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Black Death

Credit: vchalup/adobe

Credit: vchalup/adobe

By Ivy Wong

A new study suggests that some galaxies suddenly stop forming stars because the gas they use for star formation is blown away by the activity of their central supermassive black holes.

There are two types of galaxies: those that are actively forming stars, and those that are not. Galaxies that are forming stars are bluer in colour, while galaxies that no longer form stars are redder.

As a galaxy ages, it slowly runs out of the gas from which stars are formed. Eventually it will stop forming stars and start to redden as its stars begin to fade and redden.

Using results from the Sloan Digital Sky Survey and modelling the evolution of the stellar population in these galaxies, my colleagues and I have found that there are two main pathways by which galaxies stop forming stars:

  • a fast way that takes several hundred million years for star formation to stop; and
  • a slow path that takes the galaxy two billion or more years to stop forming stars.

In our current epoch, spheroidal galaxies typically display the fast track of evolution, while spiral galaxies evolve over much longer timescales. Our Milky Way is one of these middle-aged spirals that is ageing sedately and currently forming stars at the rate of just one Sun per year.

My personal interest is in the galaxies that live fast and die young. These galaxies have very strong bursts of star formation but then experience a sudden shutdown or quenching of star birth. These galaxies are called “post-starburst” galaxies.

We realised a few years ago that these post-starburst galaxies are structurally similar to elliptical and/or spheroidal galaxies. Due to their fast evolution, they will turn into red galaxies retired from star formation in the time that it takes the latest generation of young stars to fade.

So what forces these galaxies into early retirement from star formation?

Since these post-starburst galaxies are on a fast track to early retirement, it is difficult to catch the smoking gun of what causes the quenching of star formation. Therefore, we have looked at a population of galaxies that are likely to be the predecessors of post-starburst galaxies: blue early-type galaxies. These are bluer (and hence younger) versions of the post-starburst galaxies, but they have similar spheroidal shapes.

To determine what is causing the sudden quenching of star formation, we decided to investigate the cold gas content of these blue early-type galaxies. The reasons for this are twofold:

  1. stars form from an initial reservoir of cold gas, and star formation history is intrinsically linked to the availability of its fuel; and
  2. the cold gas content of a galaxy is more sensitive than the stars themselves to any form of weak interaction that affects the evolution of the galaxy. Therefore, cold gas in the form of atomic hydrogen is an excellent probe to investigate these galaxies.
    1. Although hydrogen is the most abundant element in the universe, it is very difficult to detect in galaxies that are a few hundred million light years away. To “see” atomic hydrogen, we rely on the energy it releases when the spin of its electrons changes from a more excited to a less excited state. This energy is very weak and is observed as an emission line at the specific wavelength of 21 cm.

      We typically observe light emitted by the plasma in galaxies as radio frequencies. At the specific wavelength of 21 cm we see a boost of energy if there are enough hydrogen atoms flipping their spins. This sudden blip in the light spectrum is known as an emission line. The light spectrum minus the blip from the emission line is known as the radio continuum because it’s just the continuum of light that we see at the other radio frequencies.

      Using a radio interferometer in The Netherlands called the Westerbork Synthesis Radio Telescope, we conducted a pilot survey for atomic hydrogen in four northern blue early-type galaxies spanning a range of evolutionary stages.

      Observations of the atomic hydrogen emission line also gives us the radio continuum emission from each galaxy. The radio continuum emission typically comes from star formation or is emitted as radio jets and lobes by a central supermassive black hole that is feeding or sucking in material from its surrounds.

      Our current understanding of feeding supermassive black holes is that as material accretes into its centre, strong magnetic fields launch high-speed electrons that are observed as radio jets emanating from the centres of these galaxies. As time goes by, however, these jets will evolve into discrete blobs or lobes that appear nearby and sandwich the host galaxies but will no longer be connected to or overlap with the host galaxy.

      We can infer the evolutionary stages of blue early-type galaxies from their near-ultraviolet optical colours. These reflect the ages of the current stellar population and provide an indication of recent star formation history.

      Our pilot survey of four northern blue early-type galaxies found that the ratio of the mass of gas to stars and the level of disturbance of the atomic hydrogen cloud shape and motion were consistent with the evolutionary stage of each galaxy.

      The ratio of gas to stellar mass describes how much atomic hydrogen gas there is in a galaxy relative to the amount of stars in it. This ratio is often used by astronomers to assess the ability for a galaxy to form more stars – if a galaxy has little gas left relative to the amount of stars, then the potential for future star formation is very low. Such a galaxy is likely to retire from star formation soon if it has not already done so.

      One of the great features of observing the atomic hydrogen emission line is that we can not only map the amount of gas in the galaxy, but we can also map its motion. For example, an inclined spiral galaxy spinning around its axis will slightly shift the wavelength of some of its gas. Astronomers can use these small shifts in the observed emission line to determine the motion of the atomic hydrogen gas in the galaxies.

      At the earliest stages of quenching, the distribution of atomic hydrogen gas seems relatively undisturbed and the motions of the gas show stable rotation. At later stages of evolution, the atomic hydrogen gas cloud becomes increasingly disturbed and appears in chaotic shapes and non-circular motions. At earlier stages, the atomic hydrogen gas to star fraction is also significantly higher than at later stages of evolution.

      In Figure 1, the four galaxies observed in our pilot observations are arranged from left to right in order of their evolutionary stage. The galaxy representing the earliest stage (J1237) features a large atomic hydrogen gas reservoir centred on the star component of the galaxy, while the gas reservoir of J1117 is slightly offset from the star component of its corresponding galaxy. The hydrogen gas reservoirs of the two galaxies at more evolved stages, J0900 and J0836, are completely displaced from their galaxies. In the case of the most evolved blue early-type galaxy, J0836, the entire atomic hydrogen gas cloud appears to lie in the same projected direction between the galaxy and a pair of radio lobes observed at 1.4 GHz. The pair of radio lobes indicates the past activity of J0836’s central supermassive black hole.

      In the field of galaxy evolution, the displacement of gas is often attributed to environmental factors such as galaxies interacting with their neighbours or surrounding hot gas. However, all four of our blue early-type galaxies reside in isolation and are not in close proximity to other galaxies.

      Thus we suggest that the most likely cause for the removal of gas from these galaxies lies in a mechanism within them. Our observations of J0836 suggest that the hydrogen gas reservoir may have been blown out by a previously active central supermassive black hole.

      Since this is the first time that astronomers have mapped at such high resolution the atomic hydrogen gas in fast-evolving isolated galaxies with signatures of a previously active central supermassive black hole, there aren’t many previous studies available for direct comparisons. However, our study is consistent with recent studies that found evidence for displaced reservoirs of carbon monoxide in galaxies with an active central supermassive black hole.

      To confirm the prevalence of an active supermassive black hole blowing out a galaxy’s gas reservoir, and hence accelerating the evolution of a galaxy, we have begun the monumental task of conducting our large survey for atomic hydrogen in the rest of the ~200 blue early-type galaxies in our sample. The completion of this large survey with current radio telescopes will require several thousand hours of observing time, so our team will be kept quite busy for the next few years.

      We are also attempting to understand the evolution of active supermassive black holes by assembling a large sample of feeding supermassive black holes with radio jets through a new “citizen science” project called Radio Galaxy Zoo. Visit http://radio.galaxyzoo.org if you would like to help us “dob in” the black holes responsible for emitting radio jets and lobes.

      Ivy Wong is an astronomer at the International Centre for Radio Astronomy Research in the University of Western Australia.