Surface Brightness Limit Research Articles

Extragalactic stellar tidal streams in the Dark Energy Survey

Context. Stellar tidal streams are a key tracer of galaxy evolution and have the potential to provide an indirect means for tracing dark matter. For the Local Group, many diffuse substructures have been identified and their link to galaxy evolution has been traced. However, the Local Group does not offer a statistically significant sample of stellar tidal streams. Thus, an analysis of a larger sample beyond the Local Group is required to better probe the frequency and characteristics of these streams to verify whether these properties are in agreement with the predictions of the ΛCDM model and its implementation in cosmological simulations, taking into account the impact of the baryonic physics modelling. Aims. The main scope of the Stellar Stream Legacy Survey is to obtain a statistically significant sample of stellar streams in the local Universe to be able to trace and study minor mergers and their contribution to galaxy evolution with respect to the ΛCDM theory. For that purpose, we are carrying out the first systematic survey of faint stellar debris from tidally disrupted dwarf satellites around nearby galaxies up to a distance of 100 Mpc. Methods. In this paper, we present a catalogue with the results of the first harvest of stellar tidal streams found by visual inspection in deep images of ∼700 galaxies from the Dark Energy Survey (DES). We also include, for the first time, a photometric characterisation of the streams obtained by measuring their surface brightnesses and colours. Results. We found a total of 63 streams in our sample at distances between 40 and 100 Mpc, including 58 that were not previously reported. We measured their average surface brightness for the g band, the r band and the z band, to be 28.35 ± 0.20, 27.81 ± 0.13, and 27.62 ± 0.09 mag arcsec−2, respectively. By applying a statistical analysis to our findings, we obtained a stream detection frequency of 9.1% ± 1.1% for the given surface brightness limit of the DES image sample, in agreement with previous studies. We identified stream progenitors in 5–14% of our stream sample, depending on the confidence level. Conclusions. The first catalogue of streams in the local Universe presented here will be complemented by future stream surveys within the Stellar Stream Legacy Survey and can be exploited in studies pertaining to galaxy evolution and cosmological models. In this work, we have learnt that the faintest measured stream surface brightness can be significantly brighter than the surface brightness limit of an image measured at the pixel level (in our case up to ∼1 mag arcsec−2 for the r band) mainly due to correlated noise present in the images.

1-arcsecond imaging of the ELAIS-N1 field at 144MHz using the LoTSS survey with the international LOFAR telescope

We present the first wide area ($2.5 deg ^2$) LOFAR high band antenna image at a resolution of $1.2 with a median noise of $ Jy beam $. It was made from an 8-hour International LOFAR Telescope (ILT) observation of the ELAIS-N1 field at frequencies ranging from 120 to 168\,MHz with the most up-to-date ILT imaging methods. This intermediate resolution falls between the highest possible resolution (0.3 achievable by using all ILT baselines and the standard 6-arcsecond resolution in the LOFAR Two-meter Sky Survey (LoTSS) image products utilising the LOFAR Dutch baselines only. This is the first demonstration of the feasibility of imaging using the ILT at a resolution of sim 1 which provides unique information on source morphology at scales that fall below the surface brightness limits at higher resolutions. The total calibration and imaging computational time is approximately 52,000 core hours, which is nearly five times more than required to produce a $6 resolution image. We also present a radio source catalogue containing 2263 sources detected over the $2.5 deg ^2$ image of the ELAIS-N1 field, with a peak intensity threshold of $5.5 The catalogue has been cross-matched with the LoTSS deep ELAIS-N1 field radio catalogue, and its flux density and positional accuracy have been investigated and corrected accordingly. We find that sim 80<!PCT!> of sources that we expect to be detectable based on their peak brightness in the LoTSS $6 resolution image are detected in this image, which is approximately a factor of two higher than for $0.3 resolution imaging in the Lockman Hole, implying there is a wealth of information on these intermediate scales.

The Dark Energy Camera Magellanic Clouds Emission-line Survey

We have used the Dark Energy Camera (DECam) on the CTIO Blanco 4 m telescope to perform a new emission-line survey of the Large Magellanic Cloud (LMC) using narrowband Hα and [S ii] filters in addition to a continuum band to create pure emission-line images. We refer to this new survey as DeMCELS, to distinguish it from the earlier Magellanic Cloud Emission-line Survey (MCELS). DeMCELS covers ∼54 deg2, encompassing most of the bright optical disk of the LMC. With DECam's pixel size of only 0.″27, our DeMCELS survey provides a seeing-limited improvement of 3–5 times over MCELS and is comparable in depth, with surface brightness limits of 3.3×10−17ergcm−2s−1arcsec−2 and 2.9×10−17ergcm−2s−1arcsec−2 in Hα and [S ii], respectively. DeMCELS provides detailed morphological information on nebulae of all scales, from the largest supershells to individual H ii regions and supernova remnants, to bubbles of emission surrounding individual stars, and even to faint structures in the diffuse ionized gas of the LMC. Many complex regions of emission show significant variations in the ratio of [S ii] to Hα—a sign of a mixture of shocks from stellar winds and/or supernovae with photoionization by embedded hot, young stars. We present the details of the observing strategy and data processing for this survey, and show selected results in comparison with previous data. A companion project for the Small Magellanic Cloud is in progress and will be reported separately. We are making these new data available to the community at large via NOIRLab’s Data Lab site.

H i Lyα Emission from a Metal-poor Cool Stream Fueling an Early Dusty Starburst

The GAMA J0913−0107 system is a rare conjunction of a submillimeter galaxy (SMG) at z ≈ 2.7 and two background QSOs with projected separations <200 kpc. Previous high-resolution QSO absorption-line spectroscopy has revealed high H i column density, extremely metal-poor (∼1% solar) gas streams in the circumgalactic medium of the SMG. Here we present deep optical integral-field spectroscopy of the system with the Keck Cosmic Web Imager (KCWI). Reaching a 2σ surface brightness limit ≈10−19 erg s−1 cm−2 arcsec−2 with ∼2 hr of integration time, we detect a filamentary Lyα nebula stretching ∼180 kpc from the SMG intercepting both QSO sightlines. This Lyα filament may correspond to the same cool gas stream penetrating through the hot halo seen in the absorption. In contrast to Lyα nebulae around QSOs, there is no obvious local source for photoionization due to the massive dust content. While uncertain, we consider the possibility that the nebula is ionized by shocks induced by the infall, obscured star formation, and/or a boosted UV background. The SMG–QSOs conjunction multiplied the efficiency of the KCWI observations, allowing a direct comparison of Lyα nebulae in two distinct environments. We find that the nebulae around the QSOs are much brighter and show steeper surface brightness profiles than the SMG nebula. This is consistent with the additional photoionization and Lyα scattering provided by the QSOs. While illustrating the challenges of detecting Lyα nebulae around SMGs, our work also demonstrates that important insights can be gained from comparative studies of high-z Lyα nebulae.

Deep HST/UVIS Imaging of the Candidate Dark Galaxy CDG-1

CDG-1 is a tight grouping of four likely globular clusters in the Perseus cluster, and a candidate dark galaxy with little or no diffuse light. Here we provide new constraints on the luminosity of any underlying stellar emission, using Hubble Space Telescope/UVIS F200LP imaging. No diffuse emission is detected, with a 2σ upper limit of F200LP > 28.1 mag arcsec−2 on the 5″ scale of CDG-1. This surface brightness limit corresponds to a 2σ lower limit of >0.5 for the fraction of the total luminosity that is in the form of globular clusters. The most likely alternative, although improbable, is that CDG-1 is a chance grouping of four globular clusters in the halo of the Perseus galaxy IC 312.

Distribution of Merging and Post-merger Galaxies in Nearby Galaxy Clusters

We study the incidence and spatial distribution of galaxies that are currently undergoing gravitational merging (M) or that have signs of being post-merger (PM) in six galaxy clusters (A754, A2399, A2670, A3558, A3562, and A3716) within the redshift range of 0.05 ≲ z ≲0.08. To this aim, we obtained Dark Energy Camera mosaics in the u′ , g′ , and r′ bands covering up to 3 × R 200 of the clusters, reaching 28 mag arcsec−2 surface brightness limits. We visually inspect u′ g′ r′ color-composite images of volume-limited (M r < −20) cluster member galaxies to identify whether galaxies are of M or PM type. We find 4% M-type and 7% PM-type galaxies in the galaxy clusters studied. By adding spectroscopic data and studying the projected phase-space diagram (PPSD) of the projected clustocentric radius and the line-of-sight velocity, we find that PM-type galaxies are more virialized than M-type galaxies, having a 1%–5% higher fraction within the escape-velocity region, while the fraction of M-type was ∼10% higher than the PM type in the intermediate environment. Similarly, on a substructure analysis, M types were found in groups in the outskirts, while PM-type populated groups were found in ubiquitous regions of the PPSD. Adopting literature-derived dynamical state indicator values, we observed a higher abundance of M types in dynamically relaxed clusters. This finding suggests that galaxies displaying post-merger features within clusters likely merged in low-velocity environments, including clusters outskirts and dynamically relaxed clusters.

3D tomography of the giant Lyα nebulae of z ≈ 3–5 radio-loud AGN

Lyα emission nebulae are ubiquitous around high-redshift galaxies and are tracers of the gaseous environment on scales out to ≳100 pkpc (proper kiloparsec). High-redshift radio galaxies (HzRGs, type-2 radio-loud quasars) host large-scale nebulae observed in the ionised gas differ from those seen in other types of high-redshift quasars. In this work, we exploit MUSE observations of Lyα nebulae around eight HzRGs (2.92 &lt; z &lt; 4.51). All of the HzRGs have large-scale Lyα emission nebulae with seven of them extended over 100 pkpc at the observed surface brightness limit (∼2 − 20 × 10−19 erg s−1 cm−2 arcsec−2). Because the emission line profiles are significantly affected by neutral hydrogen absorbers across the entire nebulae extent, we performed an absorption correction to infer maps of the intrinsic Lyα surface brightness, central velocity, and velocity width, all at the last scattering surface of the observed Lyα photons. We find the following: (i) that the intrinsic surface brightness radial profiles of our sample can be described by an inner exponential profile and a power law in the low luminosity extended part; (ii) our HzRGs have a higher surface brightness and more asymmetric nebulae than both radio-loud and radio-quiet type-1 quasars; (iii) intrinsic nebula kinematics of four HzRGs show evidence of jet-driven outflows but we find no general trends for the whole sample; (iv) a relation between the maximum spatial extent of the Lyα nebula and the projected distance between the active galactic nuclei (AGN) and the centroids of the Lyα nebula; and (v) an alignment between radio jet position angles and the Lyα nebula morphology. All of these findings support a scenario in which the orientation of the AGN has an impact on the observed nebular morphologies and resonant scattering may affect the shape of the surface brightness profiles, nebular kinematics, and relations between the observed Lyα morphologies. Furthermore, we find evidence showing that the outskirts of the ionised gas nebulae may be ‘contaminated’ by Lyα photons from nearby emission halos and that the radio jet affects the morphology and kinematics of the nebulae. Overall, this work provides results that allow us to compare Lyα nebulae around various classes of quasars at and beyond cosmic noon (z ∼ 3).

The MUSE-Faint survey

Context. Leo T (MV = −8.0) is a peculiar dwarf galaxy that stands out for being both the faintest and the least massive galaxy known to contain neutral gas and to display signs of recent star formation. It is also extremely dark-matter dominated. As a result, Leo T presents an invaluable opportunity to study the processes of gas and star formation at the limit where galaxies are found to have rejuvenating episodes of star formation. Aims. Our approach to studying Leo T involves analysing photometry and stellar spectra to identify member stars and gather information about their properties, such as line-of-sight velocities, stellar metallicities, and ages. By examining these characteristics, we aim to better understand the overall dynamics and stellar content of the galaxy and to compare the properties of its young and old stars. Methods. Our study of Leo T relies on data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, which we use to identify 58 member stars of the galaxy. In addition, we supplement this information with spectroscopic data from the literature to bring the total number of member stars analysed to 75. To further our analysis, we complement these data with Hubble Space Telescope (HST) photometry. With these combined datasets, we delve deeper into the galaxy’s stellar content and uncover new insights into its properties. Results. Our analysis reveals two distinct populations of stars in Leo T. The first population, with an age of ≲500 Myr, includes three emission-line Be stars comprising 15% of the total number of young stars. The second population of stars is much older, with ages ranging from &gt; 5 Gyr to as high as 10 Gyr. We combine MUSE data with literature data to obtain an overall velocity dispersion of σv = 7.07−1.12+1.29 km s−1 for Leo T. When we divide the sample of stars into young and old populations, we find that they have distinct kinematics. Specifically, the young population has a velocity dispersion of 2.31−1.65+2.68 km s−1, contrasting with that of the old population, of 8.14−1.38+1.66 km s−1. The fact that the kinematics of the cold neutral gas is in good agreement with the kinematics of the young population suggests that the recent star formation in Leo T is linked with the cold neutral gas. We assess the existence of extended emission-line regions and find none to a surface brightness limit of &lt; 1 × 10−20 erg s−1 cm−2 arcsec−2 which corresponds to an upper limit on star formation of ∼10−11 M⊙ yr−1 pc−2, implying that the star formation in Leo T has ended.

Is a Recently Discovered H i Cloud near M94 a Starless Dark Matter Halo?

Observations with the Five-hundred-meter Aperture Spherical Telescope have revealed the presence of a marginally resolved source of 21 cm emission from a location from the M94 galaxy, without a stellar counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey (∼29.15 mag arcsec−2 in the g band). The system (hereafter Cloud-9) has round column density isocontours and a line width consistent with thermal broadening from gas at T ∼ 2 × 104 K. These properties are unlike those of previously detected dark H i clouds and similar to the expected properties of REionization-Limited-H i Clouds (RELHICs), namely, starless dark matter (DM) halos filled with gas in hydrostatic equilibrium and in thermal equilibrium with the cosmic ultraviolet background. At the distance of M94, d ∼ 4.7 Mpc, we find that Cloud-9 is consistent with being a RELHIC inhabiting a Navarro–Frenk–White (NFW) DM halo of mass M 200 ∼ 5 × 109 M ⊙ and concentration c NFW ∼ 13. Although the agreement between the model and observations is good, Cloud-9 appears to be slightly, but systematically, more extended than expected for ΛCDM RELHICs. This may imply either that Cloud-9 is much closer than implied by its recessional velocity, v CL9 ∼ 300 km s−1, or that its halo density profile is flatter than NFW, with a DM mass deficit greater than a factor of 10 at radii r ≲ 1 kpc. Further observations may aid in constraining these scenarios better and help elucidate whether Cloud-9 is the first ever observed RELHIC, a cornerstone prediction of the ΛCDM model on the smallest scales.

Local Group dwarf galaxy detection limit in the CSST survey

ABSTRACT We predict the dwarf galaxy detection limits for the upcoming Chinese Space Station Telescope (CSST) survey that will cover 17 500 ${\rm \, deg}^{2}$ of the sky with a wide field of view of 1.1 deg2. The point-source depth reaches 26.3 mag in the g band and 25.9 mag in the i band. Constructing mock survey data based on the designed photometric bands, we estimate the recovery rate of artificial dwarf galaxies from mock point-source photometric catalogues. The detection of these artificial dwarf galaxies is strongly dependent on their distance, magnitude, and size, in agreement with searches in current surveys. We expect CSST to enable the detection of dwarf galaxies with MV = −3.0 and μ250 = 32.0 mag arcsec−2 (surface brightness limit for a system of half-light radius rh = 250 ${\rm \, pc}$) at $400 {\rm \, kpc}$, and MV = −4.9 and μ250 = 30.5 mag arcsec−2 around the Andromeda galaxy. Beyond the Local Group, the CSST survey will achieve MV = −5.8, and μ250 = 29.7 mag arcsec−2 in the distance range of 1–2 Mpc, opening up an exciting discovery space for faint field dwarf galaxies. With its optical bands, wide survey footprint, and space resolution, CSST will undoubtedly expand our knowledge of low-mass dwarf galaxies to an unprecedented volume.

A search for stellar tidal streams around Milky Way analogues from the SAGA sample

Context. Stellar tidal streams are the result of tidal interactions between a central galaxy and lower mass systems such as satellite galaxies or globular clusters. For the Local Group, many diffuse substructures have been identified and their link to the galaxy evolution has been traced. However, it cannot simply be assumed that the Milky Way or M 31 are representative of their galaxy class. Thus, a larger sample of analogue galaxies beyond the Local Group is required to bolster a broader generalisation of the underlying theory. Aims. We want to detect and photometrically characterise stellar streams around Milky Way (MW-) analogues in the local Universe in order to extend the observational evidence of interactions between this class of host galaxies and their satellites. This information will be applicable in a more general context around future studies on galaxy formation and evolution processes. Methods. In the present work, we identified and analysed stellar tidal streams around MW-analogue galaxies from the SAGA sample, using deep images of the DESI Legacy Imaging Surveys. For this sample, we obtained a range of r-band surface brightness limit between 27.8 and 29 mag arcsec−2. We measured the surface brightness and colours of the detected streams using GNU Astronomy Utilities software. Results. We identified 16 new stellar tidal streams around MW-analogue galaxies at distances between 25 and 40 Mpc. In applying a statistical analysis to our findings for the SAGA II galaxy sample, we obtained a frequency of 12.2% ± 2.4% for these stellar streams. We measured the surface brightness and colours of the detected streams and carried out a comparison to the dwarf satellite galaxies population around galaxies belonging to the same SAGA sample. We show that the mean colour of the streams is 0.20 mag redder than that of the SAGA satellites; in addition, the streams are, on average, 0.057 ± 0.021 mag redder that their progenitor (for cases where a likely progenitor could be identified). Conclusions. The frequency of streams detected around MW-analogues in the Local Universe is in agreement with previous studies. The difference in colour between detected streams and satellites within the SAGA host galaxy sample could be explained by a combination of both selection biases in the SAGA study and physical processes.

Massive Early-type Galaxies in the HSC-SSP: Flux Fraction of Tidal Features and Merger Rates

Here we present a statistical study on tidal features around massive early-type galaxies (ETGs). Utilizing the imaging data of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), we measure the flux fraction of tidal features (f tidal) in 2649 ETGs with stellar mass M * > 1011 M ⊙ and redshift 0.05 < z < 0.15 using automated techniques. The Wide layer of HSC-SSP reaches a depth of ∼28.5 mag arcsec−2 in the i band. Under this surface brightness limit, we find that about 28% of these galaxies harbor prominent tidal features with f tidal > 1%, among which the number of ETGs decreases exponentially with f tidal, with a logarithmic slope of ∼100. Within the stellar mass range we probe, we note that f tidal increases by a factor of 2 from M * ≈ 1011 to 1012 M ⊙. We also perform a pair count to estimate the merger rate of these massive ETGs. Combining the merger rates with f tidal, we estimate that the typical lifetime of tidal features is ∼3 Gyr, consistent with previous studies.

KMTNet Nearby Galaxy Survey: Overview and Survey Description

Recently, there has been increasing demand for deep imaging surveys to investigate the history of the mass assembly of galaxies in detail by examining the remnants of mergers and accretions, both of which have very low surface brightness (LSB). In addition, the nature of star formation in LSB regions, such as galaxy outer disks, is also an intriguing topic in terms of understanding the physical mechanisms of disk evolution. To address these issues, this study conducted a survey project, called the Korea Microlensing Telescope Network Nearby Galaxy Survey, to construct a deep imaging data set of nearby galaxies in the southern hemisphere. It provides deep and wide-field images with a field of view of ∼12 deg2 for 13 nearby galaxies drawn from the Carnegie–Irvine Galaxy Survey catalog in optical broad bands (BRI) and an Hα narrow band. Through a dedicated data reduction, the surface brightness limit in 10″ × 10″ boxes was found to reach as deep as μ 1σ ∼ 29–31 mag arcsec−2 in the optical broad bands and f 1σ ∼ 1–2 × 10−18 erg s−1 cm−2 arcsec−2 in the Hα narrow band. To conclude the paper, several possible scientific applications for this data set are described.

Tidal Distortions in NGC1052-DF2 and NGC1052-DF4: Independent Evidence for a Lack of Dark Matter

Two ultra-diffuse galaxies in the same group, NGC1052-DF2 and NGC1052-DF4, have been found to have little or no dark matter and to host unusually luminous globular cluster populations. Such low-mass diffuse objects in a group environment are easily disrupted and are expected to show evidence of tidal distortions. In this work, we present deep new imaging of the NGC1052 group, obtained with the Dragonfly Telephoto Array, to test this hypothesis. We find that both galaxies show strong position-angle twists and are significantly more elongated at their outskirts than in their interiors. The group’s central massive elliptical NGC1052 is the most likely source of these tidal disturbances. The observed distortions imply that the galaxies have a low total mass or are very close to NGC1052. Considering constraints on the galaxies’ relative distances, we infer that the dark matter halo masses of these galaxies cannot be much greater than their stellar masses. Calculating pericenters from the distortions, we find that the galaxies are on highly elliptical orbits, with a ratio of pericenter to present-day radius R peri/R 0 ∼ 0.1 if the galaxies are dark matter–free and R peri/R 0 ∼ 0.01 if they have a normal dark halo. Our findings provide strong evidence, independent of kinematic constraints, that both galaxies are dark matter–deficient. Furthermore, the similarity of the tidal features in NGC1052-DF2 and NGC1052-DF4 strongly suggests that they arose at comparable distances from NGC1052. In Appendix A, we describe sbcontrast, a robust method for determining the surface brightness limits of images.

Performance Assessment of the KASI-Deep Rolling Imaging Fast-optics Telescope Pathfinder

In a ΛCDM universe, most galaxies evolve by mergers and accretions, leaving faint and/or diffuse structures, such as tidal streams and stellar halos. Although these structures are a good indicator of galaxies’ recent mass assembly history, they have the disadvantage of being difficult to observe due to their low surface brightness (LSB). To recover these LSB features by minimizing the photometric uncertainties introduced by the optical system, we developed a new optimized telescope named K-DRIFT pathfinder, adopting a linear astigmatism free-three mirror system. Thanks to the off-axis design, it is expected to avoid the loss and scattering of light on the optical path within the telescope. To assess the performance of this prototype telescope, we investigate the photometric depth and capability to identify LSB features. We find that the surface brightness limit reaches down to μ r,1σ ∼ 28.5 mag arcsec−2 in 10″ × 10″ boxes, enabling us to identify a single stellar stream to the east of NGC 5907. We also examine the characteristics of the point-spread function (PSF) and find that the PSF wing reaches a very low level. Still, however, some internal reflections appear within a radius of ∼6′ from the center of sources. Despite a relatively small aperture (0.3 m) and short integration time (2 hr), this result demonstrates that our telescope is highly efficient in LSB detection.

The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter

ABSTRACT Due to their low surface brightness, dwarf galaxies are particularly susceptible to tidal forces. The expected degree of disturbance depends on the assumed gravity law and whether they have a dominant dark halo. This makes dwarf galaxies useful for testing different gravity models. In this project, we use the Fornax Deep Survey (FDS) dwarf galaxy catalogue to compare the properties of dwarf galaxies in the Fornax Cluster with those predicted by the Lambda cold dark matter (ΛCDM) standard model of cosmology and Milgromian dynamics (MOND). We construct a test particle simulation of the Fornax system. We then use the Markov Chain Monte Carlo (MCMC) method to fit this to the FDS distribution of tidal susceptibility η (half-mass radius divided by theoretical tidal radius), the fraction of dwarfs that visually appear disturbed as a function of η, and the distribution of projected separation from the cluster centre. This allows us to constrain the η value at which dwarfs should get destroyed by tides. Accounting for an r′-band surface brightness limit of 27.8 magnitudes per square arcsec, the required stability threshold is $\eta _{\textrm {destr}} = 0.25^{+0.07}_{-0.03}$ in ΛCDM and $1.88^{+0.85}_{-0.53}$ in MOND. The ΛCDM value is in tension with previous N-body dwarf galaxy simulations, which indicate that ηdestr ≈ 1. Our MOND N-body simulations indicate that ηdestr = 1.70 ± 0.30, which agrees well with our MCMC analysis of the FDS. We therefore conclude that the observed deformations of dwarf galaxies in the Fornax Cluster and the lack of low surface brightness dwarfs towards its centre are incompatible with ΛCDM expectations but well consistent with MOND.

Euclidpreparation

Euclidis a mission of the European Space Agency that is designed to constrain the properties of dark energy and gravity via weak gravitational lensing and galaxy clustering. It will carry out a wide area imaging and spectroscopy survey (theEuclidWide Survey: EWS) in visible and near-infrared bands, covering approximately 15 000 deg2of extragalactic sky in six years. The wide-field telescope and instruments are optimised for pristine point spread function and reduced stray light, producing very crisp images. This paper presents the building of theEuclidreference survey: the sequence of pointings of EWS, deep fields, and calibration fields, as well as spacecraft movements followed byEuclidas it operates in a step-and-stare mode from its orbit around the Lagrange point L2. Each EWS pointing has four dithered frames; we simulated the dither pattern at the pixel level to analyse the effective coverage. We used up-to-date models for the sky background to define theEuclidregion-of-interest (RoI). The building of the reference survey is highly constrained from calibration cadences, spacecraft constraints, and background levels; synergies with ground-based coverage were also considered. Via purposely built software, we first generated a schedule for the calibrations and deep fields observations. On a second stage, the RoI was tiled and scheduled with EWS observations, using an algorithm optimised to prioritise the best sky areas, produce a compact coverage, and ensure thermal stability. The result is the optimised reference survey RSD_2021A, which fulfils all constraints and is a good proxy for the final solution. The current EWS covers ≈14 500 deg2. The limiting AB magnitudes (5σpoint-like source) achieved in its footprint are estimated to be 26.2 (visible bandIE) and 24.5 (for near infrared bandsYE,JE,HE); for spectroscopy, the Hαline flux limit is 2 × 10−16erg−1cm−2s−1at 1600 nm; and for diffuse emission, the surface brightness limits are 29.8 (visible band) and 28.4 (near infrared bands) mag arcsec−2.

Characterization of low surface brightness structures in annotated deep images

Context. The identification and characterization of low surface brightness (LSB) stellar structures around galaxies such as tidal debris of ongoing or past collisions is essential to constrain models of galactic evolution. So far most efforts have focused on the numerical census of samples of varying sizes, either through visual inspection or more recently with deep learning. Detailed analyses including photometry have been carried out for a small number of objects, essentially because of the lack of convenient tools able to precisely characterize tidal structures around large samples of galaxies. Aims. Our goal is to characterize in detail, and in particular obtain quantitative measurements, of LSB structures identified in deep images of samples consisting of hundreds of galaxies. Methods. We developed an online annotation tool that enables contributors to delineate the shapes of diffuse extended stellar structures with precision, as well as artifacts or foreground structures. All parameters are automatically stored in a database which may be queried to retrieve quantitative measurements. We annotated LSB structures around 352 nearby massive galaxies with deep images obtained with the Canada-France-Hawaii Telescope as part of two large programs: Mass Assembly of early-Type GaLAxies with their fine Structures and Ultraviolet Near Infrared Optical Northern Survey/Canada-France Imaging Survey. Each LSB structure was delineated and labeled according to its likely nature: stellar shells, streams associated with a disrupted satellite, tails that formed in major mergers, ghost reflections, or cirrus. Results. From our database containing 8441 annotations, the area, size, median surface brightness, and distance to the host of 228 structures were computed. The results confirm the fact that tidal structures defined as streams are thinner than tails, as expected by numerical simulations. In addition, tidal tails appear to exhibit a higher surface brightness than streams (by about 1 mag), which may be related to different survival times for the two types of collisional debris. We did not detect any tidal feature fainter than 27.5 magarcsec−2, while the nominal surface brightness limits of our surveys range between 28.3 and 29 magarcsec−2, a difference that needs to be taken into account when estimating the sensitivity of future surveys to identify LSB structures. Conclusions. We compiled an annotation database of observed LSB structures around nearby massive galaxies including tidal features that may be used for quantitative analysis and as a training set for machine learning algorithms.

Can tides explain the low dark matter density in Fornax?

ABSTRACT The low dark matter density in the Fornax dwarf galaxy is often interpreted as being due to the presence of a constant density ‘core’, but it could also be explained by the effects of Galactic tides. The latter interpretation has been disfavoured because it is apparently inconsistent with the orbital parameters and star formation history of Fornax. We revisit these arguments with the help of the APOSTLE cosmological hydrodynamics simulations. We show that simulated dwarfs with similar properties to Fornax are able to form stars after infall, so that star formation is not necessarily a good tracer of infall time. We also examine the constraints on the pericentre of Fornax and point out that small pericentres (&amp;lt;50 kpc) are not currently ruled out by the data, allowing for Fornax to be tidally influenced on its current orbit. Furthermore, we find that some dwarfs with large orbital pericentres can be stripped prior to infall due to interactions with more massive galaxies. Tidal effects lead to a reduction in the dark matter density, while the profile remains cuspy. Navarro–Frenk–White profiles are consistent with the kinematic data within 3σ in the innermost regions, while profiles with shallow cusps or cores provide a better fit. We predict that if the reduction of the dark matter density in Fornax occurs, at least in part, because of the action of Galactic tides, then tidal tails should be visible with a surface brightness limit of ∼35–36 mag arcsec−2 over a survey area of ≳100 deg2.

A Near-infrared Search for Molecular Gas in the Fermi Bubbles

Abstract We present Gemini/NIFS near-IR integral field spectroscopy of the fields-of-view around two AGNs behind the Fermi Bubbles (PDS 456 and 1H1613-097 ) to search for molecular gas in the Milky Way’s nuclear wind. These two AGN sightlines were selected by the presence of high-velocity neutral and ionized gas seen in UV absorption. We do not detect any extended emission from the H2 rovibrational S(0) and S(1) lines at 2.224 and 2.122 μm in either direction. For the H2 1-0 S(1) line, the 3σ surface brightness limits derived from spectra extracted across the full 3″ × 3″ NIFS field-of-view are 2.4 × 10−17 erg cm−2 s−1 Å−1 arcsec−2 for PDS 456 and 4.9 × 10−18 erg cm−2 s−1 Å−1 arcsec−2 for 1H1613-097. Given these non-detections, we conclude that CO emission studies and H2 UV absorption studies are more promising approaches for characterizing molecular gas in the Fermi Bubbles.