An international group of astronomers has used the ALMA observatory to obtain an inventory of molecular gas and dust in distant galaxies at unprecedented depth in the iconic Hubble Ultra-Deep Field (H-UDF), one of the best-studied regions of the sky. The final results are being presented in a series of papers in the Astrophysical Journal. Dr. Jorge González-López, astronomer at Las Campanas Observatory of the Carnegie Institution for Science, participated in the research.
It has been well established that stars form from the gravitational collapse of dense clouds of molecular gas. To characterise galaxies’ evolution, a measurement of their molecular gas content and its evolution over cosmic time is indispensable. With this goal in mind, an international group of astronomers has led the ALMA large program ASPECS (The ALMA SPECtroscopic Survey in the Hubble Ultra-Deep Field): The first extragalactic large program approved by ALMA, designed to make an unbiased, three-dimensional survey of the molecular gas content of galaxies in the best-studied extragalactic deep field, the iconic Hubble Ultra-Deep-Field (H-UDF).
The ASPECS team chose the H-UDF as their target. It provides the ultimate observations in-depth and resolution across the electromagnetic spectrum, extending beyond traditional continuum imaging, and is ideally situated for ALMA observations. As Dr. Chris Carilli of the National Radio Astronomy Observatory (NRAO) explains: “The success of ASPECS stems from two major advances: the unparalleled sensitivity of the ALMA observatory, and the unprecedented multi-wavelength database that is available for the Hubble Ultra-Deep Field, as manifested by more than 1000 hours of observations using the most advanced astronomical facilities, in space and on ground”.
The ASPECS team further chose an observational approach commonly referred to as ‘frequency scanning’ to obtain a measurement of molecular gas in a well-defined cosmic volume. This approach was selected to maximize the cosmic volume for red-shifted Carbon Monoxide (CO) lines, a direct tracer of molecular gas, and to obtain millimeter continuum images of the HUDF at unprecedented depths. Astronomer Jorge González-López explains this method in the following way: “The frequency scanning technique is similar to tuning to your favorite AM/FM radio network, you move along the dial until you reach a signal. For us, each of these ‘radio networks’ will be a line detection of molecular gas in a distant galaxy”. The validity of the observational approach for the ASPECS large program was demonstrated through several pilot programs, both with the IRAM Plateau de Bure Interferometer and ALMA observations in earlier cycles.
3D renderings of the ASPECS band 3, and visible light. Strong cold molecular gas emitters appear as bright spots in the cubes. Linear features correspond to bright dust-continuum sources. Credits: Decarli et al, 2019, 2020
The unprecedented depth of the ALMA millimeter frequency scans and continuum images revealed the presence of dozens of distant, dusty galaxies over the H-UDF region allowing the ASPECS team to address several outstanding questions about how galaxies form and grow. Aside from just pinpointing the galaxies that contain most of the cold gas and dust reservoirs in the UDF, the wealth of multi-wavelength data available for the ASPECS/UDF field, which includes deep NASA/ESA Hubble Space Telescope imaging and ESO Very Large Telescope MUSE spectroscopy, enables a full characterization of the physical properties of the ASPECS-detected galaxies. Prof. Manuel Aravena of the Universidad Diego Portales in Santiago, Chile, concludes, “The ASPECS data provide our deepest look at the distant dusty universe to date and could recover all massive reservoirs of dust and gas in the H-UDF. Owing to our observing strategy, we could identify massive gas reservoirs in galaxies where we did not expect them given the low star formation rates and stellar masses”. Indeed, the unprecedented depth achieved by the ASPECS survey allowed the team to identify as individual galaxies almost all of the cold dust reservoirs existent out to early cosmic times in the H-UDF. The ALMA results could be well explained in the context of previous measurements through parallel modeling efforts by the ASPECS team. They also confirm that, on average, galaxies ~10 billion years ago were mostly made up of molecular gas rather than stars, compared to the opposite seen in galaxies today.
Comparison between the Hubble optical/near-infrared image in the ASPECS footprint of the H-UDF, shown to the right. Credit: STScI, gonzalez-Lopez et al, ALMA (ESO/NAOJ/NRAO)
"The ASPECS project shows how important and necessary is the synergy between different observatories and the approval of large scale programs. The hundreds of hours invested with space and ground-based telescopes allowed us to identify the galaxies detected with ALMA and thus know where the molecular gas and cold interstellar dust were", points out González-López. "The unbiased scanning done with ALMA allowed us to obtain information on the detected and undetected galaxies. The depth of the observations allowed us to place important restrictions on the amount of gas and dust in distant galaxies. Our analysis tells us that we are detecting all the reserves of cold dust in distant galaxies, which has important implications for models of galaxy formation and evolution," he adds.
The ASPECS survey also allowed for determining the evolution of the cosmic molecular gas density from the current universe back in time to within 2 Gyr of the Big Bang. As Dr. Roberto Decarli of the National Institute of Astrophysics in Bologna, Italy, explains “Our analysis unambiguously showed that the molecular gas density peaks at when the Universe was about 4 Gyr old, and then declines by almost an order of magnitude to the value measured in the local Universe.” This behavior was suggested in previous molecular deep fields, including the ASPECS pilot program. Still, now the better statistics allowed the ASPECS team to firmly conclude that there is a rise and fall for the molecular gas density with cosmic time. This peak of molecular gas density corresponds to the one in the star formation history (`the epoch of galaxy assembly’).
This behavior was put in context with other key estimates of galaxy properties, particularly the evolution of cosmic density of atomic gas, the star formation rate, and the stellar mass build-up. Dr. Fabian Walter of the Max Planck Institute for Astronomy in Heidelberg, Germany, concludes: “Putting all the information together, we were able to determine the amount of gas that had to be accreted to the centers of galaxies over cosmic time, to account for the stars that are present in galaxies.” As such, these observations provide significant constraints for galaxy evolution models and simulations.
In the future, the ASPECS team hopes to continue their studies of gas-rich galaxies by using higher-resolution ALMA data and soon to be obtained observations using NASA’s James Webb Space Telescope (scheduled to be launched in late 2021).