Baby Galaxies in the Distant Universe

A newborn baby will typically double its weight in about 4 months. Imagine an adult growing at an equally monstrous rate! Galaxies are like people in that they also grow fast when they are young, and slowly when they are old. This post is about baby galaxies discovered by CANDELS that we found to be forming stars at a ferocious rate when the universe was 4 billion years old.

Let us do a little thought experiment and scale the life time of galaxies -- essentially, the age of the universe (13.7 billion years) -- to the life span of a human (say, 80 years). The CANDELS baby galaxies are, on this scale, about a month old and will double their mass in stars by the end of the second month. In comparison, our Milky Way Galaxy would, at its current rate, double its mass in about 80 years, not too different from the current growth rate of the average weight of people in a rich country, I'm afraid.

Like their human counterparts, these baby galaxies consume vast amounts of fuel relative to their weight. This fuel consists of gas that is very simple in its composition, with a very low level of contamination in the form of elements heavier than Helium. Out of this pristine gas the galaxies form new stars continuously, quickly building up their stellar bodies.

The largest and hottest among the millions of newly formed stars dominate the energy budget of an entire galaxy. It is their energy output that we observed with CANDELS, in particular because they ionize gas which subsequently recombines and emits an enormous amount of light at a single wavelength. This line radiation makes these unexpected objects stand out among the thousands of older, more slowly growing 'kid' galaxies seen by CANDELS in the distant, that is, younger, universe, much like a crying infant is easily recognized in a crowd of gossiping teenagers.

Especially when the large, hot stars reach the end of their lives, when they explode as supernovae -- on our human time scale, these stars do not grow older than a couple of weeks, the price they pay for their fast-paced life styles -- they generate a lot of mayhem. Much of the present gas is blown out of the galaxy, making its immediate environment acutely aware of its digestive prodigiousness.

The number of baby galaxies we observed suggests that the age of the universe when it was giving birth to the observed baby galaxies is a healthy 23 years on our human time scale. (How such a young universe had already given birth to thousands of teenager galaxies is food for a discussion that we will have at some other time.) Interestingly, it was around the same time that the universe as a whole was producing new stars at the highest rate it ever would. 

At the present day, that rate is about a factor 10 smaller. The baby galaxies, in particular, are not produced in any significant number today. Even though rare examples are known, the incidence of such strongly star forming baby galaxies in the younger universe was at least 100 times larger than it is today. This commonness at earlier times is precisely one of the most interesting features. We see galaxies form their first significant batch of stars, in a way that was not anticipated or predicted by our galaxy formation models. Much work is still ahead of us to find out what causes the high star formation rate and what role these objects play in the big picture of galaxy formation. 

CANDELS has allowed us to find this new class of distant galaxies, which has already instigated much discussion and will certainly motivate further investigations. Perhaps the most pressing question is, as always, what these baby galaxies will grow up to become. Will their unhealthy growth result in their own demise, or will they settle for a more sustainable growth level and lead steady lives for many decades?