PRISM-Astro/h2f_desi_ddf_cosmos

Description

Spectroscopic galaxy sample of the COSMOS Deep Drilling Field, the field the Vera C. Rubin Observatory will observe with roughly twice the cadence of the other LSST DDFs. Redshifts come from DESI Data Release 1 and images are multi-resolution cutouts of the DESI Legacy Imaging Surveys DR10 served through hips2fits by CDS.

It contains 71,747 galaxies, one row each. Every cutout is centered on the galaxy itself, so there is no augmentation and no host_pos label: the galaxy sits at the center of every image by construction. This makes it the DESI counterpart of h2f_ps1_pasquet, built with the same pipeline and the same conventions, but on a different field, a different survey and a redshift range almost five times wider.

Sample selection

Galaxies were taken from desi_dr1.zpix inside a cone of radius 1.7481 deg around the Rubin DDF pointing (RA 150.11, Dec +2.23), the circle of equal area to the 9.6 deg² LSSTCam field of view. Only primary spectra with a reliable redshift were kept (zcat_primary, zwarn = 0, spectype = 'GALAXY'), and each was crossmatched to its nearest source in ls_dr10.tractor_s within 1.5 arcsec.

From the 83,620 rows that survived the crossmatch, three cuts were applied before downloading any image:

• 4,498 galaxies whose ls_id appeared more than once, since several DESI fibers can land on the same imaging source. The closest match was kept.
• 4,259 sources of Tractor type PSF or DUP, which carry shape_r = 0 and have no measurable morphology.
• 121 galaxies with a non-physical redshift, z <= 0.

A further 2,995 were dropped after download because at least one pixel came back as 0.0 in some band, marking a gap in HiPS coverage. Those gaps are real holes in the survey rather than transient failures: re-requesting the same cutout recovered none of the 447 cases that had a whole (level, band) plane empty. No image in this dataset contains a single zero pixel.

Cross-validation

The fold column holds a 5-fold split stratified by spectroscopic redshift. Redshift is discretized into 764 uniform bins using the same bin width as h2f_ps1_pasquet (0.4 / 180 = 0.00222), extended to cover this field's much wider redshift range. The 33 galaxies falling in bins with fewer than 5 members carry fold = 0 and belong to training only, since they cannot be stratified. They sit almost entirely at the high-redshift tail, so validation metrics say nothing about the z > 1.5 regime even though the model sees those examples.

Image format

image has shape (71747, 5, 4, 30, 30), which maps to (examples, resolution levels, photometric bands, height, width). Bands are griz in that order — four bands, not the five of the PanSTARRS counterpart. Level L has a pixel scale of 0.262 * 2^L arcsec/px, so level 0 spans 7.86 arcsec and level 4 spans 125.76 arcsec around the galaxy.

Ellipse convention

ellipse_phi is not the raw catalog angle. Legacy Surveys stores ellipticity as a complex number, e = (a-b)/(a+b) * exp(2i*phi), from which the position angle is 0.5 * atan2(shape_e2, shape_e1). That angle is defined against the sky axes, and drawing it on a pixel array requires a further 90 degree rotation, verified against second-moment measurements of the cutouts themselves. This column already carries that rotation, wrapped back into [-90, 90), so it can be fed directly to a profile generator in the same way rSerPhi is used in h2f_ps1_pasquet. The raw components are kept in shape_e1 and shape_e2 for anyone who needs the catalog convention.

Galaxies of type REX are round by construction: their ellipticity is fixed at zero, so ellipse_ab = 1 and ellipse_phi = 0. That zero is a convention, not a measurement — a circle has no orientation. Use type to mask them.

Columns

Column	Type	Description
Image
image	(5,4,30,30) float32	Multi-resolution Legacy Surveys DR10 griz cutout, centered on the galaxy.
Identification
ls_id	int64	Legacy Surveys DR10 identifier of the galaxy, unique in this dataset. Packs RELEASE, BRICKID and OBJID into one integer.
targetid	int64	DESI identifier of the spectrum the redshift comes from. The key back to desi_dr1 tables and to SPARCL.
sep_arcsec	float32	Angular separation of the DESI–Legacy Surveys crossmatch, in arcsec. Median 0.06, at most 1.496.
Sky position
ra	float32	Right ascension of the galaxy, in degrees. This is the center of the cutout.
dec	float32	Declination of the galaxy, in degrees. This is the center of the cutout.
Redshift
z	float32	Spectroscopic redshift from DESI DR1, the regression target. Ranges from 0 to 1.697, with a median of 0.534.
z_error	float32	Uncertainty reported by the DESI spectroscopic pipeline.
z_class	int64	Redshift bin, 0 to 763, of width 0.00222. Used to stratify the folds and as the classification target.
Extinction
ebv	float32	Reddening E(B−V) from the SFD dust map.
mw_transmission_g, _r, _i, _z	float32	Galactic transmission per band, in linear units between 0 and 1. Multiply the pixels by the inverse to correct for extinction.
extinction_g, _r, _i, _z	float32	The same extinction expressed in magnitudes, -2.5*log10(mw_transmission).
Morphology — Tractor fit, all bands jointly
type	string	Model the Tractor selected: REX round exponential, EXP exponential, DEV de Vaucouleurs, SER free Sérsic. All are Sérsic profiles; the label says how the index was treated.
shape_r	float32	Half-light radius of the fitted model, in arcsec. Not band-specific: the Tractor fits one morphology across all bands.
shape_e1, shape_e2	float32	Raw ellipticity components as the catalog stores them. Both exactly 0 for REX.
ellipse_radius	float32	Semi-major axis of the fitted ellipse, in arcsec. Equal to shape_r; the minor axis is ellipse_radius * ellipse_ab.
ellipse_ab	float32	Axis ratio b/a, between 0 and 1. Derived as (1-|e|)/(1+|e|) with |e| = hypot(shape_e1, shape_e2).
ellipse_phi	float32	Position angle of the fitted ellipse, in degrees over [-90, 90), already rotated to the pixel frame. See the ellipse convention section above.
Cross-validation
fold	int64	Validation fold, 1 to 5, stratified by z_class. A value of 0 means the galaxy is used for training in every split.

References

[1] DESI Collaboration 2025, Data Release 1 of the Dark Energy Spectroscopic Instrument. arXiv:2503.14745

[2] Dey, A., Schlegel, D. J., Lang, D., et al. 2019, Overview of the DESI Legacy Imaging Surveys, The Astronomical Journal, 157, 168. doi:10.3847/1538-3881/ab089d

@article{Dey_2019,
  doi = {10.3847/1538-3881/ab089d},
  url = {https://doi.org/10.3847/1538-3881/ab089d},
  year = {2019},
  month = {apr},
  publisher = {American Astronomical Society},
  volume = {157},
  number = {5},
  pages = {168},
  author = {Dey, Arjun and Schlegel, David J. and Lang, Dustin and others},
  title = {Overview of the DESI Legacy Imaging Surveys},
  journal = {The Astronomical Journal},
}