A Spectroscopic Galaxy Evolution Survey with the Hubble Space Telescope
3D-HST is a near-infrared spectroscopic survey with the Hubble Space Telescope designed to study the physical processes that shape galaxies in the distant Universe (GO-12177 & GO-12328; PI: Pieter van Dokkum). This Treasury program has been allocated 248 orbits of HST time during Cycles 18 and 19. 3D-HST is surveying ~600 square arcminutes of well-studied extragalactic survey fields (AEGIS, COSMOS, GOODS-S, UKIDSS-UDS) with two orbits of primary WFC3/G141 grism coverage and two to four orbits with ACS/G800L coverage. When completed, 3D-HST would provide the critical third dimension - redshift - for ~10,000 galaxies at z>1. This is the epoch when ~60% of the star formation in the Universe took place, the number density of quasars peaked, the first galaxies stopped forming stars and the structural regularity that we see in galaxies today emerged. The 3D-HST project details can be found in “3D-HST: A Wide-Field Grism Spectroscopic Survey with the Hubble Space Telescope”, Brammer et al., 2012, ApJ, 758L.
The survey is optimally designed for the study of galaxy evolution over 1 < z < 3.5. The science objectives include: disentangling the processes that regulate star-formation in massive galaxies, evaluating the role of environment and mergers in shaping the galaxy population, and resolving the growth of disks and bulges, spatially and spectrally.
The observations for 3D-HST are completed and high level science products will be available through MAST's HLSP section beginning mid-2013. The V3.0 release (May 10, 2013) which contains 8-17 orbit depth G141 spectra for >250 objects in the HUDF as well as F125W, F140W and F160W mosaics for AEGIS, COSMOS, GOODS-N, GOODS-S and UDS is available here.
For up-to-date information on the status on the survey, please visit the 3D-HST home page.
The 248 3D-HST orbits are divided among 124 individual pointings, each observed for two orbits (figure and table below). 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 will be 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 Hubble Ultra Deep Field (HUDF) 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.
The 28-pointing 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 ( image | regions ). Parallel ACS grism observations, however, are not available in this field.
The reduction, analysis, and interpretation of the slitless spectroscopy is not straightforward. The spectra of neighboring objects can overlap, which means a full 2D model of all spectra needs to be constructed and subtracted prior to analyzing the object of interest. Furthermore, the “PSF” of the grism spectra is effectively the (wavelength-dependent) morphology of the galaxy, complicating the fitting procedure. The 3D-HST team has developed custom packages that enable optimal modeling and fitting of interlaced 2D spectra. We find that these tools work very well, even on the deepest 17-orbit HUDF grism data that are currently available.
A further complication is that the grism spectra are often difficult to interpret without information from other wavelengths, as correct identification of faint emission lines usually requires some prior information on the likely redshifts of the objects. Data at other wavelengths are crucial for measuring stellar masses, rest-frame colors, star formation rates, and other parameters. With these goals in mind we have created photometric catalogs and derived stellar population parameters for all object in the 3D-HST fields. These will be made available along with 1D and 2D spectra, emission line fluxes, emission line maps and structural parameters as part of the 3D-HST data products.
A handful of objects exemplifying the wealth and variety of data contained within 3D-HST are presented in the following gallery:
When using data from the 3D-HST survey, please include the following acknowledgement: