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. This survey provides rest-frame optical spectra for a complete sample of ~10,000 galaxies at redshifts 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.  

3D-HST provides a spectrum for essentially every object in the field (to a magnitude limit, modulo contamination from overlapping spectra). The survey contains a great diversity of objects — from high-z  quasars to  brown dwarf stars -- but is optimally designed for the study of galaxy formation over 1 < z <  3.5. Some of the science objectives that require the unique combination of high spatial resolution, deep near-IR (and supporting optical) spectra 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.  These questions are discussed in greater detail below.


Science Overview

3D-HST in short

HST Orbits: 248

Area: 600 sq. arcmin

Redshifts: ~10,000 galaxies at 1<z<3

Redshift precision: Δz/(1+z)~0.4%

Instruments: WFC3 (G141 grism and F140W

direct) and ACS (G800 grism and F814W)

HST programs: 12177 and 12328

What causes galaxies to stop forming stars?

In the low redshift Universe many galaxies are observed to be quiescent, with current SFRs only ∼1% of their past average (e.g., Pasquali et al. 2006). These quiescent galaxies tend to be massive early-type galaxies, forming the “red sequence” in the color-mass distribution of galaxies. Recent work has shown that at z 2 many massive galaxies (M≳1011 M⊙) exhibit spectacularly high star-formation rates (SFRs) of hundreds of solar masses per year, whereas others were already quiescent, particularly those that are extremely compact for their mass (Kriek et al. 2006; van Dokkum et al. 2008; Brammer et al. 2009). AGN feedback is a possible mechanism to suppress gas cooling and star formation (e.g., Croton et al. 2006), but direct
evidence is scarce.

Diagnostics of quiescence can be correlated with stellar mass, surface density (i.e., compactness), and the environment of galaxies on Mpc scales. These diagnostics are most reliably identified spectroscopically, through the strength of the Balmer break (D4000, see Figures right) and/or the absence of emission lines such as Hα. For a 5σ limiting emission line flux of 3 × 10−17 erg s−1 cm−2, 3D-HST will reach Hα and [O II]λ3727 star-formation rate limits (Kennicutt 1998) of 1.5 and 15 M⊙ yr−1 at z=1 and z=2, respectively. If the simultaneous presence of quiescent and star-bursting massive galaxies at z 2 is the result of their surface density or their environment, correlations should exist between the SFR and these parameters.

To what extent are galaxies shaped by their environment?

The morphology-density relation (Dressler 1980) reflects that “early-type” galaxies (mostly massive quiescent galaxies) are relatively abundant in dense environments such as groups and clusters. However, at low redshift most massive galaxies are quiescent regardless of environment (Kauffmann et al. 2003; Balogh et al. 2004), so it is difficult to determine whether the environment provides a physical mechanism that alters the galaxies (e.g., through gas stripping), or whether dense environments simply are the place where quiescent galaxies tend to end up.

To disentangle the roles of mass and environment, epochs should be considered when “massive” did not yet directly imply “quiescent” (van Dokkum et al. 2011). The 3D-HST sample will be sufficiently large to determine the relation between SFR and environment in bins of fixed mass and redshift. If no environmental dependence is observed at fixed mass, then the relations between galaxy properties and environment are simply a by-product of the underlying relations of both quantities with mass. Even excellent photometric redshifts with errors δz ≈ 0.04(1 + z) have a radial error on the co-moving distance of > 150 Mpc at z = 2, which is larger than the distance of the Milky Way to the Coma cluster. With redshift errors more than an order of magnitude smaller, 3D-HST will be able to provide a sensible definition of the environmental galaxy density on the scale of a few co-moving Mpc, as well as spectroscopic diagnostics of galaxy “quiescence”.

How did disks and bulges grow?

The epoch 1 < z < 3 saw the wholesale transition from small, star-forming clumps evident in the deepest Hubble images (e.g., Elmegreen et al. 2007) to the ordered “realm of galaxies” seen today. Since z 1, most star formation has taken place in large spiral disks, but different modes may have been prevalent at earlier times, such as disks made of star-bursting clumps that coalesce to form compact spheroids directly (Dekel et al. 2009).

If bulges formed before disks, then objects with stellar masses 1–5×1010 M⊙ and star formation concentrated on 1 kpc scales should be seen at z 3. These should evolve into young compact bulges surrounded by disk-like (∼5 kpc) star formation at z 2, and then to old bulges and regular disks by z 1. By contrast, if bulges formed mostly from subsequent merging of disks, extended star-forming disks should be pervasive at z 3. The relative ages of the subcomponents of galaxies and the spatial extent of line-emitting star-forming regions can be measured from the spatially-resolved HST grism spectra (van Dokkum & Brammer 2010). Ground-based integral-field spectroscopy has demonstrated the power of spatially-resolved spectroscopy for galaxies at z > 1 (e.g., Förster Schreiber et al. 2009; Law et al. 2009), but it has been typically limited to more luminous, rare objects or relatively small samples.

What is the role of mergers in galaxy formation?

Although the merger-driven growth of massive galaxies is a common prediction of galaxy formation models, it has been difficult to test at higher redshift where mergers should be very common (e.g., Guo & White 2008). The merger rate can be determined from physical pair statistics, but these have been difficult to measure due to insufficient spatial resolution and contamination by chance superpositions of unassociated galaxies (e.g., Williams et al. 2011; Man et al. 2011).


An object in the GOODS-North field with multiple line-emitting components. Two separate spectra are shown extracted for the bright compact component (which itself has two close sub-components) and the fainter, more diffuse tail extending to the upper right of the image thumbnail. (From Brammer et al., 2012.)

Extremely massive galaxies (1011.5 and 1011.2 M⊙) at z 2 with strong continuum breaks and no visible emission lines. The inset panels show the full 0.3–8μm SEDs (photometry + spectra) and the template fit. (From Brammer et al., 2012.)

With its spatial resolution of 0.′′13 and spectral resolution of δv ≈ 1000 km s−1, 3D-HST can spectroscopically identify true physical pairs and groups down to separations < 5 kpc, weeding out projected galaxy pairs (see Figure right). Within the 3D-HST sample, the pair fraction can be measured as a function of mass and redshift, and the fractions can be turned into a merger rate using models (e.g., Kitzbichler & White 2008; Lotz et al. 2011Williams et al. 2011). The mass growth of galaxies due to mergers can be compared to  the growth due to star-formation, and the sizes, densities, AGN content of the merger  components can be determined. The star-formation rates of the spectroscopically-identified merging pairs can be compared to predictions of hydrodynamical simulations, such as that mergers should play a large role in driving star-formation activity and black hole accretion (e.g., Cox et al. 2006).