SDSS-III finds a large population of massive galaxies still alive eight billion years after the Big Bang

Building on the legacy of the Sloan Digital Sky Survey (SDSS), the SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS) is currently mapping the spatial distribution of the most massive galaxies in the Universe. MPA scientists have been involved in the SDSS for more than a decade. They have been using the galaxy spectra obtained by these experiments to infer important physical information about the stars and the gas in these systems, which illuminate how galaxies formed and evolved over the history of the Universe.

Fig. 1: This pie-diagram shows the sky distribution of the BOSS massive galaxies. In red, we show the new BOSS Galaxies that probe to a high redshift (greater distance) than the previous SDSS luminous red galaxy (LRG) sample (shown in white) and the normal (MAIN) SDSS galaxies in yellow.
Image Credit: Michael Blanton and the SDSS-III Collaboration

Fig. 2: A massive elliptical galaxy is shown together with its radio-emitting lobes of high energy particles.

Fig. 3: The solid red,black and blue lines show the fraction of galaxies that have formed more than 5,10 and 15% of their stars in the last billion years as a function of stellar mass. These results are for galaxies with median redshift z=0.1 in the SDSS low redshift MAIN sample. The dashed red,black and blue lines show the same thing for galaxies with median redshift z=0.5 in the BOSS sample.

In a recent paper, scientists from an international collaboration consisting of the MPA , the University of Wisconsin and Johns Hopkins University, as well as other members of the BOSS team, studied the masses and ages of around 300,000 massive galaxies at redshifts ranging from 0.45 to 0.7, corresponding to a time when the Universe was 60 percent of its present age. These galaxies all have stellar masses larger than 100 billion suns (1011 Msun), making this the largest sample of massive galaxies with spectra to have been analyzed thus far.

Massive galaxies are very interesting to cosmologists, because they are thought to represent the end point of galaxy evolution. The now standard Lambda-Cold Dark Matter paradigm provides detailed predictions for how structure in the dark matter component of the Universe assembles over time. In short, structure formation is a "bottom-up" process, with the smallest dark matter halos collapsing first, and then merging to form larger and larger systems such as groups and giant clusters of galaxies.

For a long period, these cosmological predictions came under attack by many observers, because the most massive galaxies that were known appeared to consist only of extremely old stars. How could this be consistent with a scenario in which the most massive structures formed last? As telescopes grew more powerful, observers were able to survey faint galaxies in the very distant Universe, where light travel times become comparable to the age of the Universe itself. The observers found that the number of massive galaxies present at early cosmic epochs was indeed far less than the number present today, confirming that massive galaxies assembled rather recently. However, they also found that the most massive galaxies in the early Universe were apparently still composed of relatively evolved stars -- in other words, no matter how far back in time observers looked, they found little evidence for recent star formation once a galaxy reached a certain threshold in stellar mass.

These results caused much consternation among theoreticians, who calculated that large quantities of gas should be cooling and forming stars in the dark matter halos surrounding massive galaxies. As a result, there has been much debate and speculation about exotic mechanisms that heat the gas and prevent it from forming stars. Examples range from giant explosions powered by material accreting onto central black holes of a billion or more solar masses, to Megaparsec-scale jets of charged particles traveling at relativistic speeds, which penetrate and heat the gas surrounding the galaxies.

The new results from SDSS-III indicate that these exotic mechanisms may have a much harder time stopping star formation in massive galaxies at higher redshifts. The MPA/Wisconsin/JHU team employed a technique that could estimate the age of the stars in a galaxy using detailed stellar absorption line features in their spectra. In massive galaxies, the youngest stars are often shrouded in cocoons of dusty gas that absorb much of the blue light emitted by young hot stars, so inferences based on galaxy colour can lead to wrong answers. The new technique and the unprecedentedly large galaxy sample, allowed the team to conclude that the fraction of the most massive galaxies with young stars has decreased by a factor of 10 over the last 4 billion years (see Figure 3). At redshift 0.5, more than 10% of all galaxies with stellar masses of around 200 billion solar masses have experienced a significant recent episode of star formation. These results are at odds with claims from some observers that the stars in massive galaxies all formed 2-3 billion years after the Big Bang. The results are also exciting, because next generation X-ray satellites will be able to detect the gas as it cools and forms stars in these massive galaxies and next generation radio surveys will track how energetic particles propelled by black holes deposit their energy into this gas. Current speculation about exotic mechanisms will then be transformed into hard science.

Guinevere Kauffmann



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