--- title: Siena Galaxy Atlas 2020 url: https://www.emergentmind.com/topics/siena-galaxy-atlas type: topic --- # Siena Galaxy Atlas 2020 The Siena Galaxy Atlas 2020 (SGA-2020) is a uniform, multi-wavelength optical and infrared imaging atlas of nearby, large angular-diameter galaxies, built from DESI Legacy Imaging Surveys Data Release 9 \(grz\) imaging and six-year unWISE coadds in four infrared bands. It contains \(N = 383{,}620\) galaxies over \(19{,}721\ \mathrm{deg}^2\) of the extragalactic sky, and was designed as a consistently processed reference data set for spatially resolved studies of the local galaxy population, for high-fidelity targeting and photometry in the DESI Bright Galaxy Survey, and for downstream applications including peculiar-velocity work, transient host-galaxy identification, and multi-messenger counterpart searches [2307.04888]. ## 1. Scope, design goals, and sample construction SGA-2020 was designed for precision studies of nearby galaxies in the regime where resolved structure is observationally accessible. Its stated purposes are to enable detailed studies of star formation and mass assembly in the local universe; provide high-quality photometry, geometry, and mosaics for large galaxies so as to mitigate shredding and bright-end incompleteness in DESI’s Bright Galaxy Survey; deliver legacy-value reference images and metadata for time-domain and multi-messenger astrophysics; and support local-velocity-field measurements via the Tully–Fisher and Fundamental Plane relations [2307.04888]. The atlas covers approximately \(20{,}000\ \mathrm{deg}^2\) in the Legacy Surveys DR9 footprint, with final angular coverage of the cataloged sample equal to \(19{,}721\ \mathrm{deg}^2\), corresponding to about \(50\%\) of the sky and \(74\%\) of the available \(|b|>20^\circ\) extragalactic sky. The selection targets nearby, large angular-diameter systems and imposes no strict redshift cut, although comparisons to HECATE \((z<0.047)\) and the DESI Peculiar Velocity program \((z<0.15)\) make the low-redshift focus explicit. Within the DESI footprint of about \(14{,}000\ \mathrm{deg}^2\), DESI is expected to obtain spectra for more than \(300{,}000\) SGA galaxies by survey completion; \(7{,}700\) spectra were already available in the DESI EDR [2307.04888]. The parent sample was built from HyperLeda and supplemented by RC3/OpenNGC, a subset of Local Group dwarfs, and Legacy Surveys DR8 large sources. An initial HyperLeda query returned \(1{,}436{,}176\) sources, which were curated to \(531{,}677\) parents. The sample is largely limited to \(D25>20''\), where \(D25\) is the major-axis diameter at the \(25\ \mathrm{mag\ arcsec^{-2}}\) optical isophote, with a tail extending down to approximately \(10''\)–\(20''\); an upper limit \(D25<180'\) excludes the LMC, SMC, and Sgr dSph. This construction emphasizes large, nearby systems while retaining enough breadth to support wide-area statistical work [2307.04888]. ## 2. Imaging basis and processing pipeline SGA-2020 combines optical imaging from the DESI Legacy Imaging Surveys DR9 with infrared imaging from unWISE six-year coadds. The optical component uses \(g\), \(r\), and \(z\) imaging from DECaLS, BASS, and MzLS; the infrared component uses W1, W2, W3, and W4, spanning \(3.4\)–\(22\ \mu\mathrm{m}\). The optical data typically have PSF FWHM of about \(1\)–\(2''\), photometric calibration from Pan-STARRS1 PSF photometry with precision better than \(\pm 10\ \mathrm{mmag}\) in \(grz\) for bright stars, and astrometry anchored to Gaia DR2 at approximately \(\pm 0.030''\) for DECam/Mosaic-3 and \(\pm 0.12''\) for 90Prime [2307.04888]. | Component | Source | Core characteristics | |---|---|---| | Optical | Legacy Surveys DR9 | \(grz\), PSF FWHM \(\sim 1\)–\(2''\), \(0.262''\)/pixel | | Infrared | unWISE six-year coadds | W1–W4, \(2.75''\)/pixel | | Catalog products | SGA-2020.fits | ELLIPSE and TRACTOR HDUs | Image reduction used the NOIRLab Community Pipeline with instrument-specific processing for DECam, Mosaic-3, and 90Prime. Large-scale sky was modeled per CCD via spline fits after masking sources, with removal of reflection or pupil-ghost patterns for DECam and Mosaic-3 and fringe corrections for MzLS \(z\)-band and 90Prime \(r\). Residual median-scaled sky patterns were also removed in DECam \(g\), \(r\), and \(z\). Small-scale sky estimation used \(512\)-pixel boxes and spline interpolation, with galaxies and Gaia stars masked during background estimation [2307.04888]. The mosaics were built as inverse-variance weighted sums, with optical coadds in tangent-plane projections using Lanczos-3 resampling. Per-group tiling scaled the mosaic diameter to group size, typically \(2\)–\(3\times\) the group diameter. In the north \((\mathrm{Dec}\ge +32.375^\circ)\) the atlas uses BASS+MzLS, while elsewhere it uses DECam imaging from DECaLS+DES. Source modeling employed The Tractor, fitting \(grz\) jointly with PSF, REX, EXP, DEV, or Sérsic models. Deblending was tuned to reduce shredding in resolved galactic structure by adopting elevated saddle-fraction and saddle-min settings, and the coadd PSF was built from the inverse-variance weighted average of the contributing pixelized PSFs [2307.04888]. ## 3. Measurement definitions and released data products The principal release file is `SGA-2020.fits`, organized into row-matched ELLIPSE and TRACTOR HDUs. The ELLIPSE HDU contains per-galaxy geometry, photometry, and surface-brightness-profile results, including RA and DEC, PGC, \(D26\), PA, BA, \(R(26)\), \(rR(26)\), average \(\mu_r\) within \(R(26)\), moment-based positions \((\mathrm{RA\_MOMENT}, \mathrm{DEC\_MOMENT})\), semi-major axes at specified isophotes from \(\mu_r=22\) to \(26\), curve-of-growth parameters per band \((m_{\mathrm{tot}}, m_0, \alpha_1, \alpha_2, \chi^2)\), and quality or failure flags in `ELLIPSEBIT`. The photometric system is AB, and fluxes are reported in nanomaggies, where \(1\) nanomaggie is the flux density of AB \(22.5\) mag. The TRACTOR HDU contains model-fitting outputs, including model family, shapes, fluxes, and depths [2307.04888]. SGA-2020 defines \(R(26)\) as the semi-major axis length at the \(r\)-band surface-brightness isophote \(\mu_r = 26\ \mathrm{mag\ arcsec^{-2}}\), and \(D(26)\) as the corresponding major-axis diameter. Azimuthally averaged optical surface-brightness profiles were measured with `photutils` on elliptical isophotes using fixed geometry from ellipse moments, sigma-clipping with `nclip=2` and `sclip=3`, and median-area integration. The quantity \(\mu_r(r)\) is reported in \(\mathrm{mag\ arcsec^{-2}}\) along semi-major axes sampled at \(1\) pixel \((0.262'')\) intervals out to about \(2\times\) the estimated semi-major axis. Radii and integrated magnitudes at \(\mu_r = 22, 22.5, \ldots, 26\ \mathrm{mag\ arcsec^{-2}}\) are recorded, and the half-light semi-major axis \(r50\) is derived analytically from the best-fit curve-of-growth parameters [2307.04888]. Beyond the tabular catalog, the release includes multi-wavelength mosaics, Tractor model stacks and residual images, PSF stamps, \(5\sigma\) depth maps, maskbit images, and a group catalog based on friends-of-friends linking with a \(10'\) linking length. Ancillary metadata include imaging depth, PSF FWHM per mosaic, and Galactic transmission per band. Public access is provided through the SGA web portal, the NOIRLab Data Lab, and an interactive viewer; file organization is per group under `RASLICE/GROUP_NAME`, and example usage includes validating masking through model and residual images, extracting \(\mu_r(r)\) profiles from the ELLIPSE HDU, deriving \(R(26)\) and \(D(26)\), and constructing Tully–Fisher samples using SGA geometric parameters and DESI rotation measures [2307.04888]. ## 4. Completeness, catalog fidelity, and known limitations The atlas is reported to be at least \(95\%\) complete for galaxies with \(R(26)\approx 25''\) and \(rR(26)<18\), measured at \(\mu_r = 26\ \mathrm{mag\ arcsec^{-2}}\), and more than \(99\%\) complete for galaxies larger than \(1'\) and brighter than \(rR(26)<16\) at the same isophotal limit. After careful vetting against HECATE and correction of catalog problems including spurious sources, shreds, overestimated diameters, and coverage gaps, only \(1.12\%\) of HECATE galaxies with \(2\times R1 > 1'\) are reported as genuinely missing. At the same time, the release notes emphasize that surface-brightness completeness varies with the heterogeneity of the input catalogs, and some sky areas without uniform three-band coverage are excluded [2307.04888]. Several systematic effects are explicitly documented. Sky-subtraction systematics can bias low-surface-brightness structure, especially for early-type outer envelopes. In BASS+MzLS regions north of \(\mathrm{Dec}\gtrsim +32.375^\circ\), Mosaic-3 pattern-noise subtraction distorts \(z\)-band profiles and colors, making galaxies appear too green in \(grz\) composites. In the infrared, W1/W2 backgrounds are sometimes over-subtracted because unWISE median background modeling uses \(1'\) grids. Crowded fields and bright stars can also degrade masking, centroiding, and profile recovery, with the Coma cluster cited as an example regime where such failures occur [2307.04888]. The release further enumerates pipeline-level failure modes. Ellipse fits were skipped for \(8{,}415\) galaxies with small Tractor sizes; \(27\) galaxies in very large groups did not complete processing; \(52\) fits were rejected after visual inspection; and \(6{,}161\) galaxies lack profiles despite `ELLIPSEBIT=0`. A catastrophic bug affected aperture photometry in the per-galaxy ellipse files, although the curves of growth were recovered from the profiles and reported in the merged catalog, so the recommended source for profiles and curve-of-growth information is the ELLIPSE HDU of `SGA-2020.fits`. A few of the largest galaxies, larger than \(5'\), also exhibit centroid offsets of a few arcseconds relative to independent infrared catalogs such as WXSC-100. Practical use therefore requires checking `ELLIPSEBIT`, maskbit images, and, in difficult fields, direct validation of centroiding and masking [2307.04888]. ## 5. DESI coupling and principal scientific use cases A central motivation for SGA-2020 is its integration with DESI spectroscopy. The atlas was built to provide robust sizes, geometries, photometry, and mosaics for large galaxies in order to improve the completeness and fidelity of the DESI Bright Galaxy Survey and to serve as an imaging framework for follow-up analyses. DESI spectra covering \(3600\)–\(9800\ \text{\AA}\) at \(R \approx 2000\)–\(5000\), with spectrophotometric precision of about \(\pm 2\%\), are expected for more than \(300{,}000\) SGA galaxies. These data support stellar-population and ISM diagnostics in the central regions of nearby galaxies when combined with the atlas’s resolved photometric information [2307.04888]. The atlas was also designed for peculiar-velocity and local-flow studies. For spirals, the intended scaling relation is the Tully–Fisher relation, \[ M = a\,\log_{10}(V_{\rm rot}) + b, \] with the equivalent luminosity form \(L \propto V_{\rm rot}^{\alpha}\). For early-type galaxies, the relevant relation is the Fundamental Plane, \[ \log R_{e} = \alpha\,\log \sigma + \beta\,\langle \mu \rangle_{e} + \gamma. \] The DESI Peculiar Velocity program targets SGA galaxies, including off-nuclear positions along major axes, to constrain distances and the \(z<0.15\) velocity field [2307.04888]. Beyond DESI’s core cosmology program, SGA-2020 functions as a reference sample for time-domain and multi-messenger astronomy. Its accurate positions, sizes, and multi-band reference images are intended to aid host-galaxy prioritization for transients and gravitational-wave events over large localization regions, including error ellipses exceeding \(100\ \mathrm{deg}^2\). Relative to earlier catalogs such as RC3 and HyperLeda, it updates positions, sizes, ellipticities, and position angles using modern deep imaging over a much larger footprint; positions generally agree within about \(0.5''\) median scatter, while \(D(25)\) values are on average about \(20\%\) larger than HyperLeda’s \(D25\), reflecting deeper imaging. Relative to NSA/SDSS, the atlas is specifically intended to avoid the bright-end incompleteness and shredding that affect SDSS catalogs at \(r \lesssim 14\)–\(15\) [2307.04888]. ## 6. Later interpretations, derivative analyses, and planned extensions Subsequent work has used SGA-2020 as a benchmark for galaxy-size methodology. The TNG50-SKIRT Atlas study compared simulated and observed half-light radii using an explicitly matched nonparametric curve-of-growth framework: concentric elliptical isophotes, semi-major-axis half-light radii, and band-by-band comparison in \(g\), \(r\), and \(z\). In all \(48\) matched bins in \(z\)-band absolute magnitude and \(g-z\) color, median effective radii decrease from \(g\) to \(r\) to \(z\). For the reddest galaxies \((1.2