Mag Arcsec Research Articles

A Mysterious Ring in Dark Space?

Abstract We report the discovery of a low-surface-brightness (27.42 mag arcsec−2 in g band) nebula, which has a ring-like shape in the Beijing–Arizona Sky Survey (BASS). Positive detections have been found in multiband data from far-ultraviolet to far-infrared, except the z band from BASS and W1, W2 from the Wide-field Infrared Survey Explorer. The reddening of the nebula E(B − V) ∼ 0.02 mag is estimated from Infrared Astronomical Satellite (IRAS) 100 μm intensity and H i column density. With the help of the 3D reddening map from Pan-STARRS 1, the Two Micron All Sky Survey, and Gaia, the distance to the nebula of about 500 pc from Earth is derived. Such a low-surface-brightness nebula whose energy can be interpreted by the diffuse Galactic light could account for the optical counterpart of the infrared cirrus, which was detected by IRAS more than 30 yr ago. The ring-like structure might be the ultimate phase of an evolved planetary nebula, while the central white dwarf star has been ejected from the nebula for an unclear reason. On the other hand, the ring structure being a superposition of two close filaments might be another reasonable explanation. Considering the lack of spectroscopic data and uncertainty in the distance measurement, these interpretations need to be checked by future observations.

Structure of Brightest Cluster Galaxies and Intracluster Light

Observations of 170 local ($z\lesssim0.08$) galaxy clusters in the northern hemisphere have been obtained with the Wendelstein Telescope Wide Field Imager (WWFI). We correct for systematic effects such as point-spread function broadening, foreground star contamination, relative bias offsets, and charge persistence. Background inhomogeneities induced by scattered light are reduced down to $\Delta {\rm SB} > 31~g'$ mag arcsec$^{-2}$ by large dithering and subtraction of night-sky flats. Residual background inhomogeneities brighter than ${\rm SB}_{\sigma}< 27.6~g'$ mag arcsec$^{-2}$ caused by galactic cirrus are detected in front of 23% of the clusters. However, the large field of view allows discrimination between accretion signatures and galactic cirrus. We detect accretion signatures in the form of tidal streams in 22%, shells in 9.4%, and multiple nuclei in 47% of the Brightest Cluster Galaxies (BCGs) and find two BCGs in 7% of the clusters. We measure semimajor-axis surface brightness profiles of the BCGs and their surrounding Intracluster Light (ICL) down to a limiting surface brightness of ${\rm SB} = 30~g'$ mag arcsec$^{-2}$. The spatial resolution in the inner regions is increased by combining the WWFI light profiles with those that we measured from archival \textit{Hubble Space Telescope} images or deconvolved WWFI images. We find that 71% of the BCG+ICL systems have surface brightness (SB) profiles that are well described by a single S\'ersic (SS) function, whereas 29% require a double S\'ersic (DS) function to obtain a good fit. We find that BCGs have scaling relations that differ markedly from those of normal ellipticals, likely due to their indistinguishable embedding in the ICL.

The haloes and environments of nearby galaxies (HERON) – II. The outer structure of edge-on galaxies

ABSTRACT The haloes and environments of nearby galaxies (HERON) project is aimed at studying haloes and low surface brightness (LSB) details near galaxies. In this second HERON paper, we consider in detail deep imaging (down to surface brightness of ∼28 mag arcsec−2 in the r band) for 35 galaxies, viewed edge-on. We confirm a range of LSB features previously described in the literature but also report new ones. We classify the observed outer shapes of the galaxies into three main types (and their prototypes): disc/diamond-like (NGC 891), oval (NGC 4302), and boxy (NGC 3628). We show that the shape of the outer disc in galaxies does not often follow the general 3D model of an exponential disc: 17 galaxies in our sample exhibit oval or even boxy isophotes at the periphery. Also, we show that the less flattened the outer disc, the more oval or boxy its structure. Many galaxies in our sample have an asymmetric outer structure. We propose that the observed diversity of the galaxy outer shapes is defined by the merger history and its intensity: if no recent multiple minor or single major merging took place, the outer shape is diamond-like or discy. On the contrary, interacting galaxies show oval outer shapes, whereas recent merging appears to transform the outer shape to boxy.

VEGAS: a VST Early-type GAlaxy Survey

Context. This paper is based on the multi-band VST Early-type GAlaxy Survey (VEGAS) with the VLT Survey Telescope (VST). We present new deep photometry of the IC 1459 group in g and r band. Aims. The main goal of this work is to investigate the photometric properties of the IC 1459 group, and to compare our results with those obtained for other galaxy groups studied in VEGAS, in order to provide an initial view of the variation of their properties as a function of the evolution of the system. Methods. For all galaxies in the IC 1459 group, we fit isophotes and extract the azimuthally averaged surface-brightness profiles, the position angle, and ellipticity profiles as a function of the semi-major axis. We also extract the average colour profile. In each band, we estimate the total magnitude, effective radius, mean colour, and total stellar mass for each galaxy in the group. We then look at the structure of the brightest galaxies and the faint features in their outskirts, considering also the intragroup component. Results. The wide field of view, long integration time, high angular resolution, and arcsec-level seeing of OmegaCAM at VST allow us to map the light distribution of IC 1459 down to a surface brightness level of 29.26 mag arcsec−2 in g band and 28.85 mag arcsec−2 in r band, and out to 7−10Re, and to detect the optical counterpart of HI gas around IC 1459. We also carry out an in-depth exploration of three low-density environments and provide information to understand how galaxy and group properties change with the group evolution stage. Conclusions. Good agreement is found between our results and predictions of numerical simulations regarding the structural properties of the brightest galaxies of the groups. We suggest that the structure of the outer envelope of he brightest cluster galaxies (i.e. the signatures of past mergers and tidal interactions), the intra-group light, and the HI amount and distribution may be used as indicators of the evolutionary stage and mass assembly of galaxy groups.

KMTNet Nearby Galaxy Survey II. Searching for Dwarf Galaxies in Deep and Wide-field Images of the NGC 1291 System

Abstract We present newly discovered dwarf galaxy candidates in deep and wide-field images of NGC 1291 obtained with the Korea Microlensing Telescope Network. We identify 15 dwarf galaxy candidates by visual inspection. Using imaging simulations, we demonstrate that the completeness rate of our detection is greater than 70% for the central surface-brightness value of μ 0,R ≲ 26 mag arcsec−2 and for magnitudes M R ≲ −10 mag. The structural and photometric properties of the dwarf galaxy candidates appear to be broadly consistent with those of ordinary dwarf galaxies in nearby groups and clusters, with μ 0,R ∼ 22.5 to 26.5 mag arcsec−2 and effective radii of 200 pc to 1 kpc. The dwarf galaxy candidates show a concentration toward NGC 1291 and tend to be redder the closer they are to the center, possibly indicating that they are associated with NGC 1291. The dwarf candidates presented in this paper appear to be bluer than those in denser environments, revealing that the quenching of star formation in dwarf galaxies is susceptible to the environment, while the morphology shaping is not.

Parker Solar Probe Observations of a Dust Trail in the Orbit of (3200) Phaethon

Abstract We present the identification and preliminary analysis of a dust trail following the orbit of (3200) Phaethon as seen in white-light images recorded by the Wide-field Imager for Parker Solar Probe (WISPR) instrument on the NASA Parker Solar Probe (PSP) mission. During PSP's first solar encounter in 2018 November, a dust trail following Phaethon’s orbit was visible for several days and crossing two fields of view. Preliminary analyses indicate this trail to have a visual magnitude of 15.8 ± 0.3 per pixel and a surface brightness of 25.0 mag arcsec−2 as seen by PSP/WISPR from a distance of ∼0.2 au from the trail. We estimate the total mass of the stream to be ∼(0.4–1.3) × 1012 kg, which is consistent with, though slightly underestimates, the assumed mass of the Geminid stream, but is far larger than the current dust production that Phaethon could support. Our results imply that we are observing a natural clustering of at least some portion of the Geminid meteor stream through its perihelion, as opposed to dust produced more recently from perihelion activity of Phaethon.

Predicting the accurate period of true dawn using a third-degree polynomial model

ABSTRACT Sky brightness data have been widely used to document the light pollution and to localise the direction of the sun during both dawn and dusk twilight. The use of the most common data analysis techniques of graphical interpretation can give less accurate final results. This study used a Sky Quality Metre to record the brightness of night sky in mag arcsec−2 at Depok city of Indonesia during 26 days for the months of June and July 2015. Analytical technique of third-degree polynomial was established to predict dawn astronomical twilight and provides a good fit to the experimental data. The modelling results have been experimentally verified that astronomical twilight in the morning at Depok city with its environment begins when the sun is at the position of −14° ± 0.6 below the eastern horizon and ends at sunrise. In conclusion, the Muslim communities of Indonesia begin their dawn prayer 6° too early and have been found to calculate the true dawn incorrectly when the centre of the sun is at a depression angle of −20° below the horizon.

Modelling the Canes Venatici I dwarf spheroidal galaxy

The aim of this work is to find a progenitor for Canes Venatici I (CVn I), under the assumption that it is a dark matter free object that is undergoing tidal disruption. With a simple point mass integrator, we searched for an orbit for this galaxy using its current position, position angle, and radial velocity in the sky as constraints. The orbit that gives the best results has the pair of proper motions μα = −0.099 mas yr−1 and μδ = −0.147 mas yr−1, that is, an apogalactic distance of 242.79 kpc and a perigalactic distance of 20.01 kpc. Using a dark matter free progenitor that undergoes tidal disruption, the best-fitting model matches the final mass, surface brightness, effective radius, and velocity dispersion of CVn I simultaneously. This model has an initial Plummer mass of 2.47 × 107 M⊙ and a Plummer radius of 653 pc, producing a remnant after 10 Gyr with a final mass of 2.45 × 105 M⊙, a central surface brightness of 26.9 mag arcsec−2, an effective radius of 545.7 pc, and a velocity dispersion with the value 7.58 km s−1. Furthermore, it is matching the position angle and ellipticity of the projected object in the sky.

The case for strangulation in low-mass hosts: DDO 113

ABSTRACT We investigate the case for environmental quenching of the Fornax-mass satellite DDO 113, which lies only 9 kpc in projection from its host, the Large-Magellanic-Cloud-mass galaxy NGC 4214. DDO 113 was quenched about 1 Gyr ago and is virtually gas-free, while analogs in the field are predominantly star-forming and gas-rich. We use deep imaging obtained with the Large Binocular Telescope to show that DDO 113 exhibits no evidence of tidal disruption to a surface brightness of μV ∼ 29 mag arcsec−2, based on both unresolved emission and resolved stars. Mass-analogs of DDO 113 in Illustris-1 with similar hosts, small projected separations, and no significant tidal stripping first fell into their host halo 2–6 Gyr ago, showing that tidal features (or lack thereof) can be used to constrain infall times in systems where there are few other constraints on the orbit of the satellite. With the infall time setting the clock for environmental quenching mechanisms, we investigate the plausibility of several such mechanisms. We find that strangulation, the cessation of cold gas inflows, is likely the dominant quenching mechanism for DDO 113, requiring a time-averaged mass-loading factor of η = 6–11 for star-formation-driven outflows that is consistent with theoretical and observational constraints. Motivated by recent numerical work, we connect DDO 113’s strangulation to the presence of a cool circumgalactic medium (CGM) around NGC 4214. This discovery shows that the CGM of low-mass galaxies can affect their satellites significantly and motivates further work on understanding the baryon cycle in low-mass galaxies.

Hunting ghosts: the iconic stellar stream(s) around NGC 5907 under scrutiny

Stellar streams are regarded as crucial objects for testing galaxy formation models because their morphology traces the underlying potentials and their occurrence tracks the assembly history of the galaxies. The existence of one of the most iconic stellar streams, the double loop around NGC 5907, has recently been questioned by new observations with the Dragonfly telescope. This new work only finds parts of the stream, even though a 1σ surface brightness limit of 30.3 mag arcsec−2 in the g band was reached. Using 7.2 h of luminance L-band imaging with the Milanković 1.4-m telescope, we have reobserved the putative double-loop part to confirm or reject this assessment. We do not find signs of the double loop, but see only a single knee-shaped stellar stream. Comparing our results to the data by the Dragonfly team, we find the same features. Our observations reach a 1σ surface brightness limit of 29.7 mag arcsec−2 in the g band. These findings emphasize the need for independent confirmation of detections of very low-surface brightness features.

Probing the merger history of red early-type galaxies with their faint stellar substructures

Several detailed observations, such as those carried out at the Canada-France-Hawaii Telescope (CFHT), have revealed prominent Low Surface Brightness (LSB) fine structures that lead to a change in the apparent morphology of galaxies. Previous photometry surveys have developed observational techniques which make use of the diffuse light detected in the external regions of galaxies. In these studies, the outer perturbations have been identified and classified. These include tidal tails, stellar streams, and shells. These structures serve as tracers for interacting events and merging events and retain some memory of the mass assembly of galaxies. Cosmological numerical simulations are required to estimate their visibility timescale, among other properties, in order to reconstruct the merger history of galaxies. In the present work, we analyze a hydrodynamical cosmological simulation to build up a comprehensive interpretation of the properties of fine structures. We present a census of several types of LSB fine structures compiled using a visual inspection of individual snapshots at various points in time. We reconstruct the evolution of the number of fine structures detected around an early-type galaxy and we compare it with the merger history of the galaxy. We find that most fine structures are associated with major and intermediate mass merger events. Their survival timescale ranges between 0.7 and 4 Gyr. Shells and streams remain visible for a longer time, while tidal tails have a shorter lifetime. These estimates for the survival time of collisional debris provide clues for the interpretation of the shape and frequency of fine structures observed in deep images with regard to their mass assembly. We find that the detectability of stellar streams is most sensitive at the surface brightness limit, demonstrating greater visibility at the deepest surface brightness level used in our simulation. We see between two and three times more streams based on a surface brightness cut of 33 mag arcsec−2 than with 29 mag arcsec−2. We find that the detection of shells is strongly dependent upon the projection angle.

A detailed study of Andromeda XIX, an extreme local analogue of ultradiffuse galaxies

ABSTRACT With a central surface brightness of μ0 = 29.3 mag arcsec−2, and half-light radius of $r_{\rm half}=3.1^{+0.9}_{-1.1}$ kpc, Andromeda XIX (And XIX) is an extremely diffuse satellite of Andromeda. We present spectra for ∼100 red giant branch stars in this galaxy, plus 16 stars in a nearby stellar stream. With this exquisite data set, we re-derive the properties of And XIX, measuring a systemic velocity of 〈vr〉 = −109.0 ± 1.6 km s−1 and a velocity dispersion of $\sigma _{vr} = 7.8^{+1.7}_{-1.5}$ km s−1 (higher than derived in our previous work). We marginally detect a velocity gradient along the major axis of $\frac{{\rm d} v}{{\rm d}\chi }= -2.1\pm 1.8~{\rm km\, s}^{-1}$ kpc−1. We find its mass-to-light ratio is higher than galaxies of comparable stellar mass (${}[M/L]_{\rm half} = 278^{+146}_{-198}\, \mathrm{M}_\odot /\mathrm{L}_\odot$), but its dynamics place it in a halo with a similar total mass to these galaxies. This could suggest that And XIX is a ‘puffed up’ dwarf galaxy, whose properties have been altered by tidal processes, similar to its Milky Way counterpart, Antlia II. For the nearby stream, we measure $v_r=-279.2\pm 3.7~{\rm km\, s}^{-1}$, and $\sigma _v=13.8^{+3.5}_{-2.6}~{\rm km\, s}^{-1}$. We measure its metallicity, and find it to be more metal rich than And XIX, implying that the two features are unrelated. Finally, And XIX’s dynamical and structural properties imply it is a local analogue to ultradiffuse galaxies (UDGs). Its complex dynamics suggest that the masses of distant UDGs measured from velocity dispersions alone should be carefully interpreted.

The Smallest Scale of Hierarchy Survey (SSH). I. Survey Description.

Abstract The Smallest Scale of Hierarchy (SSH) survey is an ongoing strategic large program at the Large Binocular Telescope, aimed at the detection of faint stellar streams and satellites around 45 late-type dwarf galaxies located in the Local Universe within ≃10 Mpc. SSH exploits the wide-field, deep photometry provided by the Large Binocular Cameras in the two wide filters g and r. This paper describes the survey, its goals, and the observational and data reduction strategies. We present preliminary scientific results for five representative cases (UGC 12613, NGC 2366, UGC 685, NGC 5477 and UGC 4426) covering the whole distance range spanned by the SSH targets. We reach a surface brightness limit as faint as μ(r) ∼ 31 mag arcsec−2 both for targets closer than 4−5 Mpc, which are resolved into individual stars, and for more distant targets through the diffuse light. Our analysis reveals the presence of extended low surface brightness stellar envelopes around the dwarfs, reaching farther out than what traced by the integrated light, and as far out as, or even beyond, the observed H I disk. Stellar streams, arcs, and peculiar features are detected in some cases, indicating possible perturbation, accretion, or merging events. We also report on the discovery of an extreme case of Ultra Diffuse Galaxy (μg(0) = 27.9 mag/arcsec2) in the background of one of our targets, to illustrate the power of the survey in revealing extremely low surface brightness systems.

Dwarf Galaxy Discoveries from the KMTNet Supernova Program. II. The NGC 3585 Group and Its Dynamical State*

Abstract We present our discovery and analysis of dwarf galaxies in the NGC 3585 galaxy group by the Korea Microlensing Telescope Network (KMTNet) Supernova Program. Using deep stack images reaching ≃28 mag arcsec−2 in BVI, we discovered 46 dwarf galaxy candidates distributed in a field of 7 square degrees. The dwarf galaxy candidates exhibit a central surface brightness as faint as μ 0,V = 26.2 mag arcsec−2, with effective radii larger than 150 pc and total absolute magnitudes brighter than M V ≈ −10 mag, if at the distance of NGC 3585. The dwarf galaxy surface number density decreases with projected distance from NGC 3585. We estimate the background contamination to be about 20% based both on the number density profile and on diffuse galaxy counts in a control field. The dwarf galaxy colors and Sérsic structural parameters are consistent with those found for other dwarf galaxies. Unusually, there is no indication of a change in color or brightness in the dwarf galaxy candidates with projected distance from the group center. Approximately 20% of the candidates contain an unresolved nucleus. The nucleated fraction is larger for brighter (and redder) galaxies, but is independent of distance from the group center. We identify four ultra-diffuse galaxy candidates, all near the group center. We interpret these spatial properties as suggesting that the NGC 3585 group might be dynamically younger than the typical group. The galaxy luminosity function of the NGC 3585 group has a faint-end slope of α ≈ −1.39, which is roughly consistent with the slopes found for other nearby groups. The possible dependence of the slope on global group properties is still unclear and continues to motivate our homogeneous deep survey of dozens of nearby groups.

A Catalog of Galaxies in the Direction of the Perseus Cluster

Abstract We present a catalog of 5437 morphologically classified sources in the direction of the Perseus galaxy cluster core, among them 496 early-type low-mass galaxy candidates. The catalog is primarily based on V-band imaging data acquired with the William Herschel Telescope, which we used to conduct automated source detection and derive photometry. We additionally reduced archival Subaru multiband imaging data in order to measure aperture colors and perform a morphological classification, benefiting from 0.″5 seeing conditions in the r-band data. Based on morphological and color properties, we extracted a sample of early-type low-mass galaxy candidates with absolute V-band magnitudes in the range of −10 to −20 mag. In the color–magnitude diagram, the galaxies are located where the red sequence for early-type cluster galaxies is expected, and they lie on the literature relation between absolute magnitude and Sérsic index. We classified the early-type dwarf candidates into nucleated and nonnucleated galaxies. For the faint candidates, we found a trend of increasing nucleation fraction toward brighter luminosity or higher surface brightness, similar to what is observed in other nearby galaxy clusters. We morphologically classified the remaining sources as likely background elliptical galaxies, late-type galaxies, edge-on disk galaxies, and likely merging systems and discussed the expected contamination fraction through non-early-type cluster galaxies in the magnitude-size surface brightness parameter space. Our catalog reaches its 50% completeness limit at an absolute V-band luminosity of −12 mag and a V-band surface brightness of 26 mag arcsec−2. This makes it the largest and deepest catalog with coherent coverage compared to previous imaging studies of the Perseus cluster.

Nature of a shell of young stars in the outskirts of the Small Magellanic Cloud

Context. Understanding the evolutionary history of the Magellanic Clouds requires an in-depth exploration and characterization of the stellar content in their outer regions, which ultimately are key to tracing the epochs and nature of past interactions. Aims. We present new deep images of a shell-like overdensity of stars in the outskirts of the Small Magellanic Cloud (SMC). The shell, also detected in photographic plates dating back to the fifties, is located at ∼1.9° from the center of the SMC in the north-east direction. Methods. The structure and stellar content of this feature were studied with multiband, optical data from the Survey of the MAgellanic Stellar History (SMASH) carried out with the Dark Energy Camera on the Blanco Telescope at Cerro Tololo Inter-American Observatory. We also investigate the kinematic of the stars in the shell using the Gaia Data Release 2. Results. The shell is composed of a young population with an age ∼150 Myr, with no contribution from an old population. Thus, it is hard to explain its origin as the remnant of a tidally disrupted stellar system. The spatial distribution of the young main-sequence stars shows a rich sub-structure, with a spiral arm-like feature emanating from the main shell and a separated small arc of young stars close to the globular cluster NGC 362. We find that the absolute g-band magnitude of the shell is Mg, shell = −10.78 ± 0.02, with a surface brightness of μg, shell = 25.81 ± 0.01 mag arcsec−2. Conclusion. We have not found any evidence that this feature is of tidal origin or a bright part of a spiral arm-like structure. Instead, we suggest that the shell formed in a recent star formation event, likely triggered by an interaction with the Large Magellanic Cloud and or the Milky Way, ∼150 Myr ago.

Impact of Space Weather on the Natural Night Sky

In 2018, Solar Cycle 24 entered a deep solar minimum. During this period, we collected night sky brightness data at Cosmic Campground International Dark Sky Sanctuary (CCIDSS) in the USA (2018 September 4–2019 January 4) and at Aotea/Great Barrier Island International Dark Sky Sanctuary (AGBIIDSS) in New Zealand (2018 March 26–August 31. These sites have artificial-light-pollution-free natural night skies. The equipment employed are identical Unihedron SQM-LU-DL meters, used as single-channel differential photometers, to scan the sky as Earth rotates on its axis. We have developed new analysis techniques which select those data points which are uninfluenced by Sun, Moon, or clouds to follow brightness changes at selected points on the celestial sphere and to measure the brightness of the airglow above its quiescent level. The 2018 natural night sky was measured to change in brightness by approximately 0.9 mag arcsec−2 at both locations. Preliminary results indicate the modulations of the light curves (brightness versus R.A.) we observed are related in complex ways to elements of space weather conditions in the near-Earth environment. In particular, episodes of increased night sky brightness are observed to be contemporaneous with geomagnetic activity, increases in mean solar wind speed, and some solar proton/electron fluence events. Charged particles in the solar wind take days to reach near-Earth environment after a coronal hole is observed to be facing in our direction. Use of this information could make it possible to predict increases in Earth’s natural night sky brightness several days in advance. What we have learned during this solar minimum leads us to search for other solar driven changes in night sky brightness as the Sun begins to move into solar maximum conditions.

Dragonfly Imaging of the Galaxy NGC 5907: A Different View of the Iconic Stellar Stream

Abstract In 2008 it was reported that the stellar stream of the edge-on spiral NGC 5907 loops twice around the galaxy, enveloping it in a giant corkscrew-like structure. Here we present imaging of this iconic object with the Dragonfly Telephoto Array, reaching a 1σ surface brightness level of μ g = 30.3 mag arcsec−2 on spatial scales of 1′ (the approximate width of the stream). We find a qualitatively different morphology from that reported in the 2008 study. The Dragonfly data do not show two loops but a single curved stream with a total length of 45′ (220 kpc). The surface brightness of the stream ranges from μ g ≈ 27.6 mag arcsec−2 to μ g ≈ 28.8 mag arcsec−2, and it extends significantly beyond the region where tidal features had previously been detected. We find a density enhancement near the luminosity-weighted midpoint of the stream which we identify as the likely remnant of a nearly disrupted progenitor galaxy. A restricted N-body simulation provides a qualitative match to the detected features. In terms of its spatial extent and stellar mass the stream is similar to Sagittarius, and our results demonstrate the efficacy of low surface brightness-optimized telescopes for obtaining maps of such large streams outside the Local Group. The census of these rare, relatively high mass events complements the census of common, low-mass ones that is provided by studies of streams in the Milky Way halo.

Detecting Thin Stellar Streams in External Galaxies: Resolved Stars and Integrated Light

Abstract The morphology of thin stellar streams can be used to test the nature of dark matter. It is therefore crucial to extend searches for globular cluster (GC) streams to other galaxies than the Milky Way. In this paper, we investigate the current and future prospects of detecting GC streams in external galaxies in resolved stars (e.g., with Wide Field InfraRed Survey Telescope (WFIRST)) and using integrated light (e.g., with Hyper Suprime-cam (HSC), the Large Synoptic Survey Telescope (LSST), and Euclid). In particular, we inject mock streams to data from the PAndAS M31 survey and produce simulated M31 backgrounds mimicking what WFIRST will observe in M31. Additionally, we estimate the distance limit to which GC streams will be observable. Our results demonstrate that for a 1 hr (1000 s) exposure, using conservative estimates, WFIRST should detect GC streams in resolved stars in galaxies out to distances of ∼3.5 Mpc (∼2 Mpc). This volume contains 199 (122) galaxies, of which &gt;90% are dwarfs. With integrated light, thin streams can be resolved out to ∼100 Mpc with HSC and LSST and to ∼600 Mpc with WFIRST and Euclid. The low surface brightness of the streams (typically &gt;30 mag arcsec−2), however, will make them difficult to detect, unless the streams originate from very young clusters. We emphasize that if the external galaxies do not host spiral arms or galactic bars, gaps in their stellar streams provide an ideal test case for evidence of interactions with dark matter subhalos. Furthermore, obtaining a large samples of thin stellar streams can help constrain the orbital structure and hence the potentials of external halos.

Properties of extragalactic thick discs recovered from ultra-deep Stripe82 imaging

Context. Thick discs can give invaluable information on the formation and evolution history of galaxies as most, if not all, disc galaxies have a thin (classical) disc and a thick disc. Aims. We study the structure of thick discs in extraordinary depth by reaching a surface brightness limit of μrdeep ∼ 28.5−29 mag arcsec−2 with combined g, r, i band images from the IAC Stripe 82 Legacy Project. Methods. We present the characterisation of the thick discs in a sample of five edge-on galaxies. A study of the radial and vertical surface brightness profiles is presented by comparing our data with point spread function (PSF) deconvolved models. Our method begins with an analysis of the background and masking processes. Then we consider the effects of the PSF through galaxy modelling. The galaxy disc components are fitted considering that the thin and thick discs are two stellar fluids that are gravitationally coupled in hydrostatic equilibrium. Results. We find that effects due to the PSF are significant when low surface brightness is reached, especially in the vertical profiles, but it can be accounted for by careful modelling. The galaxy outskirts are strongly affected by the faint wings of the PSF, mainly by PSF-redistributed light from the thin disc. This is a central problem for ultra-deep imaging. The thick-disc component is required to reach satisfactory fit results in the more complex galaxies in our sample, although it is not required for all galaxies. When the PSF is ignored, the brightness of these structures may be overestimated by up to a factor of ∼4. Conclusions. In general, our results are in good agreement with those of previous works, although we reach deeper surface brightness levels, so that the PSF effects are stronger. We obtain scale heights and mass ratios of thin and thick discs (zt, zT, and MT/Mt), which provide excellent agreement with previous studies. Our small initial sample provides evidence for aspects of a wide variety of formation theories for the thick discs in disc galaxies.