SGA-2025¶

The SGA-2025 delivers multiwavelength imaging mosaics and ellipse photometry for nearly half a million large, resolved galaxies across approximately 30,000 deg² of the extragalactic sky.

This page describes the organization of the public data release (v1.0), the naming conventions used throughout, and the data model of the primary science-ready merged catalogs.

Data Access¶

Note

The SGA-2025 data release and web visualizations are currently in draft form and still a work in progress.

Merged catalogs — one FITS file per imaging region (right-click → Save link as to download) (see Merged Catalogs for the data model and Imaging Regions for region descriptions):

SGA2025-dr11-north-v1.0.fits — 90,504 galaxies, 340 MB
SGA2025-dr11-south-v1.0.fits — 395,435 galaxies, 1.4 GB

Web visualizations (work in progress):

Per-group data files — for bulk download via wget or Globus (see Per-Group Data Files for the full file inventory):

Naming Conventions¶

Two layers of identifiers are used throughout the SGA-2025: groups (mosaics containing one or more spatially/angularly associated galaxies) and objects (individual galaxies within each group). Most objects in the sample are in singleton groups.

Groups: `GROUP_NAME` and `SGAGROUP`¶

Each SGA-2025 mosaic is processed as a group — a set of one or more galaxies whose angular sizes and separations place them within the same mosaic footprint. The mosaic is centered on the diameter-weighted centroid of its members.

Two catalog columns identify a group:

GROUP_NAME (str10, e.g. 00000m0649) — the bare group name, used as the on-disk directory name.
SGAGROUP (str18, e.g. SGA2025_00000m0649) — the prefixed form, used as the file-name prefix for all per-group output files inside that directory.

The encoding is RRRRR[p|m]DDDD where RRRRR = RA × 100 (five digits, zero-padded), the separator is p (positive Dec) or m (negative Dec), and DDDD = abs(Dec) × 100 (four digits, zero-padded), both at a precision of 0.01° (36 arcsec). For example, a group centered at RA = 0.00°, Dec = −6.49° has GROUP_NAME = 00000m0649.

In groups with more than one member (GROUP_MULT > 1), GROUP_PRIMARY identifies the largest galaxy in the group based on its initial diameter. GROUP_RA and GROUP_DEC give the diameter-weighted centroid; GROUP_DIAMETER (in arcmin) gives the diameter of the smallest circle enclosing all member ellipses.

Objects: `SGAID` and `SGANAME`¶

Individual galaxies within a group are identified by:

SGAID (int64) — a unique integer identifier assigned to each source.
SGANAME (str25, e.g. SGA2025 J000.000−006.490) — an IAU-style coordinate name at 0.001° (3.6 arcsec) precision, formed from the fitted galaxy coordinates RA and DEC.
OBJNAME (str30) — the common name from the input parent catalog (HyperLeda, NED, LVD, etc.).

Galaxy Names: `GALAXY` and `ALTNAMES`¶

GALAXY (str35) is the primary human-readable name assigned to each source. ALTNAMES (str200) holds up to three additional names, pipe-separated. Both are derived from NED’s pipe-separated CROSSIDS cross-identification list through the following steps, applied in order:

HOST fix — If the NED-primary name contains HOST (e.g. SN 2003H HOST), the first non-HOST entry in CROSSIDS is promoted to position 0 and the original HOST name is appended at the end (deduplicated case-insensitively).
Famous-name promotion — The highest-priority well-known catalog name anywhere in CROSSIDS is promoted to GALAXY, regardless of NED’s own ordering. Priority order (highest first): Messier, NGC, IC, UGCA/UGC, ESO, ESO-LV, MCG, ARP, MRK, DDO, KDG, VCC, CGCG, PGC, PGC1, AGC, FCC, HCG, HIPASS. Names absent from this list (2MASX, SDSS, WISE, ICRF, etc.) remain as GALAXY only when no higher-priority name is available.
Special-character fix — If the promoted GALAXY name contains characters outside the safe set (letters, digits, space, +, -, ., _, :) — a common pattern for NED names beginning with [ — the first ALTNAMES candidate that passes the safe-character test is promoted to GALAXY instead and the original name is moved into ALTNAMES.
Prefix normalization — Known catalog-name prefixes are fully uppercased (e.g. Mrk → MRK, Arp → ARP, UGCa → UGCA). Messier is intentionally left in mixed case.
OBJNAME fallback — If NED returned no match for the object, GALAXY is set to OBJNAME from the parent catalog.
Hand-curated overrides — A small dictionary of one-off corrections (e.g. MESSIER 109 → Messier 109) is applied after all rules above.

ALTNAMES is built from the remaining CROSSIDS entries (after the promoted GALAXY name is removed and case-insensitive duplicates are dropped), with prefix normalization applied; the field is blank when no alternatives remain.

Imaging Regions¶

SGA-2025 covers two (mostly) non-overlapping imaging regions drawn from the DESI Legacy Imaging Surveys Data Release 11 (DR11) , which together span approximately 30,000 deg² of the sky (or 90% of the extragalactic sky at Galactic latitude |b|>10).

dr11-south uses g-, r-, i-, and z-band imaging from the Dark Energy Camera (DECam) at the 4-m Victor M. Blanco Telescope at Cerro Tololo Inter-American Observatory (CTIO).

dr11-north combines two surveys covering the sky north of roughly Dec ≈ +32°:

BASS (Beijing-Arizona Sky Survey) — provides g- and r-band imaging from the 90-Prime wide-field imager at the University of Arizona’s 2.3-m Bok telescope on Kitt Peak National Observatory.
MzLS (Mayall z-band Legacy Survey) — provides z-band imaging obtained with the Mosaic-3 camera at the NOAO 4-m Mayall telescope on Kitt Peak National Observatory.

The northern region therefore has three optical bands (g, r, z) with no i-band coverage. The BANDS column in the merged catalog records which optical bands are actually available for each galaxy.

In addition to the optical imaging, the SGA-2025 also includes:

Infrared: unWISE W1 (3.4 μm) and W2 (4.6 μm) and WISE W3 (12 μm) and W4 (22 μm) mosaics, derived from the full-depth coadded WISE satellite imaging.
Ultraviolet: GALEX FUV (~1530 Å) and NUV (~2310 Å) imaging (where GALEX sky coverage exists).

Merged Catalogs¶

The per-region merged catalogs combine all per-group ellipse photometry, NASA/IPAC Extragalactic Database (NED) cross-matched results, spectroscopic redshifts from the Dark Energy Spectroscopic Instrument Data Release 1 (DESI/DR1), and direct distances from the Local Volume Database (LVD) into a single FITS file per imaging region.

These catalogs are the starting point for most science use cases.

Catalog Files¶

Two versioned FITS files are provided, one per imaging region (see Data Access for direct download links):

SGA2025-dr11-north-v1.0.fits    ( 90,504 galaxies, 340 MB)
SGA2025-dr11-south-v1.0.fits    (395,435 galaxies, 1.4 GB)

Each file has two binary table extensions:

SGA2025 — the primary science catalog containing ellipse photometry, geometry, and cross-matched redshifts and distances for every galaxy.
TRACTOR — Tractor source-fitting parameters, row-matched to SGA2025.

Reading the Catalogs¶

The catalogs are standard FITS binary tables. The recommended way to read them is via fitsio wrapped in an astropy Table:

import fitsio
from astropy.table import Table

cat     = Table(fitsio.read('SGA2025-dr11-south-v1.0.fits', ext='SGA2025'))
tractor = Table(fitsio.read('SGA2025-dr11-south-v1.0.fits', ext='TRACTOR'))

fitsio also supports reading a subset of columns:

cat = Table(fitsio.read('SGA2025-dr11-south-v1.0.fits', ext='SGA2025',
                        columns=['GALAXY', 'RA', 'DEC', 'D26', 'Z']))

Most analyses begin from the group primary galaxies (one per group):

import numpy as np

primaries = cat[cat['GROUP_PRIMARY']]

Common row selections:

# Diameter cuts (arcmin)
cut = (cat['D26'] > 1.0) & (cat['D26'] < 10.0)

# Specific galaxies by name
cut = np.isin(cat['GALAXY'], ['Messier 065', 'NGC 3621'])

# Local Volume Database objects (SAMPLE bit 1)
cut = (cat['SAMPLE'] & 1) != 0

# Groups with 2–5 members
cut = (cat['GROUP_MULT'] >= 2) & (cat['GROUP_MULT'] <= 5)

# Galaxies with a measured redshift
cut = cat['Z_IVAR'] > 0

Data Model: SGA2025 Extension¶

The SGA2025 extension contains one row per galaxy. Columns are organized into the groups below.

Identification and sky position

Column	Type	Description
`SGAID`	int64	Unique SGA identifier
`SGANAME`	str25	IAU-style name (`SGA2025 JXXX.XXX±YY.YYY`)
`GALAXY`	str35	Primary NED name (first entry from NED cross-ID list)
`ALTNAMES`	str200	Up to three additional NED cross-IDs, pipe-separated
`OBJNAME`	str30	Common galaxy name from the parent catalog
`PGC`	int32	PGC identifier (0 if unavailable)
`RA`, `DEC`	float64	Fitted galaxy coordinates (deg)
`RA_TRACTOR`, `DEC_TRACTOR`	float64	Tractor-fitted position (deg)
`MORPH`	str30	NED morphological type string

Status and quality bitmasks

Column	Type	Description
`SGAGROUP`	str18	Prefixed group name (e.g. `SGA2025_00000m0649`)
`REGION`	int16	Imaging region bitmask (see Bitmasks)
`SAMPLE`	int32	Sample classification bitmask (see Bitmasks)
`ELLIPSEBIT`	int32	Output quality and status flags (see Bitmasks)
`ELLIPSEMODE`	int32	Input fitting-mode flags requested for this object
`BANDS`	str4	Optical bands available (e.g. `griz`, `grz`)

Fitted geometry

Column	Type	Description
`D26`	float32	Isophotal diameter at μ = 26 mag arcsec⁻² (arcmin)
`D26_ERR`	float32	1σ uncertainty on D26 (arcmin)
`D26_REF`	str3	Band from which D26 was derived (one of `g`, `r`, `i`, `z`; `mom` if the moment-based SMA was the only available estimate; `fix` when the `FIXGEO` `ELLIPSEMODE` bit is set and the geometry was held fixed)
`BA`	float32	Final axis ratio b/a
`PA`	float32	Final position angle (deg, N through E)
`SMA_MOMENT`	float32	Semi-major axis from flux-weighted moments (arcsec)
`BA_MOMENT`	float32	Axis ratio b/a from moments
`PA_MOMENT`	float32	Position angle from moments (deg, N through E)

Adopted redshift (see Redshifts and Distances for resolution logic)

Column	Type	Description
`Z`	float64	Adopted redshift (0 = no measurement; use `Z_IVAR > 0` to test)
`Z_IVAR`	float64	Inverse variance of adopted redshift (0 = no measurement)
`Z_REF`	str4	Source of adopted redshift: `LVD`, `DESI`, `NED`, or blank
`Z_FLAG`	int16	Redshift quality flag bitmask (see Bitmasks)

Adopted distance (see Redshifts and Distances for resolution logic)

Column	Type	Description
`DIST`	float32	Adopted distance (Mpc; 0 = no measurement)
`DIST_IVAR`	float32	Inverse variance of adopted distance (Mpc⁻²; 0 = no measurement or no formal uncertainty reported)
`DIST_REF`	str4	Source of adopted distance: `LVD` or `NED`

DESI DR1 spectroscopy

The columns below record the single best DESI spectrum selected for each galaxy (see Redshifts and Distances). Sentinel values: TARGETID_DESI and ZWARN_DESI are −1 when no spectrum was selected; Z_DESI and Z_IVAR_DESI are 0; DELTACHI2_DESI is −1; string columns are blank.

Column	Type	Description
`TARGETID_DESI`	int64	DESI target identifier (−1 if no spectrum)
`SURVEY_DESI`	str7	DESI survey (e.g. `main`, `sv1`, `sv3`)
`PROGRAM_DESI`	str6	DESI observing program (e.g. `bright`, `dark`)
`Z_DESI`	float64	DESI spectroscopic redshift (0 if no valid spectrum)
`Z_IVAR_DESI`	float64	Inverse variance of `Z_DESI` (0 if no valid spectrum)
`ZWARN_DESI`	int64	DESI redshift warning bitmask (−1 if no spectrum)
`DELTACHI2_DESI`	float64	DESI Δχ² between best and second-best redshift fit (−1 if no spectrum)
`SPECTYPE_DESI`	str6	DESI spectral classification (e.g. `GALAXY`, `QSO`, `STAR`)
`NSPEC_DESI`	int32	Number of DESI spectra (any ZWARN) within the SGA ellipse (0 if none)

NED redshift and distance

Column	Type	Description
`Z_NED`	float64	NED redshift (0 = no NED redshift)
`Z_IVAR_NED`	float64	Inverse variance of `Z_NED`; derived from `NED_Z_UNC`. When `NED_Z_UNC = 0` (error not reported in NED), a nominal uncertainty of 100 km/s is assumed. Set to 0 only when no NED redshift exists.
`Z_REF_NED`	str	NED literature reference for the redshift
`DIST_NED`	float32	NED distance (Mpc; 0 = no NED distance)
`DIST_IVAR_NED`	float32	Inverse variance of `DIST_NED` (Mpc⁻²; 0 if no NED distance or no formal uncertainty was reported)
`NED_FLAG`	int32	NED cross-match quality flag (see Bitmasks)

LVD redshift and distance

These columns are populated only for objects in the Local Volume Database (SAMPLE & LVD != 0). Velocities are converted from km/s using Z = v_los / c.

Column	Type	Description
`Z_LVD`	float64	LVD systemic redshift from `vlos_systemic` (0 if not in LVD)
`Z_IVAR_LVD`	float64	Inverse variance of `Z_LVD` from `vlos_sigma` (0 if no LVD velocity error)
`DIST_LVD`	float32	LVD distance (Mpc, converted from kpc; 0 if not in LVD)
`DIST_IVAR_LVD`	float32	Inverse variance of `DIST_LVD` (Mpc⁻²; 0 if no LVD distance uncertainty)

Group membership

Column	Type	Description
`GROUP_NAME`	str10	Bare group name (= on-disk directory name)
`GROUP_MULT`	int16	Number of group members
`GROUP_PRIMARY`	bool	`True` for the primary (largest) group member
`GROUP_RA`, `GROUP_DEC`	float64	Group centroid coordinates (deg)
`GROUP_DIAMETER`	float32	Diameter of smallest enclosing circle (arcmin)

Imaging quality and corrections

Column	Type	Description
`EBV`	float32	Milky Way E(B−V) from Schlegel et al. (1998) (mag)
`MW_TRANSMISSION_{band}`	float32	Milky Way transmission fraction (0–1); all 10 bands
`PSFSIZE_{G,R,I,Z}`	float32	Effective PSF FWHM in each optical band (arcsec)
`PSFDEPTH_{G,R,I,Z}`	float32	5σ point-source depth in each optical band (mag)

Photometric columns — all 10 bands: G, R, I, Z, W1, W2, W3, W4, FUV, NUV

Column pattern	Description
`GINI_{band}`	Gini coefficient of the light distribution
`COG_MTOT_{band}` / `COG_MTOT_ERR_{band}`	Curve-of-growth total magnitude ± uncertainty (mag)
`COG_DMAG_{band}` / `COG_DMAG_ERR_{band}`	COG aperture correction ± uncertainty (mag)
`COG_LNALPHA1_{band}` / `COG_LNALPHA1_ERR_{band}`	COG Sérsic inner shape parameter ± uncertainty
`COG_LNALPHA2_{band}` / `COG_LNALPHA2_ERR_{band}`	COG Sérsic outer shape parameter ± uncertainty
`COG_CHI2_{band}`	COG fit χ² (float32)
`COG_NDOF_{band}`	COG fit degrees of freedom (int32)
`SMA50_{band}` / `SMA50_ERR_{band}`	Half-light semi-major axis ± uncertainty (arcsec)
`SMA_AP{00–04}`	Aperture semi-major axes (arcsec); AP00–AP04 correspond to [0.5, 1.0, 1.25, 1.5, 2.0] × `SMA_MOMENT`
`FLUX_AP{00–04}_{band}`	Aperture flux (nanomaggies)
`FLUX_ERR_AP{00–04}_{band}`	Aperture flux uncertainty (nanomaggies)
`FMASKED_AP{00–04}_{band}`	Masked pixel fraction within aperture (0–1)

Isophotal radii — optical bands G, R, I, Z only

Column pattern	Description
`R{22,23,24,25,26}_{band}`	Isophotal semi-major axis at μ = 22–26 mag arcsec⁻² (arcsec)
`R{22,23,24,25,26}_ERR_{band}`	1σ uncertainty on the isophotal radius (arcsec)

Initial geometry

These columns record the geometry adopted from the input parent catalog at the start of processing, before any ellipse fitting. They are useful for quantifying how much the pipeline has revised the initial size and shape estimates, and for identifying objects whose fitted geometry has diverged substantially from the input. INIT_REF encodes the source of the initial diameter: one of SGA2020, HYPERLEDA, LIT, LVD, SMUDGes, RC3, or VI (visually inspected and added during processing).

Column	Type	Description
`RA_INIT`, `DEC_INIT`	float64	Parent-catalog coordinates used to initialize processing (deg; −1 if not available)
`DIAM_INIT`	float32	Initial diameter from the parent catalog (arcmin; −1 if not available)
`BA_INIT`	float32	Initial axis ratio b/a from the parent catalog (−1 if not available)
`PA_INIT`	float32	Initial position angle from the parent catalog (deg, N through E; −1 if not available)
`MAG_INIT`	float32	Initial magnitude from the parent catalog (mag; −1 if not available)
`OPTFLUX`	float32	Optical reference flux used to set source-masking priority within a group (nanomaggies; 0 if not set)
`SMA_MASK`	float32	Semi-major axis used for neighbor masking during ellipse fitting (arcsec; 0 if not set); overrides the moment-based SMA when positive
`INIT_REF`	str14	Source catalog for `DIAM_INIT` (blank if not available)

Data Model: TRACTOR Extension¶

The TRACTOR extension is row-matched to SGA2025 and contains Tractor source-fitting parameters for each SGA galaxy and all sources detected within its ellipse. See the Legacy Surveys documentation for the full Tractor data model.

Redshifts and Distances¶

The merged catalogs cross-match three independent redshift sources — DESI DR1, NED, and the Local Volume Database (LVD) — and one additional distance source (NED) for galaxies without LVD distances.

Adopted redshift (Z, Z_IVAR, Z_REF)

The adopted redshift follows the priority order LVD > DESI > NED:

Z_REF = LVD when Z_IVAR_LVD > 0 (LVD systemic velocity with a reported uncertainty).
Z_REF = DESI when Z_IVAR_DESI > 0 (a valid DESI spectrum was selected; see below).
Z_REF = NED when Z_IVAR_NED > 0 (a NED redshift exists).
Z_REF = '' (blank) when no valid redshift is available from any source.

Important

Always use Z_IVAR > 0 to test for a valid redshift. Z = 0 is ambiguous: it is the sentinel for no measurement but is also a valid (if rare) redshift for very nearby galaxies.

DESI DR1 spectrum selection

A single best DESI spectrum is selected per galaxy using the following steps, applied to all spectra within 3 arcsec of the SGA position:

Retain only ZCAT_PRIMARY = True rows (highest-S/N coadd per TARGETID).
Drop physically implausible spectra regardless of ZWARN:
- SPECTYPE = STAR — always a mis-classification for galaxies.
- SPECTYPE = GALAXY and Z > 1.5 — pipeline redshift error.
- SPECTYPE = QSO and Z > 1.0 — outside the redshift range of this sample.
Prefer SPECTYPE = GALAXY and ZWARN = 0.
Fall back to any ZWARN = 0 spectrum regardless of SPECTYPE.
Among survivors, select the spectrum with the highest DELTACHI2.

If no spectrum survives all filters, no DESI redshift is adopted for that galaxy (Z_IVAR_DESI = 0). The 3-arcsec proximity cut excludes background or foreground objects whose fiber happens to fall inside a large SGA ellipse. NSPEC_DESI records the total number of DESI spectra (any ZWARN) found within the SGA ellipse, regardless of the selection outcome.

Z_FLAG (redshift discrepancy)

Bit 0x01 of Z_FLAG is set when any two of the three measured redshifts (LVD, DESI, NED) differ by more than 1000 km/s in the rest frame. This flags sources where independent measurements are inconsistent and warrants closer inspection.

Adopted distance (DIST, DIST_IVAR, DIST_REF)

The adopted distance follows the priority order LVD > NED:

DIST_REF = LVD when DIST_LVD > 0 (an LVD distance exists).
DIST_REF = NED when DIST_NED > 0 (a NED distance exists).

DIST_IVAR = 0 may mean either no distance is available or that no formal uncertainty was reported (unlike redshifts, no nominal default uncertainty is assumed for distances). LVD distances are converted from kpc to Mpc.

Sentinel conventions

Column type	Sentinel / missing value
Redshifts and their IVARs (`Z`, `Z_IVAR`, etc.)	0 — use the paired `*_IVAR > 0` column to test for a valid measurement
Distances and their IVARs (`DIST`, `DIST_IVAR`, etc.)	0 — same convention as redshifts
Other float columns	−1 (strictly negative, unphysical for distances and separations)
Integer IDs (`TARGETID_DESI`, etc.)	−1
String columns	blank (empty string)

Bitmasks¶

`SAMPLE`¶

Set in the parent catalog and propagated to the merged catalogs. A source may have multiple bits set simultaneously.

Bit name	Value	Description
`LVD`	1	Source from the Local Volume Database of nearby galaxies
`MCLOUDS`	2	Located within the Magellanic Cloud footprint
`GCLPNE`	4	Located within a globular cluster or planetary nebula mask
`NEARSTAR`	8	Near a bright star (star–galaxy separation < 1.2 × Gaia/Tycho mask)
`INSTAR`	16	Inside a bright star (star–galaxy separation < 0.5 × Gaia/Tycho mask)
`OVERLAP`	32	Initial ellipse overlaps another (initial) SGA ellipse

`ELLIPSEMODE` — input fitting flags¶

Requested before processing; describes the intended fitting strategy.

Bit name	Value	Description
`FIXGEO`	1	Fix the ellipse geometry to initial values (no free fitting)
`RESOLVED`	2	Milky Way satellite (effectively resolved into stars); Tractor fitting not performed
`FORCEPSF`	4	Force all Tractor sources to be PSF
`FORCEGAIA`	8	Force Tractor to only fit Gaia sources (not used)
`LESSMASKING`	16	Apply less aggressive masking during ellipse-fitting
`MOREMASKING`	32	Apply more aggressive masking during ellipse-fitting
`MOMENTPOS`	64	Use moment-based position instead of Tractor position
`TRACTORGEO`	128	Adopt Tractor geometry instead of ellipse-fit geometry
`NORADWEIGHT`	256	Disable radial weighting in when determining light-weighted geometry

`ELLIPSEBIT` — output fitting and quality flags¶

Set after processing; records the actual fitting outcome and data quality.

Bit name	Value	Description
`NOTRACTOR`	1	No Tractor model (dropped or never fitted)
`TRACTORPSF`	2	Tractor fit this source as a point source (PSF model)
`FIXGEO`	4	Ellipse geometry was fixed, not freely fitted
`BLENDED`	8	Source center is within the ellipse of another SGA source
`LARGESHIFT`	16	Large positional shift between initial and fitted ellipse center
`LARGESHIFT_TRACTOR`	32	Large positional shift between initial and Tractor center
`MAJORGAL`	64	Major galaxy requiring special processing treatment
`OVERLAP`	128	Fitted ellipse overlaps another SGA ellipse
`SATELLITE`	256	Satellite galaxy located within the ellipse of a larger system
`MOMENTPOS`	512	Moment-based position was adopted
`TRACTORGEO`	1024	Tractor geometry was adopted
`NORADWEIGHT`	2048	Radial weighting was not applied to isophote fitting
`LESSMASKING`	4096	Less aggressive neighbor masking was applied
`MOREMASKING`	8192	More aggressive neighbor masking was applied
`FAILGEO`	16384	Isophote fitting failed; D26 may be unreliable
`SKIPTRACTOR`	32768	Tractor fitting was skipped entirely for this mosaic

`NED_FLAG` — NED cross-match quality¶

Records how the NED cross-match was resolved for each galaxy. The byname query searches NED by galaxy name; the bypos query searches by sky position. Multiple bits may be set simultaneously.

Bit name	Value	Description
`LARGE_SEP`	1	Byname positional offset exceeds threshold (3 arcsec); bypos result used instead
`BYNAME_FAIL`	2	Byname query returned an error or no match; bypos result used instead
`BYPOS_FAIL`	4	Bypos query also failed; NED cross-match result may be incomplete

Note

LVD galaxies are exempt from the LARGE_SEP flag: their NED coordinates can legitimately differ from the SGA centroid (extended dwarfs), so a successful byname match is always accepted regardless of positional offset.

`Z_FLAG` — redshift consistency¶

Bit name	Value	Description
`DISCREPANCY`	1	Any two measured redshifts (LVD, DESI, NED) differ by more than 1000 km/s in the rest frame; the adopted redshift should be treated with caution

`OPTMASKBITS`, `GALEXMASKBITS`, `UNWISEMASKBITS`¶

Per-band imaging mask bits stored in the per-group -maskbits.fits[.fz] files and the per-galaxy -ellipse-{dataset}.fits files. All three share the same low-order bits:

Bit name	Value	Description
`brightstar`	1	Bright star, medium star, or star cluster (legacypipe MASKBITS)
`gaiastar`	2	Gaia point source (type = PSF)
`galaxy`	4	Extended (non-reference) source
`reference`	8	SGA reference source

Per-band bits:

OPTMASKBITS: g = 16, r = 32, i = 64, z = 128
GALEXMASKBITS: FUV = 16, NUV = 32
UNWISEMASKBITS: W1 = 16, W2 = 32, W3 = 64, W4 = 128

Per-Group Data Files¶

Directory Layout¶

The public data release is organized as:

data/
  {region}/                  # dr11-south or dr11-north
    {raslice}/               # 3-digit integer part of GROUP_RA (000–359)
      {group_name}/          # e.g. 00000m0649
        SGA2025_{group_name}-*

The raslice is the zero-padded integer part of GROUP_RA (e.g. RA = 33.98° → 033). The group_name is the bare GROUP_NAME column value.

To construct the path to a group directory in Python:

import os

datadir = '/path/to/data'   # local copy of the data release
region  = 'dr11-south'      # or 'dr11-north'

for row in cat:
    raslice   = f'{int(row["GROUP_RA"]):03d}'
    group_name = row['GROUP_NAME'].strip()
    galaxydir  = os.path.join(datadir, region, raslice, group_name)
    prefix     = f'SGA2025_{group_name}'

File Inventory¶

All files in a group directory are prefixed by g = SGA2025_{group_name}.

Two processing modes exist. Full mode applies to groups where Tractor source fitting was performed (the file {g}-tractor.fits is present). Coadds-only mode applies to groups where only image mosaics were produced (no Tractor fitting).

Files present in both modes:

File	Description
`{g}-sample.fits`	Parent-catalog rows for this group
`{g}-ccds.fits`	Metadata for all CCDs overlapping this mosaic
`{g}-coadds.isdone`	Sentinel file written when coadd step completes
`{g}-ellipse.isdone`	Sentinel file written when ellipse step completes
`{g}-coadds.log`	Coadd processing log
`{g}-ellipse.log`	Ellipse processing log
`{g}-image.jpg`	Optical grz color composite
`{g}-image-W1W2.jpg`	unWISE W1/W2 color composite
`{g}-image-FUVNUV.jpg`	GALEX FUV/NUV color composite

Full mode only:

File	Description
`{g}-tractor.fits`	Tractor source catalog for this mosaic
`{g}-maskbits.fits.fz`	Imaging mask-bit array (compressed)
`{g}-image-{band}.fits.fz`	Image mosaic per band (compressed); bands = g, r, i, z, FUV, NUV, W1–W4
`{g}-invvar-{band}.fits.fz`	Inverse-variance map per band (compressed)
`{g}-model-{band}.fits.fz`	Tractor model image per band (compressed)
`{g}-psf-{band}.fits.fz`	PSF image per band (compressed)
`{g}-model.jpg`	Optical grz Tractor model composite
`{g}-model-W1W2.jpg`	unWISE W1/W2 Tractor model composite
`{g}-model-FUVNUV.jpg`	GALEX FUV/NUV Tractor model composite
`{g}-resid.jpg`	Optical grz residual (image − model)
`{g}-resid-W1W2.jpg`	unWISE W1/W2 residual
`{g}-resid-FUVNUV.jpg`	GALEX FUV/NUV residual

Coadds-only mode only:

File	Description
`{g}-maskbits.fits`	Imaging mask-bit array (uncompressed)
`{g}-image-{band}.fits`	Image mosaic per band (uncompressed); at least one of g, r, i, z plus FUV, NUV, W1–W4

Per-Galaxy Ellipse Files¶

In addition to the per-group files, each galaxy within a group has up to three per-galaxy ellipse files named after the galaxy’s IAU coordinate name (without spaces):

SGA2025_J{RA}[+|-]{DEC}-ellipse-griz.fits
SGA2025_J{RA}[+|-]{DEC}-ellipse-galex.fits
SGA2025_J{RA}[+|-]{DEC}-ellipse-unwise.fits

Each file contains four FITS extensions:

Extension	Description
`MODELS`	Image cube of the ellipse model and data for each band
`MASKBITS`	Per-band imaging mask array (same footprint as `MODELS`)
`ELLIPSE`	Ellipse photometry table (one row per ellipse semi-major axis step)
`SBPROFILES`	Azimuthally averaged surface-brightness and color profiles

Release History¶

v1.0 (2025)¶

First public release.

SGA-2025¶

Data Access¶

Naming Conventions¶

Groups: GROUP_NAME and SGAGROUP¶

Objects: SGAID and SGANAME¶

Galaxy Names: GALAXY and ALTNAMES¶

Imaging Regions¶

Merged Catalogs¶

Catalog Files¶

Reading the Catalogs¶

Data Model: SGA2025 Extension¶

Data Model: TRACTOR Extension¶

Redshifts and Distances¶

Bitmasks¶

SAMPLE¶

ELLIPSEMODE — input fitting flags¶

ELLIPSEBIT — output fitting and quality flags¶

NED_FLAG — NED cross-match quality¶

Z_FLAG — redshift consistency¶

OPTMASKBITS, GALEXMASKBITS, UNWISEMASKBITS¶

Per-Group Data Files¶

Directory Layout¶

File Inventory¶

Per-Galaxy Ellipse Files¶

Release History¶

v1.0 (2025)¶

Groups: `GROUP_NAME` and `SGAGROUP`¶

Objects: `SGAID` and `SGANAME`¶

Galaxy Names: `GALAXY` and `ALTNAMES`¶

`SAMPLE`¶

`ELLIPSEMODE` — input fitting flags¶

`ELLIPSEBIT` — output fitting and quality flags¶

`NED_FLAG` — NED cross-match quality¶

`Z_FLAG` — redshift consistency¶

`OPTMASKBITS`, `GALEXMASKBITS`, `UNWISEMASKBITS`¶