3D-HST: A wide-field grism spectroscopic survey with the Hubble Space Telescope (1204.2829v2)

Abstract: We present 3D-HST, a near-infrared spectroscopic Treasury program with the Hubble Space Telescope for studying the processes that shape galaxies in the distant Universe. 3D-HST provides rest-frame optical spectra for a sample of ~7000 galaxies at 1<z<3.5, the epoch when 60% of all star formation took place, the number density of quasars peaked, the first galaxies stopped forming stars, and the structural regularity that we see in galaxies today must have emerged. 3D-HST will cover 3/4 (625 sq.arcmin) of the CANDELS survey area with two orbits of primary WFC3/G141 grism coverage and two to four parallel orbits with the ACS/G800L grism. In the IR these exposure times yield a continuum signal-to-noise of ~5 per resolution element at H~23.1 and a 5sigma emission line sensitivity of 5x10-17 erg/s/cm2 for typical objects, improving by a factor of ~2 for compact sources in images with low sky background levels. The WFC3/G141 spectra provide continuous wavelength coverage from 1.1-1.6 um at a spatial resolution of ~0."13, which, combined with their depth, makes them a unique resource for studying galaxy evolution. We present the preliminary reduction and analysis of the grism observations, including emission line and redshift measurements from combined fits to the extracted grism spectra and photometry from ancillary multi-wavelength catalogs. The present analysis yields redshift estimates with a precision of sigma(z)=0.0034(1+z), or sigma(v)~1000 km/s. We illustrate how the generalized nature of the survey yields near-infrared spectra of remarkable quality for many different types of objects, including a quasar at z=4.7, quiescent galaxies at z~2, and the most distant T-type brown dwarf star known. The CANDELS and 3D-HST surveys combined will provide the definitive imaging and spectroscopic dataset for studies of the 1<z<3.5 Universe until the launch of the James Webb Space Telescope.

• The paper introduces a novel near-infrared spectroscopic survey using HST’s WFC3 grism to capture rest-frame optical spectra of approximately 7000 galaxies between redshifts 1 and 3.5.
• The methodology features precise background subtraction, contamination control, and integration with deep multi-wavelength photometry for robust redshift and emission line measurements.
• The survey yields unprecedented insights into galaxy formation, star formation suppression, and morphological transformation, establishing a key resource prior to JWST.

Analysis of the 3D-HST Grism Survey with the Hubble Space Telescope

The 3D-HST survey is a strategic near-infrared spectroscopic Treasury program carried out with the Hubble Space Telescope (HST). The program aims to investigate the physical processes governing the formation and evolution of galaxies during the critical epoch at redshifts $1 < z < 3.5$. This timeframe captures significant cosmic events, including the peak of quasar activity and the emergence of structural regularity among galaxies that resemble today's universe. The survey integrates with the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) imaging to provide detailed spectroscopic data over an area of 625 arcmin², encompassing the most deeply studied extragalactic fields such as AEGIS, COSMOS, GOODS-N, GOODS-S, and UKIDSS-UDS.

The survey utilizes the Wide Field Camera 3 (WFC3) grism to collect slitless spectra, offering a unique opportunity to examine approximately 7000 galaxies' rest-frame optical properties at a depth and resolution unprecedented for studies within this redshift range. The WFC3/G141 grism provides continuous wavelength coverage from 1.1 to 1.6 $\mu\mathrm{m}$, with a resolution element yielding a signal-to-noise ratio (S/N) of ~5 at $H_{140} \approx 23.1$, enabling the detection of emission lines down to $5 \times 10^{-17}\ \mathrm{erg\ s^{-1}\ cm^{-2}}$.

Results and Methodology

The paper presents a comprehensive account of the methods used for grism data acquisition and analysis, emphasizing the critical steps in background subtraction and contamination control, which are pivotal for ensuring the accuracy of spectroscopic data in a slitless configuration. Utilizing the MultiDrizzle and aXe software, the team developed a pipeline to process raw grism data efficiently. This automated processing generates high-fidelity two-dimensional (2D) spectra and optimally weighted one-dimensional (1D) extractions.

A key feature of 3D-HST is its integration with deep multi-wavelength photometric datasets, which are crucial for precise redshift estimation. The photometric ancillary data allow for robust spectroscopic redshift measurements with an impressive precision of $\sigma(z) = 0.0034(1+z)$. This accuracy surpasses that achievable with broadband photometry alone by an order of magnitude. The survey successfully disentangles redshift uncertainties associated with single-line identifications and photometric degeneracies by incorporating detailed spectral-energy distribution (SED) fitting.

Emission line strength serves as an essential component in the analysis, with 3D-HST exhibiting sensitivity to key features such as H$\alpha$ and [O III] emission lines. These diagnostics allow investigations into varying star formation activities and AGN contributions across redshift. The data reveal redshift trends and correlations between emission line properties and galaxy morphology and environment, thereby providing insight into the mechanisms driving star formation suppression and morphological transformation.

Implications and Future Prospects

The 3D-HST survey's dataset, when combined with CANDELS imaging, forms the definitive framework for rest-frame optical studies of high-redshift galaxies until the launch of the James Webb Space Telescope (JWST). The extensive spatial and spectral coverage supports a broad range of scientific inquiries, from understanding the cessation of star formation in massive galaxies to dissecting environmental influences on galaxy evolution.

In practical terms, the survey enables a detailed search for quiescent galaxies, compellingly supporting investigations into the processes underlying starburst activity and quiescence transition. It facilitates robust studies of the morphology-density relationship and the role of major mergers, contributing significantly to our understanding of high-redshift galaxy assembly dynamics.

Overall, 3D-HST sets a precedent for future spectroscopic surveys, demonstrating the feasibility and scientific value of combining grism spectroscopy with multi-band photometry. As data continue to be refined and analyzed, the survey's comprehensive catalog and spectra will remain a valuable resource for ongoing and future research efforts aimed at unraveling the complexities of galaxy formation and evolution in the universe's formative epoch.