Beacons or Fireworks?

In the local, old Universe we only see intense bursts of star formation when galaxies collide - but the young Universe was a very different place, it seems. Georgios Magdis is using the Herschel and VLA telescopes to measure the amount of gas in massive galaxies 1-4 billion years after the Big Bang, and finding that mergers weren't so important back then. Some galaxies had so much gas they could sustain long periods of star formation, without the need for a merger to compress the gas at all - they look more like beacons than fireworks. Phil Marshall caught up with Georgios and asked him a few questions about his work.

Magdis presented the results of his study at the National Astronomy Meeting this week - you can read more about it at the RAS press release and in his recent paper. He and his team selected distant galaxies from HST images of the GOODS survey fields, and then measured the dust mass in each with Herschel space telescope, and the gas mass using the VLA radio telescope, and from all these data estimated the efficiency of the star formation in each galaxy. The star formation efficiency in the merger system, GN20, is about 5-10 times higher than in BzK-21000, a similar mass galaxy not undergoing any interactions - making GN20 a starburst, more like a firework than a beacon. BzK-21000 contains about the same mass of gas, but is not undergoing any interactions - despite this, it is forming stars at about 10% the rate seen in GN20, and 100 times what we see in our own non-interacting galaxy, the Milky Way. It's more like a beacon, less luminous than a firework but longer-lasting.

Hi Georgios! Thanks for agreeing to answer our questions. What's special about this era 1-4 billion years after the Big Bang? Are these the first galaxies? The galaxies in which most stars formed?

We know that as we look further back in time, galaxies were more active with higher star formation rates. Still, the peak of the star formation activity of the Universe, i.e. the epoch when most of the stars were born, was more recent, at around z=1-2. This is exactly the redshift range of the normal galaxies in our sample. We study galaxies like BzK-21000 at this time in order to put constraints on the star formation mode that dominated the fate of the Universe.

GN20 on the other hand, the merger/starburst prototype, is at z~4 and has a star formation rate of 1000-2000 M/yr, about 10 times higher than BzK-21000. Seen at just 1 billion years after reionization (when the gas in the Universe was ionized by the first stars and quasars), it is indeed one of the first massive galaxies in the Universe.

What will these galaxies have evolved into in the present-day universe? Will they have become galaxies like our own?

That's a tough one. We actually do not know. However we can make some educated guesses. For example, GN20 already has a stellar mass of 2x10^11 Msun, and is still forming stars at a rate of 2000M/y. However, its starburst nature indicates it will soon run out of gas and stop forming stars, becoming a massive elliptical galaxy. On the other hand the normal galaxies like BzK-21000 have enough gas to maintain their star formation for long periods, and could evolve into those galaxies in the local universe that are not dead yet but still have low star formation rates, as the gas supplies run low with cosmic time - rather like our own Milky Way.

How can you conclude much at all from just two galaxies? Do you plan to extend this study to many more?

The study was based on two carefully selecting systems, each one being a prototype of the two populations (normal, non-interacting disks, and starbursts triggered by mergers). We have more data now, and at the moment I am about to submit a paper with a much larger sample, confirming our results. In particular, we now have 9 normal and 3 starburst systems. Still, the number is not impressive, but detecting CO molecular gas by its radio emission, and continuum emission at mm wavelengths for such distant sources is quite hard! Also, things get more difficult since we choose to focus not on the extreme objects of the universe, but on the ''ordinary galaxies'', which are less luminous and hence more difficult (time-wise) to detect.

Thanks, Georgios!

RAS press release
Magdis et al (2011), Astrophysical Journal Letters