Radial Brightness Profiles Research Articles

Constraints on the Physical Origin of Large Cavities in Transition Disks from Multiwavelength Dust Continuum Emission

The physical origin of the large cavities observed in transition disks is to date still unclear. Different physical mechanisms (e.g., a companion, dead zones, enhanced grain growth) produce disk cavities of different depth, and the expected spatial distribution of gas and solids in each mechanism is not the same. In this work, we analyze the multiwavelength interferometric visibilities of dust continuum observations obtained with Atacama Large Millimeter/submillimeter Array and Very Large Array for six transition disks: CQTau, UXTau A, LkCa15, RXJ1615, SR24S, and DMTau, and calculate brightness radial profiles, where diverse emission morphology is revealed at different wavelengths. The multiwavelength data are used to model the spectral energy distribution and compute constraints on the radial profile of the dust surface density, maximum grain size, and dust temperature in each disk. They are compared with the observational signatures expected from various physical mechanisms responsible for disk cavities. The observational signatures suggest that the cavities observed in the disks around UXTau A, LkCa15, and RXJ1615 could potentially originate from a dust trap created by a companion. Conversely, in the disks around CQTau, SR24S, DMTau, the origin of the cavity remains unclear, although it is compatible with a pressure bump and grain growth within the cavity.

Shock imprints on the radio mini halo in RBS 797

Aims. In this work, we analysed new LOw Frequency ARray observations of the mini halo in the cluster RBS 797, together with archival Very Large Array observations and the recent Chandra results. This cluster is known to host a powerful active galactic nucleus (AGN) at its centre, with two pairs of jets propagating in orthogonal directions. Recent X-ray observations have detected three pairs of shock fronts within 125 kpc from the cluster centre, connected with the activity of the central AGN. Our aim is to investigate the connection between the mini halo emission and the activity of the central source. Methods. We have used different methods to separate the emission of the central source from the diffuse mini halo emission, and we have derived the radial spectral index trend of the mini halo. Results. We find that the diffuse radio emission is elongated in different directions at 144 MHz (east-west) with respect to 1.4 GHz (north-south), tracing the orientation of the two pairs of jets. The mini halo emission is characterised by an average spectral index α = −1.02 ± 0.05. The spectral index profile of the mini halo shows a gradual flattening from the centre to the periphery. Such a trend is unique among the mini halos studied to date, and resembles the spectral index trend typical of particles re-accelerated by shocks. However, the estimated contribution to the radio brightness profile coming from shock re-acceleration is found to be insufficient to account for the radial brightness profile of the mini halo. Conclusions. We propose three scenarios that could explain the observed trend: (i) the AGN-driven shocks are propagating onto an already existing mini halo, re-energising the electrons and leaving clear imprints in the mini halo spectral properties. We estimate that the polarisation induced by the shocks could be detected at 6 GHz and above; (ii) we could be witnessing turbulent re-acceleration in a high magnetic field cluster; and (iii) the mini halo could have a hadronic origin, in which the particles are injected by the central AGN and the diffusion coefficient depends of the cosmic ray proton momentum. Future observations in polarisation would be fundamental to understand the role of shocks and of the magnetic field.

X-ray metal line emission from the hot circumgalactic medium: probing the effects of supermassive black hole feedback

ABSTRACT We derive predictions from state-of-the-art cosmological galaxy simulations for the spatial distribution of the hot circumgalactic medium (CGM, [0.1–1]R200c) through its emission lines in the X-ray soft band ([0.3–1.3] keV). In particular, we compare IllustrisTNG, EAGLE, and SIMBA and focus on galaxies with stellar mass $10^{10-11.6}\, \rm {M}_{\odot }$ at z = 0. The three simulation models return significantly different surface brightness radial profiles of prominent emission lines from ionized metals such as O vii(f), O viii, and Fe xvii as a function of galaxy mass. Likewise, the three simulations predict varying azimuthal distributions of line emission with respect to the galactic stellar planes, with IllustrisTNG predicting the strongest angular modulation of CGM physical properties at radial range ${{\gtrsim}0.3{-}0.5\, R_{200c}}$. This anisotropic signal is more prominent for higher energy lines, where it can manifest as X-ray eROSITA-like bubbles. Despite different models of stellar and supermassive black hole (SMBH) feedback, the three simulations consistently predict a dichotomy between star-forming and quiescent galaxies at the Milky Way and Andromeda mass range, where the former are X-ray brighter than the latter. This is a signature of SMBH-driven outflows, which are responsible for quenching star formation. Finally, we explore the prospect of testing these predictions with a microcalorimeter-based X-ray mission concept with a large field of view. Such a mission would probe the extended hot CGM via soft X-ray line emission, determine the physical properties of the CGM, including temperature, from the measurement of line ratios, and provide critical constraints on the efficiency and impact of SMBH feedback on the CGM.

Deprojecting and constraining the vertical thickness of exoKuiper belts

ABSTRACT Constraining the vertical and radial structure of debris discs is crucial to understanding their formation, evolution, and dynamics. To measure both the radial and vertical structure, a disc must be sufficiently inclined. However, if a disc is too close to edge-on, deprojecting its emission becomes non-trivial. In this paper we show how Frankenstein, a non-parametric tool to extract the radial brightness profile of circumstellar discs, can be used to deproject their emission at any inclination as long as they are optically thin and axisymmetric. Furthermore, we extend Frankenstein to account for the vertical thickness of an optically thin disc (H(r)) and show how it can be constrained by sampling its posterior probability distribution and assuming a functional form (e.g. constant h = H/r), while fitting the radial profile non-parametrically. We use this new method to determine the radial and vertical structures of 16 highly inclined debris discs observed by ALMA. We find a wide range of vertical aspect ratios, h, ranging from 0.020 ± 0.002 (AU Mic) to 0.20 ± 0.03 (HD 110058), which are consistent with parametric models. We find a tentative correlation between h and the disc fractional width, as expected if wide discs were more stirred. Assuming discs are self-stirred, the thinnest discs would require the presence of at least 500-km-sized planetesimals. The thickest discs would likely require the presence of planets. We also recover previously inferred and new radial structures, including a potential gap in the radial distribution of HD 61005. Finally, our new extension of Frankenstein also allows constraining how h varies as a function of radius, which we test on 49 Ceti, finding that h is consistent with being constant.

Examining Uranus’ ζ Ring in Voyager 2 Wide-angle Camera Observations: Quantifying the Ring’s Structure in 1986 and Its Modifications Prior to the Year 2007

The ζ ring is the innermost component of the Uranian ring system. It is of scientific interest because its morphology changed significantly between the Voyager 2 encounter in 1986 and subsequent Earth-based observations around 2007. It is also of practical interest because some Uranus mission concepts have the spacecraft pass through the inner flank of this ring. Recent reexaminations of the Voyager 2 images have revealed additional information about this ring that provide a more complete picture of the ring’s radial brightness profile and phase function. These data reveal that this ring’s brightness varies with phase angle in a manner similar to other tenuous rings, consistent with it being composed primarily of submillimeter-sized particles. The total cross section of particles within this ring can also be estimated from these data, but translating that number into the actual risk to a spacecraft flying through this region depends on a number of model-dependent parameters. Fortunately, comparisons with Saturn’s G and D rings allows the ζ ring’s particle number density to be compared with regions previously encountered by the Voyager and Cassini spacecraft. Finally, these data indicate that the observed changes in the ζ ring’s structure between 1986 and 2007 are primarily due to a substantial increase in the amount of dust at distances between 38,000 and 40,000 km from Uranus’ center.

Modest Dust Settling in the IRAS04302+2247 Class I Protoplanetary Disk

We present new Very Large Array observations, between 6.8 and 66 mm, of the edge-on Class I disk IRAS04302+2247. Observations at 6.8 mm and 9.2 mm lead to the detection of thermal emission from the disk, while shallow observations at the other wavelengths are used to correct for emission from other processes. The disk radial brightness profile transitions from broadly extended in previous Atacama Large Millimeter/submillimeter Array 0.9 mm and 2.1 mm observations to much more centrally brightened at 6.8 mm and 9.2 mm, which can be explained by optical depth effects. The radiative transfer modeling of the 0.9 mm, 2.1 mm, and 9.2 mm data suggests that the grains are smaller than 1 cm in the outer regions of the disk, allowing us to obtain the first lower limit for the scale height of grains emitting at millimeter wavelengths in a protoplanetary disk. We find that the millimeter dust scale height is between 1 au and 6 au at a radius 100 au from the central star, while the gas scale height is estimated to be about 7 au, indicating a modest level of settling. The estimated dust height is intermediate between less evolved Class 0 sources, which are found to be vertically thick, and more evolved Class II sources, which show a significant level of settling. This suggests that we are witnessing an intermediate stage of dust settling.

Structure and kinematics of a massive galaxy at z ∼ 7

Context. Observations of the rest-frame UV emission of high-redshift galaxies suggest that the early stages of galaxy formation involve disturbed structures. Imaging the cold interstellar medium (ISM) can provide a unique view of the kinematics associated with the assembly of galaxies. Aims. In this paper, we analyze the spatial distribution and kinematics of the cold ionized gas of the normal star-forming galaxy COS-2987030247 at z = 6.8076, based on new high-resolution observations of the [C II] 158 μm line emission obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Methods. The analysis of these observations allowed us to: compare the spatial distribution and extension of the [C II] and rest-frame UV emission, model the [C II] line data-cube using the 3DBAROLO code, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions. Results. The system is found to be composed of a main central source, a fainter north extension, and candidate [C II] companions located 10-kpc away. We find similar rest-frame UV and [C II] spatial distributions, suggesting that the [C II] emission emerges from the star-forming regions. The agreement between the UV and [C II] surface brightness radial profiles rules out diffuse, extended [C II] emission (often called a [C II] halo) in the main galaxy component. The [C II] velocity map reveals a velocity gradient in the north-south direction, suggesting ordered motion, as commonly found in rotating-disk galaxies. However, higher resolution observations would be needed to rule out a compact merger scenario. Our model indicates an almost face-on galaxy (i ∼ 20°), with a average rotational velocity of 86 ± 16 km s−1 and a low average velocity dispersion, σ < 30 km s−1. This result implies a dispersion lower than the expected value from observations and semi-analytic models of high redshift galaxies. Furthermore, our measurements indicate that COS-2987030247 and its individual regions systematically lie within the local L[CII]-SFR relationship, yet slightly below the local Σ[CII]-ΣUV relation. Conclusions. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will possibly become perturbed at later times by accreting sources.

Millimeter gap contrast as a probe for turbulence level in protoplanetary disks

Turbulent motions are believed to regulate angular momentum transport and influence dust evolution in protoplanetary disks. Measuring the strength of turbulence is challenging through gas line observations because of the requirement for high spatial and spectral resolution data, and an exquisite determination of the temperature. In this work, taking the well-known HD 163296 disk as an example, we investigated the contrast of gaps identified in high angular resolution continuum images as a probe for the level of turbulence. With self-consistent radiative transfer models, we simultaneously analyzed the radial brightness profiles along the disk major and minor axes, and the azimuthal brightness profiles of the B67 and B100 rings. By fitting all the gap contrasts measured from these profiles, we constrained the gas-to-dust scale height ratio $\Lambda$ to be $3.0_{-0.8}^{+0.3}$, $1.2_{-0.1}^{+0.1}$ and ${\ge}\,6.5$ for the D48, B67 and B100 regions, respectively. The varying gas-to-dust scale height ratios indicate that the degree of dust settling changes with radius. The inferred values for $\Lambda$ translate into a turbulence level of $\alpha_{\rm turb}\,{<}\,3\times10^{-3}$ in the D48 and B100 regions, which is consistent with previous upper limits set by gas line observations. However, turbulent motions in the B67 ring are strong with $\alpha_{\rm turb}\,{\sim}1.2\,{\times}\,10^{-2}$. Due to the degeneracy between $\Lambda$ and the depth of dust surface density drops, the turbulence strength in the D86 gap region is not constrained.

SILVERRUSH. XII. Intensity Mapping for Lyα Emission Extending over 100–1000 Comoving Kpc around z ∼ 2−7 LAEs with Subaru HSC-SSP and CHORUS Data

We conduct intensity mapping to probe for extended diffuse Lyα emission around Lyα emitters (LAEs) at z ∼2−7, exploiting very deep (∼26 mag at 5σ) and large-area (∼4.5 deg2) Subaru/Hyper Suprime-Cam narrowband (NB) images and large LAE catalogs consisting of a total of 1540 LAEs at z = 2.2, 3.3, 5.7, and 6.6 obtained by the HSC-SSP and CHORUS projects. We calculate the spatial correlations of these LAEs with ∼1–2 billion pixel flux values of the NB images, deriving the average Lyα surface brightness (SBLyα ) radial profiles around the LAEs. By carefully estimating systematics such as fluctuations of sky background and point-spread functions, we detect Lyα emission at 100–1000 comoving kpc around z = 3.3 and 5.7 LAEs at the 3.2σ and 3.7σ levels, respectively, and tentatively (=2.0σ) at z = 6.6. The emission is as diffuse as ∼10−20–10−19 erg s−1 cm−2 arcsec−2 and extended beyond the virial radius of a dark matter halo with a mass of 1011 M ⊙. While the observed SBLyα profiles have similar amplitudes at z = 2.2–6.6 within the uncertainties, the intrinsic SBLyα profiles (corrected for the cosmological dimming effect) increase toward high redshifts. This trend may be explained by increasing hydrogen gas density due to the evolution of the cosmic volume. Comparisons with theoretical models suggest that extended Lyα emission around an LAE is powered by resonantly scattered Lyα photons in the CGM and IGM that originate from the inner part of the LAE and/or neighboring galaxies around the LAE.

On the surface brightness radial profile of the extended γ-ray sources

The morphology of the extended $\gamma$-ray source is governed by the propagation process of parent relativistic particles. In this paper, we investigate the surface brightness radial profile of extended $\gamma$-ray sources illuminated by cosmic ray protons and electrons, considering the radiation mechanisms, projection effects, and the response of instruments. We found that the parent particle species and the propagation process can cause considerable differences in the observed radial profiles. Thus, the surface brightness profile can be used as a unique tool to identify the radiation mechanism and the propagation process of the parent particles. In addition, We also discuss the possible implications regarding the latest discoveries from very/ultra-high energy $\gamma$-ray instruments like LHAASO and HAWC.

A Disk and No Signatures of Tidal Distortion in the Galaxy “Lacking” Dark Matter NGC 1052-DF2

Abstract Using ultra-deep imaging (μ g = 30.4 mag arcsec−2; 3σ, 10″ × 10″), we probed the surroundings of the first galaxy “lacking” dark matter (DM) KKS2000[04] (NGC 1052–DF2). Signs of tidal stripping in this galaxy would explain its claimed low content of DM. However, we find no evidence of tidal tails. In fact, the galaxy remains undisturbed down to a radial distance of 80″. This radial distance triples previous spatial explorations of the stellar distribution of this galaxy. In addition, the distribution of its globular clusters (GCs) is not extended in relation to the bulk of the galaxy (the radius containing half of the GCs is 21″). We also found that the surface brightness radial profiles of this galaxy in the g and r bands decline exponentially from 35″ to 80″. Together with a constant ellipticity and position angle in the outer parts of the galaxy, this strongly suggests the presence of a low-inclination disk. This is consistent with the evidence of rotation found for this object. This finding implies that the dynamical mass of this galaxy is a factor of 2 higher than previously reported, which brings the DM content of this galaxy in line with galaxies of similar stellar mass.

PSP/WISPR observations of dust density depletion near the Sun

Context. In 1929, Russell predicted that dust particles cannot survive in a region close to any star, hence giving justification for a dust free zone to exist inside a certain distance from the star. This theoretical prediction has not been confirmed, even with our Sun. Aims. We use the unique vantage points and new perspectives of the Parker Solar Probe (PSP) mission to study the dust environment close to the Sun with imaging observations from the Wide Field Imager for Solar Probe (WISPR) as PSP orbits, progressively closer to the Sun (PSP will ultimately reach a perihelion distance of 9.86 R⊙). Methods. We analyze the radial brightness profile of the axis of symmetry of the F-corona in the WISPR images obtained from heliocentric distances between about 0.350 AU (75 R⊙) and 0.129 AU (28 R⊙) to detect any change from earlier observations. Historically, at observer locations between 1 and 0.3 AU, the brightness of the axis of symmetry has been shown to fall off as a power law of solar distance, r−n, with an exponent of n = 2.3. Results. We show that as PSP approaches its perihelion distance of 28 R⊙ (orbits 4 and 5), the radial gradient of the brightness profile of the axis of symmetry of the F-corona gradually becomes less steep, starting at about 19 R⊙ down to the shortest elongations reached with current WISPR observations at about 7.65 R⊙. This observational signature is modeled with an ad hoc homogeneous dust density model (i.e., it is not based on any physical model) along the symmetry axis of the zodiacal dust cloud, which (1) varies as the historical density profile, r−1.3, down to 19 R⊙, then (2) stays approximately constant down to 10 R⊙, and finally (3) decreases exponentially to become zero at 3 R⊙. The density profile below 19 R⊙ is accomplished by using a multiplier on the historical density profile that decreases linearly down to 3 R⊙. The distance dependence and range of the multiplier were chosen to best match the brightness observations below 30 R⊙. Conclusions. The observed brightness decrease in the axis of symmetry is interpreted as the signature of the existence of a dust density depletion zone between about 19 R⊙ and 3 R⊙, which at the inner limit of WISPR’s field of view of 7.65 R⊙ has a dust density that is ~5% lower than the density at 19 R⊙, instead of the expected density which is three times if no depletion zone exists. No noticeable variations in the brightness of the F-corona axis of symmetry were observed from 2018 to 2020.

The Structure of Stellar Disks in Isolated Lenticular Galaxies∗

Abstract We have obtained imaging data in two photometric bands, g and r, for a sample of 42 isolated lenticular galaxies with the Las Cumbres Observatory 1 m telescope network. We have analyzed the structure of their large-scale stellar disks. The parameters of surface-brightness distributions have been determined including the radial profile shapes and disk thicknesses. After inspecting the radial brightness profiles, all of the galaxies have been classified into pure exponential (Type I), truncated (Type II), and antitruncated (Type III) disks. By comparing the derived statistics of the radial profile shapes with our previous sample of cluster S0s, we noted a prominent difference between stellar disks of S0s galaxies in quite rarefied environments and in clusters: it is only in sparse environments that Type II disks, with profile truncations, can be found. This finding implies probable different dynamical history of S0 galaxies in different environments.

Surface Brightness Plateau in S4G Galaxies

Using 3.6-μm data from 2112 galaxies, we show that, contrary to widely held expectations of a continuous steep decline, radial surface brightness profiles of galaxies tend to flatten and form extended plateaus beyond 27–28 magAB/arcsec2. This phenomenon could be explained by the presence of extended stellar populations dominated by low-mass stars in galactic outskirts. The flattening of radial brightness profiles questions the artificial exponential extrapolations of brightness data and the automatic assumption that light always declines considerably faster than mass density, presenting an empirical challenge for the dark matter hypothesis.

Frankenstein: protoplanetary disc brightness profile reconstruction at sub-beam resolution with a rapid Gaussian process

ABSTRACT Interferometric observations of the mm dust distribution in protoplanetary discs are now showing a ubiquity of annular gap and ring substructures. Their identification and accurate characterization are critical to probing the physical processes responsible. We present frankenstein (frank), an open source code that recovers axisymmetric disc structures at a sub-beam resolution. By fitting the visibilities directly, the model reconstructs a disc’s 1D radial brightness profile non-parametrically using a fast (≲1 min) Gaussian process. The code avoids limitations of current methods that obtain the radial brightness profile either by extracting it from the disc image via non-linear deconvolution at the cost of reduced fit resolution or by assumptions placed on the functional forms of disc structures to fit the visibilities parametrically. We use mock Atacama Large Millimeter Array observations to quantify the method’s intrinsic capability and its performance as a function of baseline-dependent signal-to-noise ratio. Comparing the technique to profile extraction from a clean image, we motivate how our fits accurately recover disc structures at a sub-beam resolution. Demonstrating the model’s utility in fitting real high- and moderate-resolution observations, we conclude by proposing applications to address open questions on protoplanetary disc structure and processes.

Searching for dark matter signals from local dwarf spheroidal galaxies at low radio frequencies in the GLEAM survey

ABSTRACT The search for emission from weakly interacting massive particle (WIMP) dark matter annihilation and decay has become a multipronged area of research not only targeting a diverse selection of astrophysical objects, but also taking advantage of the entire electromagnetic spectrum. The decay of WIMP particles into standard model particles has been suggested as a possible channel for synchrotron emission to be detected at low radio frequencies. Here, we present the stacking analysis of a sample of 33 dwarf spheroidal (dSph) galaxies with low-frequency (72–231 MHz) radio images from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. We produce radial surface brightness profiles of images centred upon each dSph galaxy with background radio sources masked. We remove 10 fields from the stacking due to contamination from either poorly subtracted, bright radio sources or strong background gradients across the field. The remaining 23 dSph galaxies are stacked in an attempt to obtain a statistical detection of any WIMP-induced synchrotron emission in these systems. We find that the stacked radial brightness profile does not exhibit a statistically significant detection above the 95 per cent confidence level of ∼1.5 mJy beam−1. This novel technique shows the potential of using low-frequency radio images to constrain fundamental properties of particle dark matter.

Consistent Dust and Gas Models for Protoplanetary Disks. III. Models for Selected Objects from the FP7 DIANA Project**EU FP7-SPACE-2011 project 284405 “DiscAnalysis” (Analysis and Modeling of Multi-wavelength Observational Data from Protoplanetary Discs).

The European FP7 project DIANA has performed a coherent analysis of a large set of observational data of protoplanetary disks by means of thermo-chemical disk models. The collected data include extinction-corrected stellar UV and X-ray input spectra (as seen by the disk), photometric fluxes, low and high resolution spectra, interferometric data, emission line fluxes, line velocity profiles and line maps, which probe the dust, polycyclic aromatic hydrocarbons (PAHs) and the gas in these objects. We define and apply a standardized modeling procedure to fit these data by state-of-the-art modeling codes (ProDiMo, MCFOST, MCMax), solving continuum and line radiative transfer (RT), disk chemistry, and the heating and cooling balance for both the gas and the dust. 3D diagnostic RT tools (e.g., FLiTs) are eventually used to predict all available observations from the same disk model, the DIANA-standard model. Our aim is to determine the physical parameters of the disks, such as total gas and dust masses, the dust properties, the disk shape, and the chemical structure in these disks. We allow for up to two radial disk zones to obtain our best-fitting models that have about 20 free parameters. This approach is novel and unique in its completeness and level of consistency. It allows us to break some of the degeneracies arising from pure Spectral Energy Distribution (SED) modeling. In this paper, we present the results from pure SED fitting for 27 objects and from the all inclusive DIANA-standard models for 14 objects. Our analysis shows a number of Herbig Ae and T Tauri stars with very cold and massive outer disks which are situated at least partly in the shadow of a tall and gas-rich inner disk. The disk masses derived are often in excess to previously published values, since these disks are partially optically thick even at millimeter wavelength and so cold that they emit less than in the Rayleigh–Jeans limit. We fit most infrared to millimeter emission line fluxes within a factor better than 3, simultaneously with SED, PAH features and radial brightness profiles extracted from images at various wavelengths. However, some line fluxes may deviate by a larger factor, and sometimes we find puzzling data which the models cannot reproduce. Some of these issues are probably caused by foreground cloud absorption or object variability. Our data collection, the fitted physical disk parameters as well as the full model output are available to the community through an online database (http://www.univie.ac.at/diana).

Stellar cusp and warm dust at the heart of NGC 1068

Aims. Establishing precisely how stars and interstellar medium distribute within the central 100 pc area around an AGN, down to the pc scale, is key to understanding the late stages of transfer of matter onto the accretion disc. Methods. Using adaptive optics-assisted (SPHERE-VLT) near-IR images in the H band, Ks band, and several narrow bands of the Seyfert 2 galaxy NGC 1068, we analysed the radial distribution of brightness in the central r &lt; 100 pc area down to the pc scale. The median-averaged radial profiles are fitted by a cusp (power law) plus a central point source. A simple radiative transfer model is also used to interpret the data. Results. We find that the fit of radial brightness profiles beyond 10 pc is done quite precisely at Ks band by a cusp of exponent −2.0 plus a central point source and by a cusp of exponent −1.2 at H. The difference of exponents between H band and Ks band can be explained by differential extinction, provided that the distribution of dust is itself cuspy, with an exponent −1.0. However, the derived stellar density is found to follow a r−4 cusp, which is much steeper than any other cusp, either theoretically predicted around a massive black hole, or observed in the centre of early- and late-type galaxies or in mergers. Introducing a segregation in the stellar population with a central excess of giant stars leads to a somewhat less steep exponent; however, the de-reddened luminosity of the stellar cusp, as well as the mass of dust and gas all appear much too high to be realistic. An alternative scenario, where the Ks-band profile is well fitted by a combination of radiation from a stellar cusp identical to the H-band profile and thermal emission of warm/hot dust heated by the central engine appears much more satisfactory. NGC 1068 is shown to satisfy a relationship between half-light radius, cusp luminosity, and exponent that we established using a sample of luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies ULIRGs. This suggests that the cusp is the remnant of a recent starburst. We identify the central point-like source with the very hot dust at the internal wall of the putative torus and derive an intrinsic luminosity that requires a central extinction AK ≈ 8, a value consistent with predictions by several torus models. Conclusions. The overall picture revealed by this study is closely consistent with the scheme of a central rather steep stellar cusp, embedded in a diluted medium of warm dust, while a compact and dense structure identified with the putative torus is required to interpret a highly reddened point-like central source of very hot dust.

The scattered disc of HR 8799

HR~8799 is a young F0-type star with four directly imaged giant planets and two debris belts, one located exterior and another one interior to the region occupied by the planetary orbits. Having an architecture similar to that of our Solar System, but also revealing dissimilarities such as high masses of planets, a huge radial extent and a high mass of the outer debris belt, HR~8799 is considered to be a benchmark to test formation and evolution models of planetary systems. Here we focus on the outer debris ring and its relation to the planets. We demonstrate that the models of the outer disc, proposed previously to reproduce Herschel observations, are inconsistent with the ALMA data, and vice versa. In an attempt to find a physically motivated model that would agree with both observational sets, we perform collisional simulations. We show that a narrow planetesimal belt and a radiation pressure induced dust halo cannot account for the observed radial brightness profiles. A single, wide planetesimal disc does not reproduce the data either. Instead, we propose a two-population model, comprising a Kuiper-Belt-like structure of a low-eccentricity planetesimal population ("the classical Kuiper Belt") and a high-eccentricity population of comets ("scattered disc"). We argue that such a structure of the exo-Kuiper belt of HR~8799 could be explained with planet migration scenarios analogous to those proposed for the Kuiper Belt of the Solar System.

Dwarf galaxies: evidence of differential tidal effects in the Large Magellanic Cloud

AbstractWe built the most extended stellar density and/or surface brightness radial profiles for 13 old Large Magellanic Cloud (LMC) globular clusters (GCs). The studied GCs located farther than ~ 5 kpc from the LMC center would not seem to present any hint of extended stellar structures, while those closer than ~ 5 kpc do show extended structures. Such an excess of stars tightly depends on the position of the GCs, so that the closer the GC to the LMC center, the larger the excess of stars. Furthermore, the GC radii also show a remarkable trend with the position of the GC in the LMC disc. These outcomes can be fully interpreted in the light of the known GC radial velocity disc-like kinematics, from which GCs have been somehow mostly experiencing the influence of the LMC gravitational field at their respective mean distances from the LMC center.