The 3D-HST Survey: Hubble Space Telescope WFC3/G141 grism spectra, redshifts, and emission line measurements for $\sim 100,000$ galaxies

Abstract: We present reduced data and data products from the 3D-HST survey, a 248-orbit HST Treasury program. The survey obtained WFC3 G141 grism spectroscopy in four of the five CANDELS fields: AEGIS, COSMOS, GOODS-S, and UDS, along with WFC3 $H_{140}$ imaging, parallel ACS G800L spectroscopy, and parallel $I_{814}$ imaging. In a previous paper (Skelton et al. 2014) we presented photometric catalogs in these four fields and in GOODS-N, the fifth CANDELS field. Here we describe and present the WFC3 G141 spectroscopic data, again augmented with data from GO-1600 in GOODS-N. The data analysis is complicated by the fact that no slits are used: all objects in the WFC3 field are dispersed, and many spectra overlap. We developed software to automatically and optimally extract interlaced 2D and 1D spectra for all objects in the Skelton et al. (2014) photometric catalogs. The 2D spectra and the multi-band photometry were fit simultaneously to determine redshifts and emission line strengths, taking the morphology of the galaxies explicitly into account. The resulting catalog has 98,663 measured redshifts and line strengths down to $JH_{IR}\leq 26$ and 22,548 with $JH_{IR}\leq 24$, where we comfortably detect continuum emission. Of this sample 5,459 galaxies are at $z>1.5$ and 9,621 are at $0.7<z<1.5$, where H$\alpha$ falls in the G141 wavelength coverage. Based on comparisons with ground-based spectroscopic redshifts, and on analyses of paired galaxies and repeat observations, the typical redshift error for $JH_{IR}\leq 24$ galaxies in our catalog is $\sigma_z \approx 0.003 \times (1+z)$, i.e., one native WFC3 pixel. The $3\sigma$ limit for emission line fluxes of point sources is $1.5\times10^{-17}$ ergs s$^{-1}$ cm$^{-2}$. We show various representations of the full dataset, as well as individual examples that highlight the range of spectra that we find in the survey.

• The paper achieves precise redshift measurements (≈0.003×(1+z)) and accurate emission line fluxes for nearly 100,000 galaxies using HST grism spectroscopy.
• It employs innovative non-slitted grism techniques and advanced software to extract interlaced 2D and 1D spectra, effectively managing spectral overlaps.
• The comprehensive dataset from four CANDELS fields supports detailed studies of galaxy evolution, star formation, and cosmic structure over 11 billion years.

Analysis and Implications of the 3D-HST Survey: A Comprehensive Redshift and Emission Line Catalog

The paper "The 3D-HST Survey: Hubble Space Telescope WFC3/G141 grism spectra, redshifts, and emission line measurements for $\sim 100,000$ galaxies" provides a meticulous account of the 3D-HST survey conducted using the Hubble Space Telescope (HST). This survey is a crucial endeavor to create a comprehensive catalog of near-infrared grism spectroscopy data, targeting around 100,000 galaxies. It utilizes the WFC3 G141 grism to cover significant portions of the CANDELS fields, making pivotal contributions to our understanding of galaxy properties and evolution.

The survey's ambitious scope encompasses four of the five CANDELS fields, with precise attention to data uniformity and spectral accuracy. The survey's primary aim is to gather redshifts and emission line fluxes, crucial for characterizing galaxies over a cosmic timeline of approximately 11 billion years, specifically focusing on epochs when substantial stellar formation occurred.

Key methodological innovations in the survey include the utilization of a non-slitted grism, which facilitates simultaneous spectral capture of all objects within the field. This approach maximizes efficiency in data acquisition but introduces complexity due to spectral overlaps. The authors mitigate these challenges by developing and implementing sophisticated software to optimally extract and analyze interlaced 2D and 1D spectra.

A significant achievement noted in the paper is the measurement of redshifts with an accuracy of approximately $\sigma_z \approx 0.003 \times (1+z)$ for galaxies with $IR\leq 24$. This precision is a function of the survey's ability to hybridize spectral data with multi-wavelength photometry. The skillful integration of broad-band photometry with grism spectroscopy allows for a more holistic capture of galaxy properties, including morphology-dependent spectral features. As such, the data not only provide robust redshifts but also deliver comprehensive line strength measurements crucial for studying star formation rates and metallicity gradients.

The survey's statistical reach is impressive, measuring redshifts for nearly 100,000 galaxies, with a keen focus on crucial redshift intervals where H$\alpha$ falls within the G141 wavelength coverage. The spectrophotometric precision achieved underscores the utility of the 3D-HST survey in evaluating large-scale cosmological structures and galaxy evolution.

The implications of this dataset are profound for both practical astrophysics and theoretical modeling. Practically, the dataset supports large-scale structural studies of galaxy clusters and environmental effects on galaxy morphology. Theoretically, it provides empirical foundations to refine models of galaxy formation and evolution, helping to calibrate simulations of the universe's expansion history. Moreover, the survey's public data release invites further scientific inquiry, enabling broader community-driven research.

In forecasting future developments, the integration of 3D-HST data with upcoming missions like the James Webb Space Telescope (JWST) presents promising avenues for exploring even fainter galaxies and more distant cosmic epochs with unprecedented detail.

In conclusion, the 3D-HST survey stands as a significant milestone in extragalactic astrophysics, providing a meticulously curated and accessible dataset that bridges observational gaps and enriches our understanding of the universe's grand tapestry.