Mission Overview

A Wide-Field WFC3 Imaging Survey in the COSMOS Field ("COSMOS-DASH")

Primary Investigator:  Pieter van Dokkum

HLSP Authors: Lamiya Mowla, Ivelina Momcheva, Gabriel Brammer, Sam Cutler, Katherine Whitaker

Released: 2019-07-01

Updated: 2023-10-04

Primary Reference(s): Mowla et al. (2019), Cutler et al. (2021)

DOI:  10.17909/T96Q5M

Citations: See ADS Statistics


Read Me (Morphological Catalog)

Source Data:



The COSMOS-DASH mosaic in sky coordinates. Coverage area mostly comprises three tall rectangles.

The H160 science image is shown in the main figure, along with insets for the exposure map and zoomed-in portions of the science image. The area contributed by COSMOS-DASH is 0.49 deg2; the total area with H160 data (including COSMOS-DASH, the deeper CANDELS imaging, and other archival data) is 0.66 deg2. The dark blue outline shows the 1.64 degree2 covered by ACS I814.

Low-resolution ground-based images and high-resolution COSMOS-DASH images of the same galaxies.
Comparison with Ground-Based Photometry

Images of massive galaxies at z~1.5 observed with UltraVISTA (top panel; H+i) and COSMOS-DASH (bottom panel; H160 + I814). Many single objects observed from the ground reveal more complex structures in high resolution HST images, allowing for more analysis of galaxy structures, close-pair fraction, etc.

Images of galaxies in COSMOS-DASH showing a wide variety of shapes.
Galaxy Images

Grid of some galaxy images from the COSMOS-DASH survey (6”x6”; H160+I814).

Fit lines to scatter plot of parameter H160 limit vs. 5-sigma depth, flat at depth<24.5 then linear.
Morphological Catalog Depth

Magnitude limit-depth relation for effective radius and Sérsic index used in generating parameter robustness flags in the COSMOS-DASH morphological catalog. Effective radius limits are shown with circles and a solid line, and Sérsic index limits are shown with squares and a dashed line. Limits from COSMOS-DASH are indicated by filled points (this work), while open points indicate values derived from the CANDELS/3D-HST morphological catalog (van der Wel et al. 2014).


COSMOS-Drift And SHift (COSMOS-DASH) is a Hubble Space Telescope WFC3 imaging survey of the COSMOS field in the F160W filter, comprising 456 WFC3 pointings covering an area of 0.49 deg2, giving 0.66 deg2 of total data in the archive. By taking advantage of the unique Drift And SHift (DASH) observing mode, COSMOS-DASH is the widest HST/WFC3 imaging survey in F160W, tripling the extragalactic survey area in the near-infrared at HST resolution. Generally, guide star acquisitions make it impractical to observe multiple pointings per orbit, so very wide near-IR surveys with HST are inefficient. However, this limitation can be overcome by guiding with gyros alone, which is possible when HST operates with three gyros. COSMOS-DASH observed eight independent WFC3-IR pointings per orbit, taking advantage of the fact that HST drifts by only a very small amount in the 25 seconds between non-destructive reads of unguided exposures.

Because the DASH method leads to shifts during exposures, reducing DASH data is more complex than for guided exposures. For full details, see Momcheva et al. (2017) and Appendix A1 of Mowla et al. (2019). Results from the COSMOS-DASH program show that with appropriate data reduction methods, the DASH technique preserves the WFC3 spatial resolution, and that galaxy structural parameters are consistent with those measured from guided surveys.

The science goals of COSMOS-DASH include a study of the relation between the star formation rate and structure of massive galaxies; the identification of the star forming progenitors of massive compact galaxies; an analysis of the merger rate using close pairs; and an analysis of structures of other rare objects such as post-starburst galaxies.

Towards these ends, the team has also provided the COSMOS-DASH Morphological Catalog, presenting structural measurements from 2D Sersic profile fits to galaxies with SNR>5 in COSMOS-DASH. Early science results from this catalog include evidence for an environment-independent flattening of the galaxy size-mass relation persisting out to z ~ 2 for log M ≲ 10.5. For full details on the morphological catalog, see Cutler et al. (2021).

v1.3.b update to morphological catalog

The 2023-10-04 update from v1.3 to v1.3.b represents a minor correction to the COSMOS-DASH Morphological Catalog:

  • Position angle errors (dpa) are now median error instead of mean error, yielding more reasonable errors for several sources.
  • Position angles (pa) for some flag=2 and flag=3 sources are set to -99.
  • One flag=3 source with negative e_F160W_tot has been set to -99 in multiple columns.

This minor update is described in more detail in the ReadMe file. The v1.3.b catalog at MAST matches the 10-04-2023 update to the copy in VizieR (corresponding to the "Corrections on e_PA and ID188056 sent by the author" annotation in the VizieR ReadMe) , and is still based on v1.3 mosaic products.

The last major update to COSMOS-DASH products was on 2021-12-01, corresponding to the original release of the morphological catalog.

Data Products

The main COSMOS-DASH product is a wide H160 imaging mosaic and weight map, currently available as version 1.2.10. The COSMOS-DASH Morphological Catalog was derived from an updated, redrizzled version of these mosaic products containing more archival data (version 1.3, not yet publicly available). Version 0.1 of the mosaic products refers to the preliminary reduction of the first 32 pointings. All COSMOS-DASH file names begin with the following string:


where <ver> = version number, and <ext> is one of the following extensions:

drz-sci.fits drizzled, calibrated mosaic of the COSMOS field in F160W
drz-wht.fits weight map corresponding to the COSMOS-DASH mosaic in F160W

morphological catalog of galaxies, derived from the version <ver> mosaic


For column descriptions and important recommendations on selection filters to use when working with the morphological catalog, see the morphological catalog's ReadMe file.

Data Access

v0.1 Images

(first 32 pointings)

v1.2.10 Images

(full mosaic)

Morphological Catalog






Supporting Files Description
pointings-info.txt A list of all FLT files that were used in the mosaics with information extracted from the headers and sky background value calculated during reduction.
shifts.tar.gz A folder containing shifts and rotations calculated for each image during alignment relative to the external catalogs. 
pointings-reg.txt DS9 regions file of the footprints of all pointings. 
drizzled-psf.tar.gz A folder containing cutouts of all the stars used to construct a PSF. Also included is a CSV file listing all the stars used in the PSF.



Please remember to cite the appropriate paper(s) below and the DOI 10.17909/T96Q5M if you use these data in a published work. 

Note: These HLSP data products are licensed for use under CC BY 4.0.