Observations and theoretical simulations have now established a framework for galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the center of collapsing dark matter halos, and subsequently grew through mergers and collisions leading to the hierarchical build-up of galaxy mass. It remains unclear, however, when and how disks and spheroids — the primary components of today's galaxies — were formed, over what timescales, and which processes (internal or external) were driving the early evolution of galaxies. Moreover, as a result of new empirical evidence — notably from our "SINS survey" — and recent theoretical developments, the emphasis is shifting from a merger-driven picture towards one in which smoother yet efficient modes of gas accretion and internal dynamical processes also played an important role in the formation and evolution of distant massive galaxies. To help make progress, it is crucial to gain further knowledge of the relevant mechanisms that control the phase, angular momentum, cooling, and dynamics of the baryonic matter.
The SINS survey
The SINS survey now sheds new light on these issues, from detailed information on the dynamics, morphologies, and physical properties of distant massive star-forming galaxies at early stages of evolution. The survey focusses on the crucial epochs at lookback times of 8–12 billion years ago (cosmological redshifts z ~ 1–4), when a major fraction of the mass in stars seen in present-day galaxies is believed to have been put in place.
Observations at near-infrared (1–2.5 μm) wavelengths are well-suited to study galaxies at z ~ 1–4 because many key diagnostic spectral features that are present in the rest-frame optical spectrum of galaxies are redshifted in the near-infrared bands. This includes important emission lines from ionized gas tracing the sites of intense star-formation activity or shock-excited material (such as the hydrogen Balmer recombination lines of Hα and Hβ, and the forbidden lines from atomic fine-structure transitions of [NII], [OIII], [OII], [SII]).
Until only a few years ago, spatially-resolved information of such key spectral diagnostics were virtually non-existent. This has now changed with the advent of sensitive near-IR integral field spectrographs mounted on 8–10 m ground-based telescopes, such as SINFONI at the ESO Very Large Telescope on Cerro Paranal, Chile. SINFONI consists of SPIFFI (built by the IR/submm group at MPE), a cryogenic near-infrared integral field spectrometer that provides the spectrum of a full atmospheric band simultaneously at all positions over the entire square field of view. SPIFFI is coupled to a curvature-sensing adaptive optics (AO) module, MACAO (built by ESO), which can be used with a natural or laser guide star and enable diffraction-limited observations at the Very Large Telescope (VLT) 8-m telescope.
The unique capabilities afforded by SINFONI make it possible to fully map the relative motions and spatial distribution of the gas and stars within distant galaxies and to relate directly spatial variations of the kinematics and physical properties with morphological features. To obtain a panoramic view of the properties of high redshift galaxies, we drew our SINS targets from samples selected by a variety of methods based on integrated colours and luminosities (for instance, "BM/BX" and "BzK" objects at z ~ 2, Lyman-break galaxies at z ~ 3, and K- and submillimeter-bright galaxies at z~ 1.5–3.5). More specifically, we used SINFONI to:
- measure source sizes and morphologies in line and continuum emission,
- characterize the velocity fields as ordered rotational/orbital motions such as for disk galaxies, or irregular/perturbed motions as expected for interacting/merging systems,
- measure the dynamical mass and mass-to-light ratios,
- constrain the angular momenta,
- investigate the dynamical state and stability,
- relate the star formation properties to the structure, dynamics, and gas content on kpc scales,
- map out spatial variations in the gas-phase chemical abundances and in the age of the stellar population from maps of line ratios and line-to-continuum ratios,
- explore the presence of outflows from gradients in line ratios and spectral line profiles,
- determine line and continuum luminosities, classify the dominant activity type (starburst versus AGN), velocity dispersions, and interstellar dust extinction, from both the integrated spectrum and the various maps.
- constrain the relative role of major mergers, smooth gas accretion, and feedback from star formation and AGN in the early phases of massive galaxy evolution.
The results from the SINS survey are providing unprecedented insights into the nature of distant massive star-forming galaxies and the processes involved in their mass assembly and early evolution; a selection is summarized in these highlights and all published results can be found in these papers.
The survey status
The observations carried out as part of the MPE SINFONI GTO observing time are now completed. SINS represents the largest survey of fully mapped emission line kinematics and morphologies of galaxies at z ~ 1.5 – 3, with 80 galaxies observed and over 63 detected in at least one rest-frame optical emission line. The histogram below shows the distribution of SINS galaxies according to their class, i.e. the primary selection criteria applied for the photometric surveys from which we drew our targets. The majority of the galaxies were observed in seeing-limited mode with typically excellent seeing conditions resulting in a spatial resolution of ≈ 4–5 kpc. In addition, 12 of the galaxies were also observed with the help of AO (using either a natural guide star or the LGSF), resolving structures on scales as small as ≈ 1–2 kpc, and AO-assisted observations of several more sources are pending.
Overview of the distribution of the 80 galaxies observed as part of SINS with SINFONI among different classes of galaxy populations at z ~ 1 – 4 (hatched histograms). Of the total, 63 galaxies were detected in rest-frame optical line emission (solid-filled histograms) for a success rate of 80%.
Building on the success of our SINS survey, our focus is now shifting towards addressing very specific scientific questions with the highest quality data from SINFONI as well as from other instruments providing key complementary information. On the observational front, we are:
- expanding our current SINFONI data sets, with particular emphasis on AO-assisted observations in order to resolve a majority of our targets on scales of ~ 1 kpc,
- complementing the main data on Hα and [NII] emission with data in other bands targeting further key diagnostic emission lines to constrain the physical conditions of the ISM,
- planning ultra-deep SINFONI integrations on selected representative targets in order to study individual regions with the best S/N ratio possible,
- combining the SINFONI data on line emission with very high angular resolution broad-band imaging tracing the distribution of stellar mass, using both the NICMOS/NIC2 camera on board the Hubble Space Telescope and the NACO instrument in combination with the LGSFat ESO's VLT,
- exploiting the strong synergies between the SINS survey and a long-term program involving several members of our team of spatially-resolved distribution and dynamics of the molecular gas in distant galaxies using the IRAM/Plateau de Bure millimeter interferometer.
To analyze the data and interpret the results, large efforts are dedicated into the application and development of sophisticated analysis tools, theoretical modeling, and comparisons to simulations of the formation and evolution of galaxies. These include:
- extraction of robust kinematic parameters through the application of kinemetry (originally developed by the SAURON team for local galaxies but now adapting it to high-redshift studies) and adaptive binning based on Voronoi tesselations, and on genetic algorithms for fitting the full 2D kinematic maps,
- quantitative characterization of the dynamics and of the stellar and gaseous components using dynamical and evolutionary synthesis modeling codes,
- detailed comparisons to predictions from numerical N-body/hydrodynamical and semi-analytical cosmological simulations, and to chemodynamical simulations of the evolution of galaxies.
With this broad observational program and these unique data, we aim at gaining new insights into some of the key issues of modern cosmology:
- the importance of violent major mergers versus smoother infall or minor mergers in the mass accretion process,
- the timescale for mass accretion and conversion of gas into stars,
- the dynamical evolution and stability of young galaxies,
- the relationship between bulge and disk formation,
- the efficiency of star formation at early stages of evolution,
- the role of angular momentum exchange and loss,
- the role of feedback processes from star formation and AGN powered by massive accreting black holes,
- the chemical abundances and enrichment of galaxies from stellar nucleosynthesis,
- the relationship between the young galaxies and their parent dark matter halos.