A Spectroscopic Galaxy Evolution Survey with the Hubble Space Telescope


Target Fields

3D-HST targets four well-studied extragalactic fields:  EGS/AEGIS, COSMOS, UKIDSS-UDS, and GOODS-South. The G141 grism coverage of most of the GOODS-N field from program GO-11600 (PI: B. Weiner) is incorporated into 3D-HST as the observational strategy of the GOODS-N observations are nearly identical to that of 3D-HST. In total 3D-HST surveys 625 square arcminutes with two orbits of primary WFC3/G141 grism coverage and two to four orbits with ACS/G800L coverage. The survey covers roughly 75% of the area imaged by the CANDELS survey.  The areas of the sky targeted by 3D-HST are the best-studied extragalactic fields, offering a wealth of deep, multi-wavelength imaging and spectroscopic observations from many previous ground-based and space-based surveys. Such ancillary data sets, spanning from X-ray to radio wavelengths, are crucial for the interpretation of both the WFC3 imaging and grism spectroscopy datasets.  

The 248 3D-HST orbits are divided among 124 individual pointings, each observed for two orbits, as shown in Figure 1 and summarized in Table 1. In order to schedule 3D-HST concurrently with CANDELS observations of the same fields over Cycles 18 and 19, the orientations of the fields were determined only after the observations were scheduled. The positions of the individual pointings were optimized to provide contiguous mosaics and maximum overlap between the primary WFC3 G141 and parallel ACS G800L observations. Owing to this optimization, fully 90% of the G141 mosaic (excluding GOODS-N) is covered by between two and four orbits of the ACS grism. Within the GOODS-South mosaic, four of the two-orbit visits are centered on the Ultra Deep Field (UDF) at the same orientation. The GOODS-South pointings outside of the area with CANDELS coverage provide WFC3 grism spectroscopy of the HUDF09  and WFC3-ERS fields.

Figure 1. Layout of the 124 3D-HST pointings. Primary WFC3 F140W+G141 pointings are shown in blue with the pointing ID numbers as defined in the HST Phase II file. The locations of the parallel ACS F814W+G800L observations are shown in light green. Also indicated is the distribution of the 28 pointings covering the GOODS-North field from program GO-11600 that are incorporated into 3D-HST. The light gray polygons indicate the footprint of the CANDELS WFC3 imaging, including both the “wide” and “deep” components of that survey. Note that the relative sizes of the separate fields are not shown exactly to scale. (Figure from Brammer et al., 2012)


The WFC3 G141 grism is the primary spectral element used for the 3D-HST survey. The combined transmission of the HST  optical telescope assembly and the primary spectral order of the G141 grism (“+ 1st ”) is greater than 30% from 1.10 to 1.65 μm, reaching a peak of nearly 50% at 1.45 μm. The mean dispersion of the + 1st  order is 46.5 Å/pixel (R~130) and varies by a few percent across the field of view. The uncertainties of the wavelength zeropoint and dispersion of the G141 grism are 8Å and 0.06Å/pixel, respectively. The spectral features covered by the G141 grism include Hα at 0.7<z<1.5, [O III ] 5007 at 1.2 < z < 2.3, [O II ] 3727 at 2.0 < z < 3.4, and the Balmer break at 1.8 < z < 3.1 (Figure 2). The nominal G141 dispersion corresponds to ~1000 km/s for Hα at z > 1; however, in practice the resolution of the slitless grism spectra is determined by the physical extent of a given object.

Observations with the HST  grisms typically require an accompanying image taken with an imaging filter to establish the wavelength zeropoint of the spectra. For 3D-HST, we obtain these “direct” images in the broad F140W filter that spans the gap between the standard J  and H  passbands and lies roughly in the center of the G141 sensitivity (Figure 2). While CANDELS will ultimately provide significantly deeper imaging of the 3D-HST fields in the F125W and F160W WFC3 filters, the 3D-HST F140W direct images can be useful for scientific analysis in addition to calibrating the grism, as they reach depths competitive with even the deepest ground-based surveys (H~26.1; 5σ and spatial resolution ~0.13 arcsec).  In addition to the primary WFC3 observations, 3D-HST obtains parallel ACS F814W imaging and G800L grism spectroscopy. The G800L grism covers wavelengths 0.55 to 1.00 μm with a dispersion of 40 Å/pixel and a resolution of 80 Å for point-like sources. The parallel spectroscopy extends the Hα line sensitivity of the survey to z  = 0 and will provide coverage of additional lines at certain redshift intervals where only a single line is visible in WFC3/G141, for example [O III ] in G800L and Hα in G141 at 0.7 < z < 1.

Each of the 3D-HST two-orbit visits with WFC3 is structured in an identical fashion: four pairs of a short F140W direct image followed by a longer G141 grism exposure. The four pairs of direct+grism exposures are separated by small telescope offsets to enable the rejection of hot pixels and pixels affected by cosmic-rays, as well as dithering over some WFC3 cosmetic defects such as the “IR-blobs” (Pirzkal et al. 2010 ). The sub-pixel offsets of the dither pattern are chosen to improve sampling of the WFC3 PSF, which enables some recovery of the image quality lost by the pixels that undersample the PSF by a factor of 2.

Figure 2. Throughput curves of the WFC3/G141 (primary) and ACS/G800L (parallel) grisms and the WFC3/F140W and ACS/F814W imaging filters used to define the reference for the grisms. The shaded bands in the top panel indicate the redshift range in which rest-frame optical spectral lines of O II, Hβ, O III, and Hα fall within the WFC3 (red) and ACS (blue) grisms. (Figure from Brammer et al., 2012.)


The 3D-HST observations were executed in HST Cycles 18 and 19. The observations were divided between programs 12177 and 12328. The first 3D-HST exposures were obtained in October 2010 and the program was completed in April 2013. The 28 grism pointings in GOODS-N were completed in April 2011. The scheduling windows from STScI can be found here:

  1. Scheduling for 12177

  2. Scheduling for 12328

Data Reduction

We have developed a custom data reduction pipeline which can quickly and automatically reduce any typical direct and grism image sequence.  The 3D-HST survey fields provide a wealth of ancillary multi-wavelength observations that are crucial for interpreting the grism spectra, which frequently only contain a single emission line, if any. We have adapted the EAZY code (Brammer et al. 2008) to measure redshifts from SEDs composed of both the grism spectra and matched photometry spanning 0.3-8 μm. These spectrophotometric redshifts are more than an order of magnitude more precise than typical broad-band photometric redshift estimates, with σ = 0.0034(1+z). The redshift-fitting code also provides measurements of emission line fluxes and equivalent widths.

Details on the 3D-HST data reduction and analysis are provided by Brammer et al. (2012, 2013). A detailed desciption of the methods used in the final data release is given in Momcheva et al. (2016). The two libraries developed by Gabriel Brammer for the 3D-HST data reductions, threedhst and unicorn, are now public.