}$. Using [${\rm O\, \small {III}}$] line measurements and black hole masses from the literature, we show that LINERs, ${\rm H\, \small {II}}$ galaxies and ALGs follow similar correlations to low luminosities, suggesting that some ‘inactive’ galaxies may harbour AGN.

Changing looks of the nucleus of the Seyfert galaxy NGC 1566 compared with other changing‐look AGNs

AbstractWe present results of a long‐term optical, UV, and X‐ray study of variability of the nearby changing‐look (CL) Seyfert NGC 1566 which was observed with the Swift Observatory from 2007 to 2020. We summarize our previously published spectroscopic and photometric results and present new observations. We reported on the alteration in the spectral type of NGC 1566 in 2018 (REF1). Moreover, we focused on the exceptional postmaximum behavior after 2018 July, when all bands dropped with some fluctuations (REF2). We observed four significant re‐brightenings in the postmaximum period. We have found differences in X‐ray and UV/Optical variability. The ratio was decreased during 2018–2020. New postmaximum spectra covering the period November 31, 2018 to September 23, 2019 show dramatic changes compared to August 2, 2018, with fading of the broad lines and [Fe X] λ6374 until 2019 March (REF2). Effectively, two changing‐look (CL) states were observed for this object: Changing to type 1.2 and then returning to the low state as a type 1.8 Sy. We suggest that the changes are mostly due to fluctuations in the energy generation. Variability properties of NGC1566 are compared with our results for other CL active galactic nuclei.

Discovery of methanimine (CH2NH) megamasers toward compact obscured galaxy nuclei

We present the first search for the 5.29 GHz methanimine (CH2NH) 110 − 111 transition toward a sample of galaxy nuclei. We target seven galaxies that host compact obscured nuclei (CONs) with the Karl G. Jansky Very Large Array. These galaxies are characterized by Compton-thick cores. CH2NH emission is detected toward six CONs. The brightness temperatures measured toward Arp 220 indicate maser emission. Isotropic luminosities of the CH2NH transition, from all sources where it is detected, exceed 1 L⊙ and thus may be considered megamasers. We also detect formaldehyde (H2CO) emission toward three CONs. The isotropic CH2NH luminosities are weakly correlated with the infrared luminosity of the host galaxy and strongly correlated with OH megamaser luminosities from the same galaxies. Non-local thermodynamic equilibrium radiative transfer models suggest that the maser is pumped by the intense millimeter-to-submillimeter Our study suggests that CH2NH megamasers are linked to the nuclear processes within 100 pc of the Compton-thick nucleus within CONs.

A deep search for faint Chandra X-ray sources, radio sources, and optical counterparts in NGC 6752

ABSTRACT We report the results of a deep search for faint Chandra X-ray sources, radio sources, and optical counterparts in the nearby, core-collapsed globular cluster, NGC 6752. We combined new and archival Chandra imaging to detect 51 X-ray sources (12 of which are new) within the 1.9 arcmin half-light radius. Three radio sources in deep ATCA 5 and 9 GHz radio images match with Chandra sources. We have searched for optical identifications for the expanded Chandra source list using deep Hubble Space Telescope photometry in B435, R625, H α, UV275, and U336. Among the entire sample of 51 Chandra sources, we identify 18 cataclysmic variables (CVs), 9 chromospherically active binaries (ABs), 3 red giants (RGs), 3 galaxies (GLXs), and 6 active galactic nuclei (AGNs). Three of the sources are associated with millisecond pulsars (MSPs). As in our previous study of NGC 6752, we find that the brightest CVs appear to be more centrally concentrated than the faintest CVs, although the effect is no longer statistically significant as a consequence of the inclusion in the faint group of two intermediate brightness CVs. This possible difference in the radial distributions of the bright and faint CV groups appears to indicate that mass segregation has separated them. We note that photometric incompleteness in the crowded central region of the cluster may also play a role. Both groups of CVs have an inferred mass above that of the main-sequence turnoff stars. We discuss the implications for the masses of the CV components.

Calibration of mid- to far-infrared spectral lines in galaxies

Context. Mid- to far-infrared (IR) lines are suitable in the study of dust-obscured regions in galaxies because dust extinction strongly decreases with wavelength, and therefore IR spectroscopy allows us to explore the most hidden regions of galaxies, where heavily obscured star formation as well as accretion onto supermassive black holes at the nuclei of galaxies occur. This is mostly important for the so-called cosmic noon (i.e. at redshifts of 1 < z < 3), at which point most of the baryonic mass in galaxies has been assembled. Aims. Our goal is to provide reliable calibrations of the mid- to far-IR ionic fine-structure lines, the brightest H2 pure rotational lines, and the polycyclic aromatic hydrocarbon (PAH) features, which we used to analyse current and future observations in the mm-submm range from the ground, as well as mid-IR spectroscopy from the upcoming James Webb Space Telescope. Methods. We used three samples of galaxies observed in the local Universe: star-forming galaxies (SFGs, 196), active galactic nuclei (AGN; 90−150 for various observables), and low-metallicity dwarf galaxies (40). For each population, we derive different calibrations of the observed line luminosities versus the total IR luminosities. Results. Through the resulting calibrations, we derive spectroscopic measurements of the star formation rate (SFR) and of the black hole accretion rate (BHAR) in galaxies using mid- and far-IR fine-structure lines, H2 pure rotational lines and PAH features. In particular, we derive robust star formation tracers based on the following: the [CII]158 μm line; the sum of the two far-IR oxygen lines, the [OI]63 μm line, and the [OIII]88 μm line; a combination of the neon and sulfur mid-IR lines; the bright PAH features at 6.2 and 11.3 μm; as well as – for the first time – the H2 rotational lines at 9.7, 12.3, and 17 μm. We propose the [CII]158 μm line, the combination of the two neon lines ([NeII]12.8 μm and [NeIII]15.5 μm), and, for solar-like metallicity galaxies that may harbour an AGN, the PAH 11.3 μm feature as the best SFR tracers. On the other hand, a reliable measure of the BHAR can be obtained using the [OIV]25.9 μm and the [NeV]14.3 and 24.3 μm lines. For the most commonly observed fine-structure lines in the far-IR, we compare our calibration with the existing ALMA observations of high-redshift galaxies. We find an overall good agreement for the [CII]158 μm line for both AGN and SFGs, while the [OIII]88 μm line in high-z galaxies is in better agreement with the low-metallicity local galaxies (dwarf galaxy sample) than with the SFGs, suggesting that high-z galaxies might have strong radiation fields due to low metal abundances, as expected.

CAPOS: The bulge Cluster APOgee Survey

We present results from a study of 15 red giant members of the intermediate-metallicity globular cluster (GC) FSR 1758 using high-resolution, near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2) that were obtained as part of CAPOS (the bulge Cluster APOgee Survey). Since its very recent discovery as a massive GC in the bulge region, evoking the name Sequoia, this has been an intriguing object with a highly debated origin, and initially led to the suggestion of a purported progenitor dwarf galaxy of the same name. In this work, we use new spectroscopic and astrometric data to provide additional clues as to the nature of FSR 1758. Our study confirms the GC nature of FSR 1758, and as such we report the existence of the characteristic N-C anticorrelation and Al-N correlation for the first time. We thereby reveal the existence of the multiple-population phenomenon, similar to that observed in virtually all GCs. Furthermore, the presence of a population with strongly enriched aluminum makes it unlikely that FSR 1758 is the remnant nucleus of a dwarf galaxy because Al-enhanced stars are uncommon in dwarf galaxies. We find that FSR 1758 is slightly more metal rich than previously reported in the literature; this source has a mean metallicity [Fe/H] between −1.43 to −1.36, depending on the adopted atmospheric parameters and a scatter within observational error, again pointing to its GC nature. Overall, the α-enrichment (≳ + 0.3 dex), Fe-peak (Fe, Ni), light (C, N), and odd-Z (Al) elements follow the trend of intermediate-metallicity GCs. Isochrone fitting in the Gaia bands yields an estimated age of ∼11.6 Gyr. We used the exquisite kinematic data, including our CAPOS radial velocities and Gaia eDR3 proper motions, to constrain the N-body density profile of FSR 1758, and found that it is as massive (∼2.9 ± 0.6 × 105 M⊙) as NGC 6752. We confirm a retrograde and eccentric orbit for FSR 1758. A new examination of its dynamical properties with the GravPot16 model favors an association with the Gaia-Enceladus-Sausage accretion event. Thus, paradoxically, the cluster that gave rise to the name of the Sequoia dwarf galaxy does not appear to belong to this specific merging event.

Structure and morphology of the MATLAS dwarf galaxies and their central nuclei

ABSTRACT We present a photometric study of the dwarf galaxy population in the low to moderate density environments of the MATLAS (Mass Assembly of early-Type gaLAxies with their fine Structures) deep imaging survey. The sample consists of 2210 dwarfs, including 508 nucleated. We define a nucleus as a compact source that is close to the galaxy photocentre (within 0.5 $R_\mathrm{ e}$) which is also the brightest such source within the galaxy’s effective radius. The morphological analysis is performed using a 2D surface brightness profile modelling on the g-band images of both the galaxies and nuclei. Our study reveals that, for similar luminosities, the MATLAS dwarfs show ranges in the distribution of structural properties comparable to cluster (Virgo and Fornax) dwarfs and a range of sizes comparable to the Local Group and Local Volume dwarfs. Colour measurements using the r- and i-band images indicate that the dwarfs in low and moderate density environments are as red as cluster dwarfs on average. The observed similarities between dwarf ellipticals in vastly different environments imply that dEs are not uniquely the product of morphological transformation due to ram-pressure stripping and galaxy harassment in high density environments. We measure that the dwarf nuclei are located predominantly in massive, bright and round dwarfs and observe fewer nuclei in dwarfs with a faint centre and a small size. The colour of the galaxy nucleus shows no clear relation to the colour of the dwarf, in agreement with the migration and wet migration nucleus formation scenarios. The catalogues of the MATLAS dwarfs photometric and structural properties are provided.

A search for active galactic nuclei in low-mass compact galaxies

ABSTRACT Low-mass compact galaxies (ultracompact dwarfs, UCDs, and compact ellipticals, cEs) populate the stellar size–mass plane between globular clusters and early-type galaxies. Known to be formed either in situ with an intrinsically low mass or resulting from the stripping of a more massive galaxy, the presence of a supermassive or an intermediate-mass black hole (BH) could help discriminate between these possible scenarios. With this aim, we have performed a multiwavelength search of active BH activity, i.e. active galactic nuclei (AGN), in a sample of 937 low-mass compact galaxies (580 UCDs and 357 cEs). This constitutes the largest study of AGN activity in these types of galaxies. Based on their X-ray luminosity, radio luminosity, and morphology, and/or optical emission line diagnostic diagrams, we find a total of 11 cEs that host an AGN. We also study for the first time the location of both low-mass compact galaxies (UCDs and cEs) and dwarf galaxies hosting AGN on the BH–galaxy scaling relations, finding that low-mass compact galaxies tend to be overmassive in the BH mass–stellar mass plane but not as much in the BH mass–stellar velocity dispersion correlation. This, together with available BH mass measurements for some of the low-mass compact galaxies, supports a stripping origin for the majority of these objects that would contribute to the scatter seen at the low-mass end of the BH–galaxy scaling relations. However, the differences are too large to be explained solely by this scatter, and thus our results suggest that a flattening at such low masses is also plausible, happening at a velocity dispersion of ∼20–40 km s−1.

Testing the tidal stripping scenario of ultracompact dwarf galaxy formation by using internal properties

ABSTRACT We use the hydrodynamical EAGLE simulation to test if ultracompact dwarf galaxies (UCDs) can form by tidal stripping by predicting the ages and metallicities of tidally stripped galaxy nuclei in massive galaxy clusters, and compare these results to compiled observations of age and metallicities of observed UCDs. We further calculate the colours of our sample of simulated stripped nuclei using single stellar population models and compare these colours to observations of UCDs in the Virgo Cluster. We find that the ages of observed UCDs are consistent with simulated stripped nuclei, with both groups of objects having a mean age >9 Gyr. Both stripped nuclei and UCDs follow a similar mass–metallicity relationship, and the metallicities of observed UCDs are consistent with those of simulated stripped nuclei for objects with M > 107 M⊙. The colours of observed UCDs are also consistent with our simulated stripped nuclei, for objects with M > 107 M⊙, with more massive objects being redder. We find that the colours of stripped nuclei exhibit a bimodal red and blue distribution that can be explained by the dependency of colour on age and metallicity, and by the mass–colour relation. We additionally find that our low-mass stripped nuclei sample is consistent with the colour of blue globular clusters. We conclude that the internal properties of simulated nuclei support the tidal stripping model of UCD formation.

The Uchuu simulations: Data Release 1 and dark matter halo concentrations

ABSTRACT We introduce the Uchuu suite of large high-resolution cosmological N-body simulations. The largest simulation, named Uchuu, consists of 2.1 trillion (12 8003) dark matter particles in a box of side-length 2.0 $\, h^{-1} \, \rm Gpc$, with particle mass of 3.27 × 108$\, h^{-1}\, \rm M_{\odot }$. The highest resolution simulation, Shin-Uchuu, consists of 262 billion (64003) particles in a box of side-length 140 $\, h^{-1} \, \rm Mpc$, with particle mass of 8.97 × 105$\, h^{-1}\, \rm M_{\odot }$. Combining these simulations, we can follow the evolution of dark matter haloes and subhaloes spanning those hosting dwarf galaxies to massive galaxy clusters across an unprecedented volume. In this first paper, we present basic statistics, dark matter power spectra, and the halo and subhalo mass functions, which demonstrate the wide dynamic range and superb statistics of the Uchuu suite. From an analysis of the evolution of the power spectra, we conclude that our simulations remain accurate from the baryon acoustic oscillation scale down to the very small. We also provide parameters of a mass–concentration model, which describes the evolution of halo concentration and reproduces our simulation data to within 5 per cent for haloes with masses spanning nearly eight orders of magnitude at redshift 0 ≤ z ≤ 14. There is an upturn in the mass–concentration relation for the population of all haloes and of relaxed haloes at z ≳ 0.5, whereas no upturn is detected at z < 0.5. We make publicly available various N-body products as part of Uchuu Data Release 1 on the Skies & Universes site.1 Future releases will include gravitational lensing maps and mock galaxy, X-ray cluster, and active galactic nucleus catalogues.

A Chandra and HST View of WISE-selected AGN Candidates in Dwarf Galaxies

Abstract Reliably identifying active galactic nuclei (AGNs) in dwarf galaxies is key to understanding black hole (BH) demographics at low masses and constraining models for BH seed formation. Here we present Chandra X-ray Observatory observations of 11 dwarf galaxies that were chosen as AGN candidates using Wide-field Infrared Survey Explorer (WISE) mid-infrared (mid-IR) color–color selection. Hubble Space Telescope images are also presented for 10 of the galaxies. Based on Sloan Digital Sky Survey spectroscopy, six galaxies in our sample have optical evidence for hosting AGNs and five are classified as star-forming. We detect X-ray point sources with luminosities above that expected from X-ray binaries in the nuclei of five of the six galaxies with optical evidence of AGNs. However, the X-ray emission from these AGNs is generally much lower than expected based on AGN scaling relations with infrared and optical tracers. We do not find compelling evidence for AGNs in the five optically-selected star-forming galaxies despite having red mid-IR colors. Only two are detected in X-rays and their properties are consistent with stellar-mass X-ray binaries. Based on this multiwavelength study, we conclude that two-color mid-IR AGN diagnostics at the resolution of WISE cannot be used to reliably select AGNs in optically-star-forming dwarf galaxies. Future observations in the infrared with the James Webb Space Telescope offer a promising path forward.

Deep Observations of CO and Free–Free Emission in Ultraluminous Infrared QSO IRAS F07599+6508

Abstract Infrared quasi-stellar objects (IR QSOs) are a rare subpopulation selected from ultraluminous infrared galaxies (ULIRGs) and have been regarded as promising candidates of ULIRG-to-optical QSO transition objects. Here we present NOEMA observations of the CO (1−0) line and 3 mm continuum emission in IR QSO IRAS F07599+6508 at z = 0.1486, which has many properties in common with Mrk 231. The CO emission is found to be resolved with a major axis of ∼6.1 kpc that is larger than the size of ∼4.0 kpc derived for 3 mm continuum. We identify two faint CO features located at a projected distance of ∼11.4 and 19.1 kpc from the galaxy nucleus, both of which are found to have counterparts in the optical and radio bands and may have a merger origin. A systematic velocity gradient is found in the CO main component, suggesting that the bulk of molecular gas is likely rotationally supported. Based on the radio-to-millimeter spectral energy distribution and IR data, we estimate that about 30% of the flux at 3 mm arises from free–free emission and infer a free–free-derived star formation rate of 77 M ⊙ yr−1, close to the IR estimate corrected for the AGN contribution. We find a high-velocity CO emission feature at the velocity range of about −1300 to −2000 km s−1. Additional deep CO observations are needed to confirm the presence of a possible very high velocity CO extension of the OH outflow in this IR QSO.

Core-collapse supernova subtypes in luminous infrared galaxies

The fraction of core-collapse supernovae (CCSNe) occurring in the central regions of galaxies is not well constrained at present. This is partly because large-scale transient surveys operate at optical wavelengths, making it challenging to detect transient sources that occur in regions susceptible to high extinction factors. Here we present the discovery and follow-up observations of two CCSNe that occurred in the luminous infrared galaxy (LIRG) NGC 3256. The first, SN 2018ec, was discovered using the ESO HAWK-I/GRAAL adaptive optics seeing enhancer, and was classified as a Type Ic with a host galaxy extinction ofAV= 2.1−0.1+0.3mag. The second, AT 2018cux, was discovered during the course of follow-up observations of SN 2018ec, and is consistent with a subluminous Type IIP classification with anAV = 2.1 ± 0.4 mag of host extinction. A third CCSN, PSN J10275082−4354034 in NGC 3256, was previously reported in 2014, and we recovered the source in late-time archivalHubbleSpace Telescope imaging. Based on template light curve fitting, we favour a Type IIn classification for it with modest host galaxy extinction ofAV= 0.3−0.3+0.4mag. We also extend our study with follow-up data of the recent Type IIb SN 2019lqo and Type Ib SN 2020fkb that occurred in the LIRG system Arp 299 with host extinctions ofAV= 2.1−0.3+0.1andAV= 0.4−0.2+0.1mag, respectively. Motivated by the above, we inspected, for the first time, a sample of 29 CCSNe located within a projected distance of 2.5 kpc from the host galaxy nuclei in a sample of 16 LIRGs. We find, if star formation within these galaxies is modelled assuming a global starburst episode and normal IMF, that there is evidence of a correlation between the starburst age and the CCSN subtype. We infer that the two subgroups of 14 H-poor (Type IIb/Ib/Ic/Ibn) and 15 H-rich (Type II/IIn) CCSNe have different underlying progenitor age distributions, with the H-poor progenitors being younger at 3σsignificance. However, we note that the currently available sample sizes of CCSNe and host LIRGs are small, and the statistical comparisons between subgroups do not take into account possible systematic or model errors related to the estimated starburst ages.

Aspects of GRMHD in high-energy astrophysics: geometrically thick disks and tori agglomerates around spinning black holes

This work focuses on some key aspects of the general relativistic (GR)—magneto-hydrodynamic (MHD) applications in high-energy astrophysics. We discuss the relevance of the GRHD counterparts formulation exploring the geometrically thick disk models and constraints of the GRMHD shaping the physics of accreting configurations. Models of clusters of tori orbiting a central super-massive black hole (SMBH) are described. These orbiting tori aggregates form sets of geometrically thick, pressure supported, perfect fluid tori, associated to complex instability processes including tori collision emergence and empowering a wide range of activities related expectantly to the embedding matter environment of Active Galaxy Nuclei. Some notes are included on aggregates combined with proto-jets, represented by open cusped solutions associated to the geometrically thick tori. This exploration of some key concepts of the GRMHD formulation in its applications to High-Energy Astrophysics starts with the discussion of the initial data problem for a most general Einstein–Euler–Maxwell system addressing the problem with a relativistic geometric background. The system is then set in quasi linear hyperbolic form, and the reduction procedure is argumented. Then, considerations follow on the analysis of the stability problem for self-gravitating systems with determined symmetries considering the perturbations also of the geometry part on the quasi linear hyperbolic onset. Thus we focus on the ideal GRMHD and self-gravitating plasma ball. We conclude with the models of geometrically thick GRHD disks gravitating around a Kerr SMBH in their GRHD formulation and including in the force balance equation of the disks the influence of a toroidal magnetic field, determining its impact in tori topology and stability.

Ultra-compact dwarfs beyond the centre of the Fornax galaxy cluster: hints of UCD formation in low-density environments

ABSTRACT Ultra-compact dwarf galaxies (UCDs) were serendipitously discovered by spectroscopic surveys in the Fornax cluster 20 yr ago. Nowadays, it is commonly accepted that many bright UCDs are the nuclei of galaxies that have been stripped. However, this conclusion might be driven by biased samples of UCDs in high-density environments, on which most searches are based. With the deep optical images of the Fornax Deep Survey, combined with public near-infrared data, we revisit the UCD population of the Fornax cluster and search for UCD candidates, for the first time, systematically out to the virial radius of the galaxy cluster. Our search is complete down to magnitude mg = 21 mag or M$_g\, \sim$ −10.5 mag at the distance of the Fornax cluster. The UCD candidates are identified and separated from foreground stars and background galaxies by their optical and near-infrared colours. This primarily utilizes the u−i/i−Ks diagram and a machine learning technique is employed to incorporate other colour combinations to reduce the number of contaminants. The newly identified candidates (44) in addition to the spectroscopically confirmed UCDs (61), increase the number of known Fornax UCD considerably (105). Almost all of the new UCD candidates are located outside the Fornax cluster core (360 kpc), where all of the known UCDs were found. The distribution of UCDs within the Fornax cluster shows that a population of UCDs may form in low-density environments. This most likely challenges the current models of UCD formation.

Discovery of tidal debris stars from G1/Mayall II in M31

ABSTRACT The object Mayall II or G1 is the brightest globular cluster belonging to M31. Because of its extreme properties for a globular cluster, it has been speculated that G1 is the remnant nucleus of a dwarf galaxy that has been stripped by the tidal field of M31. Using the Keck DEIMOS spectrograph, we have conducted a survey for tidally stripped stars from G1, obtaining a sample of 351 stellar velocities over ∼320 sq. arcmin of sky centred on G1. 13 are within $25~{\, \rm km\, s^{-1}\, }$ of the systemic velocity of G1, and exhibit spatial and velocity correlations consistent with being dynamically associated with G1, and all 13 are well outside the tidal radius of the cluster. These 13 stars could be either (i) the remnants of an almost completely evaporated stellar envelope or (ii) G1 member stars lost through tidal interaction with M31. Estimates of the implied mass-loss rate based on our data suggest a short dissolution time-scale for G1, thus favouring the stellar envelope hypothesis for the origin of the tidal tail stars, or, at the very least, an advanced stage of cluster dissolution. In either case, G1 and by extension compact stellar systems in general have likely played a significant role in building the halo of M31.

An XMM–Newton study of active–inactive galaxy pairs

ABSTRACT While theory and simulations indicate that galaxy mergers play an important role in the cosmological evolution of accreting black holes and their host galaxies, samples of active galactic nuclei (AGNs) in galaxies at close separations are still small. In order to increase the sample of AGN pairs, we undertook an archival project to investigate the X-ray properties of an SDSS-selected sample of 32 galaxy pairs with separations ≤150 kpc containing one optically identified AGN, which were serendipitously observed by XMM–Newton. We discovered only one X-ray counterpart among the optically classified non-active galaxies, with a weak X-ray luminosity (≃ 5 × 1041 erg s−1). 59 per cent (19 out of 32) of the AGNs in our galaxy pair sample exhibit an X-ray counterpart, covering a wide range in absorption-corrected X-ray luminosity (5 × 1041–2 × 1044 erg s−1). More than 79 per cent of these AGNs are obscured (column density NH > 1022 cm−2), with more than half thereof (i.e. about 47 per cent of the total AGN sample) being Compton-thick. AGN/no-AGN pairs are therefore more frequently X-ray obscured (by a factor ≃1.5) than isolated AGNs. When compared to a luminosity and redshift-matched sample of bona fide dual AGN, AGN/no-AGN pairs exhibit one order-of-magnitude lower X-ray column density in the same separation range (>10 kpc). A small sample (4 objects) of AGN/no-AGN pairs with sub-pc separation is all heavily obscured, driving a formal anticorrelation between the X-ray column density and the galaxy pair separation in these systems. These findings suggest that the galactic environment has a key influence on the triggering of nuclear activity in merging galaxies.

Gemini NIFS survey of feeding and feedback in nearby active galaxies – IV. Excitation

ABSTRACT The near-infrared spectra of active galactic nuclei (AGN) present emission lines of different atomic and molecular species. The mechanisms involved in the origin of these emission lines in AGN are still not fully understood. We use J- and K-band integral field spectra of six luminous ($43.1\lt \log L_{\rm bol}/({\rm erg\, s^{-1}})\lt 44.4$) Seyfert galaxies (NGC 788, Mrk 607, NGC 3227, NGC 3516, NGC 5506, and NGC 5899) in the local Universe (0.0039 < z < 0.0136) to investigate the gas excitation within the inner 100–300 pc radius of the galaxies at spatial resolutions of a few tens of parsecs. In all galaxies, the H2 emission originates from thermal processes with excitation temperatures in the range 2400–5200 K. In the high-line ratio (HLR) region of the H2/Brγ versus [Fe ii]/Paβ diagnostic diagram, which includes 29 per cent of the spaxels, shocks are the main excitation mechanism, as indicated by the correlation between the line widths and line ratios. In the AGN region of the diagram (64 per cent of the spaxels) the H2 emission is due to the AGN radiation. The [Fe ii] emission is produced by a combination of photoionization by the AGN radiation and shocks in five galaxies and is dominated by photoionization in NGC 788. The [S ix]1.2523 μm coronal emission line is present in all galaxies, and its flux distributions are extended from 80 to 185 pc from the galaxy nuclei, except for NGC 5899, in which this line is detected only in the integrated spectrum.