Mireia Montes

Current and forthcoming deep optical imaging surveys reach surface brightness levels fainter than 30 mag/arcsec ² , where Galactic cirrus emission becomes a significant contaminant. This poses a major challenge for the study of extremely low-surface-brightness extragalactic structures. We investigated the relationship between far-infrared emission, as traced by the Herschel 250 μm band, and optical surface brightness in the Sloan g and r bands. We find that, for Galactic cirrus with μ _g ≳ 27 mag/arcsec ² , a linear relation is consistent with the data, assuming optically thin dust, with optical cirrus emission amounting to ∼0.1% of the far-infrared flux. Using Herschel far-infrared data to trace Galactic cirrus enables its identification over wide areas but at the cost of significantly reducing spatial resolution. This limitation introduces source confusion and restricts the effective surface brightness depth that can be robustly probed in optical data. We show that, when cirrus emission is modeled using Herschel 250 μm data, the limiting surface brightness is μ _V ∼ 28 mag/arcsec ² . This is substantially shallower than the nominal depth of surveys such as LIGHTS or the 10-year co-added LSST ( μ _V ∼ 30.5 mag/arcsec ² ). These results highlight a fundamental limitation in the use of currently available far-infrared data for cirrus mitigation in ultra-deep optical imaging and emphasize the need for higher-resolution cirrus tracers to fully exploit the depth of upcoming surveys.

Deep imaging surveys are opening a new window on the low surface brightness universe. However, the recovery of diffuse emission depends on the survey depth and how the sky background is treated. Estimating the sky on scales comparable to astrophysical sources risks biasing flux measurements. This effect is particularly significant for intracluster light (ICL), the most extended diffuse baryonic component of galaxy clusters. We show in a M ₅₀₀ – z plane that the sky subtraction scales employed in the Legacy Survey of Space and Time (LSST) Data Preview 2 and Hyper Suprime-Cam Public Data Release 3 (HSC-PDR3) impact clusters with M ₅₀₀ ≳ 10 ^13.4 M _⊙ at all redshifts, whereas HSC-PDR2 only oversubtracts the most massive systems at z < 0.1. The larger sky subtraction scale anticipated for the full LSST survey will mitigate this bias.

Low-mass galaxies beyond the Local Group can only be studied through their integrated light, yet it remains unclear whether structural parameters inferred from integrated light are directly comparable to those obtained from resolved-star studies. Using new deep imaging obtained with small-aperture amateur telescopes, we analyse the classical dwarf spheroidals Leo II, Sculptor, and Fornax, whose stellar distributions have been extensively characterised through resolved-star studies. We find that the radial light distributions and effective radii inferred from integrated light are in excellent agreement with those derived from resolved stars, showing that resolved-star and integrated-light analyses can be placed within a common observational framework applicable across a broad range of distances. The two approaches are highly complementary: resolved-star analyses trace galaxies to very large radii but are limited by stellar crowding in their central regions, whereas integrated-light observations are insensitive to crowding and allow the inner stellar distribution to be traced to substantially smaller radii. The integrated-light analysis extends the structural analysis to radii up to an order of magnitude smaller than previous star-counting studies while significantly reducing the scatter in the measured profiles.

The maximum physical extent that galaxies can reach is poorly understood. In this regard, IC 1101, one of the most extended and massive galaxies known, provides a valuable opportunity to constrain the upper limit of galaxy sizes at the present epoch. Previous deep imaging of the system confirmed its enormous extension, but did not indicate whether it has an edge. We explore this issue using the deepest images ever taken of this galaxy; ultra-deep g and r bands imaging from the INT/WFC, reaching µ ∼ 30 mag arcsec ⁻² (3 σ in an area equivalent to 10×10 arcsec ² ). We model and subtract the scattered light from both stars and the galaxy itself using an extended point spread function characterisation and a hybrid wavelet-based deconvolution. Using a combination of surface brightness, colour, and stellar mass density profiles oriented at different position angles, we find that the main body of IC 1101 extends to R _edge ≈ 260 kpc along the semi-major axis (assuming the redshift of Abell 2029, z = 0.077), enclosing ∼3.4×10 ¹² M _⊙ in stars. This R _edge is among the largest edge radii measured for any galaxy to date, placing IC 1101 at the extreme upper end of the mass–size relation. In addition, we report a large number of asymmetrical, very low-surface-brightness features around the galaxy that are spatially consistent with the large-scale disturbances observed in the intracluster medium through X-ray studies of the Abell 2029 cluster, in which IC 1101 is embedded. With a confirmed diameter of around 520 kpc, IC 1101 stands as the largest galaxy known to date; yet, its outskirts show clear signatures of ongoing mass assembly, indicating that its spatial extent is still growing.

Analogues of extreme Local Group galaxies with very low surface brightnesses and large effective radii must exist elsewhere, still hidden due to current detection limits. We report the serendipitous discovery of the low-mass almost-dark galaxy TTT J1237327+143535 in the Virgo Cluster, observed using the Two-meter Twin Telescope at the Teide Observatory. TTT J1237327+143535 has a central surface brightness of 27.9 ± 0.2 mag arcsec ⁻² in the g ′ band, an effective radius of 0.9 ± 0.1 kpc, and a total stellar mass of (2.2 ± 0.4)×10 ⁶ M _⊙ . Its effective radius and absolute magnitude are similar to those of galaxies And XXI and And XXIII. The discovery of this extremely low-surface-brightness, extended, low-mass galaxy suggests the existence of a significant population of almost-dark galaxies in the Virgo Cluster. An in-depth analysis of the Next Generation Virgo Cluster Survey, as well as upcoming Rubin data releases and the 10-year Rubin Legacy Survey of Space and Time, is expected to reveal large samples of this extreme galaxy population, which will offer insights into galaxy formation in extreme conditions.

We report the non-detection of stellar light associated with the dark galaxy candidate Cloud-9. Ultra-deep imaging obtained with HiPERCAM at the Gran Telescopio Canarias (GTC) reaches surface-brightness limits of 31.4 and 31.0 mag arcsec ⁻² in g and r , respectively, approximately ten times deeper than previous deep imaging of the region. No stellar emission is detected within the central 1 ′ × 1 ′ (∼1.3 × 1.3 kpc) region of Cloud-9. Assuming an old, metal-poor stellar population, these limits imply a stellar surface mass density of no more than ∼0.01 M _⊙ pc ⁻² and an upper limit on the total stellar mass of 1.6 × 10 ⁴ M _⊙ . This independent constraint from integrated light complements previous limits based on resolved-star counts and strengthens the case for Cloud-9 as a candidate starless galaxy.

Measuring galaxy sizes is fundamental to understanding how galaxies grow and evolve. Traditional methods for measuring sizes trace the concentration of light (i.e., the effective radius) better or are limited by the depth of the survey (isophotal methods). With the advent of deep, wide surveys, a new and physically motivated definition of size has arisen: the edge of the galaxy, which is the most distant location where star formation has or is occurring. In this work, we take advantage of the extraordinary depth and spatial resolution of the Hubble and James Webb space telescopes to conduct the most accurate study of galaxy edges at z ∼ 1. Using 22 photometric bands, we derived the radial age and metallicity profiles of two disk galaxies in the GOODS South field with stellar masses of ∼4 × 10 ¹⁰ M _⊙ . The age profiles display the characteristic U-shape, while the metallicity profiles steadily decrease over galactocentric distance. The turnover of the age profile occurs near the galaxy’s edge, suggesting that stellar migration is responsible for the stars beyond the edges of these galaxies. A comparison to z = 0 disk galaxies reveals that z = 1 galaxies grow inside out, with little to no increase in mass in the inner 8 kpc but a significant increase (∼300%) in the outer parts.

The Euclid space telescope of the European Space Agency (ESA) is designed to provide sensitive and accurate measurements of weak gravitational lensing distortions over wide areas on the sky. Here, we present a weak gravitational lensing analysis of early Euclid observations obtained for the field around the massive galaxy cluster Abell 2390 as part of the Euclid Early Release Observations (ERO) programme. We conducted shape measurements for galaxies down to I _E ≲ 26.5 using three independent algorithms ( LensMC , KSB+ , and SourceXtractor++ ). Incorporating multi-band photometry from Euclid and Subaru/Suprime-Cam, we estimated photometric redshifts to preferentially select background sources from tomographic redshift bins, for which we calibrated the redshift distributions using the self-organising map approach and data from the Cosmic Evolution Survey (COSMOS). We quantified the residual cluster member contamination and corrected for it in bins of photometric redshift and magnitude using their source density profiles, including corrections for source obscuration and magnification. We reconstructed the cluster mass distribution and jointly fit the tangential reduced shear profiles of the different tomographic bins with spherical Navarro-Frenk-White profile predictions to constrain the cluster mass, finding consistent results for the three shape catalogues and good agreement with earlier measurements. As an important validation test, we compared these joint constraints to mass measurements obtained individually for the different tomographic bins, finding a good level of consistency. More detailed constraints on the cluster properties are presented in a companion paper, which additionally incorporates strong lensing measurements. Our analysis provides a first demonstration of the outstanding capabilities of Euclid for tomographic weak lensing measurements.