arXiv subjects

Samane Raji

Publications and source records attributed to Samane Raji.

9 recordsLinked to original sources

arXiv2026

How large can galaxies be? Ultra-deep imaging of IC 1101, the most extended known galaxy

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- band imaging from the INT/WFC, reaching {\mu} = 30 mag arcsec^-2 (3{\sigma} in an area equivalent to 10 x 10 arcsec^2). We model and subtract the scattered light from both stars and the galaxy itself using an extended PSF characterization 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 Redge = 260 kpc along the semi-major axis (assuming the redshift of Abell 2029, z = 0.077), enclosing 3.4 x 10^12 M_sun in stars. This Redge 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.

astro-ph.GA↗

arXiv2026

Witnessing the rapid growth of disk galaxies over cosmic time using JWST and HST

Measuring galaxy sizes is fundamental to understanding how galaxies grow and evolve. Traditional methods to measure sizes either trace the concentration of light (i.e., effective radius) or are limited by the depth of the survey (isophotal methods). With the advent of deep, wide surveys, a new physically motivated definition of size has emerged: the edge of the galaxy, defined as the most distant location where star formation has occurred or is still occurring. In this work, we take advantage of the extraordinary depth and spatial resolution of the Hubble and James Webb Space Telescopes to perform an accurate study of galaxy edges at $z=1$. Using 22 photometric bands, we derive radial age and metallicity profiles for two disk galaxies in the GOODS-South field with stellar masses of around $4\times10^{10}\ M_\odot$. The age profiles display a characteristic U-shape, while the metallicity profiles steadily decrease with galactocentric distance. The turnover in the age profile occurs near the galaxy edge, suggesting that stellar migration is responsible for the stars beyond the edge of these galaxies. Comparison with $z=0$ disk galaxies suggests that galaxies at $z=1$ grow inside-out, with little or no increase in mass within the inner 8 kpc, but a significant increase (approximately 300\%) in the outer regions.

astro-ph.GA↗

arXiv2026

LIGHTS. The Thin Encircling Stellar Stream of NGC 3938

We present a stellar stream found in images of the nearby, nearly face-on, late-type galaxy, NGC 3938 obtained for the LBT Imaging of Galactic Halos and Tidal Structures (LIGHTS) survey that is thin, has very low mean surface brightness ($\langle\mu_g\rangle \approx$ 28.7 mag arcsec$^{-2}$ and $\langle\mu_r\rangle \approx$ 28.1 mag arcsec$^{-2}$), appears to lie nearly on the plane of the sky, and wraps more than half way around a host galaxy that is otherwise apparently isolated. We estimate that the progenitor had a stellar mass of $\sim 3.7\times 10^7$ M$_\odot$. Despite an intriguing apparent offset between the centroid of the host galaxy and the apparent center of the stream orbit, we find that we can reproduce the morphology, including this apparent off-centering, with simple models and standard assumptions about the host (thin disk centered within a canonical spherical dark matter halo) and the progenitor satellite orbit. We identify a number of detailed features of the stream, such as changes in curvature and density, that will require more complex models to reproduce. Even this rather simple system provides a rich set of constraints with which to explore the accretion history and gravitational potential of an otherwise unremarkable late-type galaxy. Given the depth of the LIGHTS images, this system is an example of the types of stellar stream that could be found in a majority of nearby giant galaxies with the 10-year stack of Rubin/LSST data.

astro-ph.GA↗

arXiv2025

Revisiting the structure of galactic disks with deep imaging

Thanks to new advances in astronomical imaging, we can now routinely explore disk galaxy profiles about two magnitudes deeper than the data available 20 years ago. In this regard, it is an opportune time to reevaluate the past classifications of different surface brightness Types. In this paper, we explore the stellar mass profiles of a sample of disk galaxies with similar stellar masses ($\sim$10$^{10}$ M$_{\odot}$) using IAC Stripe82 Legacy Project data. We find that Type I, II-CT, and III-d galaxies exhibit edges at surface brightnesses $\mu_g$(R$_{edge}$)$\sim$26.5 mag/arcsec$^2$ and surface mass densities $\Sigma$(R$_{edge}$)$\sim$0.5-1 M$_{\odot}$/pc$^2$. These surface brightnesses were outside the range of typical SDSS images and, therefore, unstudied. The present data suggest (although the statistics are inconclusive) that the main difference between the previous profile Types is the presence of either a more or a less intense bulge. Half of the Type II-CT galaxies are Sc, whereas half of the Type III-d galaxies are Sb. Finally, Type III-d galaxies have low mass surface density disks ($\Sigma$($\Delta$ $R_{\mathrm{exp}}$)$\sim$0.5 M$_{\odot}$/pc$^2$) and are bluer $(g-r)_0$=0.32$\pm$0.02 than the other Types ($(g-r)_0$=0.42$\pm$0.02 for Type I and II-CT), suggesting that their disks probably formed later in time.

astro-ph.GA↗

arXiv2025

LIGHTS. A robust technique to identify galaxy edges

The LIGHTS survey is imaging galaxies at a depth and spatial resolution comparable to what the Legacy Survey of Space and Time (LSST) will produce in 10 years (i.e., $\sim$31 mag/arcsec$^2$; 3$\sigma$ in areas equivalent to 10$^{\prime\prime}$$\times$ 10$^{\prime\prime}$). This opens up the possibility of probing the edge of galaxies, as the farthest location of in-situ star formation, with a precision that we have been unable to achieve in the past. Traditionally, galaxy edges have been analyzed in one-dimension through ellipse averaging or visual inspection. Our approach allows for a two-dimensional exploration of galaxy edges, which is crucial for understanding deviations from disc symmetry and the environmental effects on galaxy growth. In this paper, we propose a novel method using the second derivative of the surface mass density map of a galaxy to determine its edges. This offers a robust quantitative alternative to traditional edge-detection methods when deep imaging is available. Our technique incorporates Wiener-Hunt deconvolution to remove the effect of the Point Spread Function (PSF) by the galaxy itself. By applying our methodology to the LIGHTS galaxy NGC 3486, we identify the edge at 205$^{\prime\prime}$ $\pm$ 5$^{\prime\prime}$. At this radius, the stellar surface mass density is $\sim$1 M$_\odot$/pc$^2$, supporting a potential connection between galaxy edges and a threshold for in-situ star formation. Our two-dimensional analysis on NGC 3486 reveals an edge asymmetry of $\sim$5$\%$. These techniques will be of paramount importance for a physically motivated determination of the sizes of galaxies in ultra-deep surveys such as LSST, Euclid and Roman.

astro-ph.GA↗

arXiv2025

UV LIGHTS. New tools for revealing the low surface brightness regime in the ultraviolet

Ultra-deep optical surveys have reached unprecedented depths, facilitating the study of faint galactic structures. However, the ultraviolet bands, crucial for stellar population studies, remain essentially unexplored at these depths. We present a detailed surface brightness and color analysis of 20 nearby galaxies in the LIGHTS fields observed by GALEX in the FUV and NUV. We adapt and apply a low surface brightness oriented methodology that has proven effective in ultra-deep optical surveys. A novel approach to background subtraction is proposed for UV imaging. Instead of subtracting a constant value from the background, we subtract a Poisson distribution that transforms the background into a pseudo-Gaussian distribution centered at zero. Furthermore, the PSF deconvolution algorithms developed for optical data are applied to our sample, using a novel set of very extended (R=750 arcsec) PSFs for the GALEX bands. This methodology allows us to obtain depths ranging from 28.5 to 30 mag arcsec^{-2}, with reliable surface brightness profiles up to 31 mag arcsec^{-2}. This is about 1 mag deeper than with standard UV techniques. We use the surface brightness and color profiles to show that the application of PSF deconvolution, especially in the FUV, effectively mitigates the excess of light present in the outer regions of certain galaxies compared to the standard GALEX pipeline. This finding is crucial for any accurate stellar population inference from the color profiles. Additionally, a qualitative analysis of the results is presented, with particular emphasis on surface brightness and color properties of the galaxies beyond their optical edges. Our work highlights the importance of developing innovative low surface brightness methods for UV surveys.

astro-ph.GA↗

arXiv2024

LIGHTS. The extended point spread functions of the LIGHTS survey at the LBT

With the arrival of the next generation of ultra-deep optical imaging surveys reaching $\mu_V$$\sim$30 mag/arcsec$^2$ (3$\sigma$; 10"$\times$10"), the removal of scattered light due to the point spread function (PSF) effect remains a critical step for the scientific exploitation of the low surface brightness information contained in these data. Because virtually all pixels in the ground-based images are affected by an unwanted screen of light with a brightness greater than $\mu_V$$\sim$29 mag/arcsec$^2$, the characterization of the extended PSF (R$>$5 arcmin) is mandatory. We describe the procedure used to construct the extended PSFs of the LIGHTS survey in the g- and r-band images taken with the Large Binocular Cameras (LBCs) of the Large Binocular Telescope (LBT). We produce PSFs with a radial extension of 6.5 arcmins. These are later extended to 30 arcmins following an empirically motivated power-law extrapolation of their behaviour in their outermost regions. As an example of the application of our methodology, we subtract the scattered light around the galaxy NGC3198. The result of this subtraction clearly shows the outermost parts of the galaxy's disc, which have been obscured by the influence of nearby bright stars. We make all the PSF (compact and extended) models publicly available.

astro-ph.GA↗

arXiv2023

Gnuastro: Estimating the Zero Point Magnitude in Astronomical Imaging

Calibration of pixel values is a fundamental step for accurate measurements in astronomical imaging. In astronomical jargon this is known as estimating zero point magnitude. Here, we introduce a newly added script in GNU Astronomy Utilities (Gnuastro) version 0.20 for the zero point magnitude estimation, named: astscript-zeropoint. The script offers numerous features, such as the flexibility to use either image(s) or a catalog as the reference dataset. Additionally, steps are parallelized to enhance efficiency for big data. Thanks to Gnuastro's minimal dependencies, the script is both flexible and portable. The figures of this research note are reproducible with Maneage, on the Git commit c89275e.

astro-ph.IM↗

arXiv2021

Once in a blue stream: Detection of recent star formation in the NGC 7241 stellar stream with MEGARA

In this work we study the striking case of a narrow blue stream around the NGC 7241 galaxy and its foreground dwarf companion. We want to figure out if the stream was generated by tidal interaction with NGC 7241 or it first interacted with the foreground dwarf companion and later both fell together towards NGC 7241. We use four sets of observations, including a follow-up spectroscopic study with the MEGARA instrument at the 10.4-m Gran Telescopio Canarias. Our data suggest that the compact object we detected in the stream is a foreground Milky Way halo star. Near this compact object we detect emission lines overlapping a bluer and fainter blob of the stream that is clearly visible in both ultra-violet and optical deep images. From its heliocentric systemic radial velocity (Vsyst= 1548.58+/-1.80 km s^-1) and new UV and optical broad-band photometry, we conclude that this over-density could be the actual core of the stream, with an absolute magnitude of M_g ~ -10 and a (g-r) = 0.08 +/- 0.11, consistent with a remnant of a low-mass dwarf satellite undergoing a current episode of star formation. From the width of the stream and assuming a circular orbit, we calculate that the progenitor mass can be the typical of a dwarf galaxy, but it could also be substantially lower if the stream is on a very radial orbit or it was created by tidal interaction with the companion dwarf instead of with NGC 7241. Finally, we find that blue stellar streams containing star formation regions are commonly predicted by high-resolution cosmological simulations of galaxies lighter than the Milky Way. This scenario is consistent with the processes explaining the bursty star formation history of some dwarf satellites, which are followed by a gas depletion and a fast quenching once they enter within the virial radius of their host galaxies for the first time.

astro-ph.GA↗