The First Galaxies: What We Know And What We Still Need To Learn
“As we look farther back in time, we find that younger galaxies formed stars at faster rates than galaxies do today. We can measure the star-formation rate, and find that at earlier and earlier times, it was more intense. But then we find it hits a peak when the Universe is about two billion years old. Go younger than that, and the rate goes down again.”
We’ve come incredibly far in our quest to learn how the Universe came to be the way it is today. We can see out in space for tens of billions of light years, to galaxies as they were when the Universe was only a few percent of its present age. We can see how galaxies evolve, merge and the stars inside change. And we can see to even before that, when no stars or galaxies existed at all. But how did we get from there to here? There are still plenty of gaps in the story. We’ve never seen the first stars or galaxies; we’ve never witnessed the start of cosmic reionization; we’ve never seen the star formation rate jump from zero to a real, finite number. Yet with James Webb and WFIRST on the horizon, these gaps in our knowledge may – if we’re lucky – all disappear.
The Sunflower Galaxy, Messier 63 sports a bright yellowish core in this sharp composite image from space- and ground-based telescopes. Its sweeping blue spiral arms are streaked with cosmic dust lanes and dotted with pink star forming regions.
A bright spiral galaxy of the northern sky, M63 is about 25 million light-years distant in the loyal constellation Canes Venatici. A dominant member of a known galaxy group, M63 has faint, extended features that are likely star streams from tidally disrupted satellite galaxies. M63 shines across the electromagnetic spectrum and is thought to have undergone bursts of intense star formation.
Image Credit & Copyright: Data - Hubble Legacy Archive, Subaru Telescope (NAOJ), Don Goldman
Processing - Robert Gendler, Roberto Colombari, Don Goldman
Hubble Catches New Stars, Individually, Forming In Galaxies Beyond The Milky Way
“There are a massive variety of star-forming regions nearby, and Hubble’s new Legacy ExtraGalactic UV Survey (LEGUS) is now the sharpest, most comprehensive one ever.
By imaging 50 nearby, star-forming spiral and dwarf galaxies, astronomers can see how the galactic environment affects star-formation.”
Within galaxies, new stars are going to be formed from the existing population of gas. But how that gas collapses and forms stars, as well as the types, numbers, and locations of the stars that will arise, is highly dependent on the galactic environment into which they are born. Dwarf galaxies, for example, tend to form stars when a nearby gravitational interaction triggers them. These bursts occur periodically, leading to multiple populations of stars of different ages. Spirals, on the other hand, form their new stars mostly along the lines traced by their arms, where the dust and gas is densest. Thanks to the Hubble Space Telescope, we’re capable of finding these stars and resolving them individually, using a combination of optical and ultraviolet data.
The beautiful Trifid Nebula, also known as Messier 20, lies about 5,000 light-years away, a colorful cosmic sky. It shares this field with open star cluster Messier 21 (top left).
The Trifid nebula is about 40 light-years across and a mere 300,000 years old. That makes it one of the youngest star forming regions in our sky. M20 and M21 are easy to find with even a small telescope in the nebula rich constellation Sagittarius. In fact, this well-composed scene is a composite from two different telescopes.
New Stars Turn Galaxies Pink, Even Though There Are No ‘Pink Stars’
“New star-forming regions produce lots of ultraviolet light, which ionizes atoms by kicking electrons off of their nuclei.
These electrons then find other nuclei, creating neutral atoms again, eventually cascading down through its energy levels.
Hydrogen is the most common element in the Universe, and the strongest visible light-emitting transition is at 656.3 nanometers.
The combination of this red emission line — known as the Balmer alpha (or Hα) line — with white starlight adds up to pink.”
When you look through a telescope’s eyepiece at a distant galaxy, it will always appear white to you. That’s because, on average, starlight is white, and your eyes are more sensitive to white light than any color in particular. But with the advent of a CCD camera, collecting individual photons one-at-a-time, you can more accurately gauge an astronomical object’s natural color. Even though new stars are predominantly blue in color, star-forming regions and galaxies appear pink. The problem compounds itself when you realize there isn’t any such thing as a pink star! And yet, there’s a straightforward physical explanation for what we see.
The Earliest Galaxies Spin Just Like Our Milky Way, Defying Expectations
“As our data sets improve, we should begin to measure the internal motions of large numbers of galaxies like this, which will answer many questions and raise others. Do most/all galaxies at these early stages rotate in a whirlpool-like plane? Is there a variety and multiple sets of populations that exhibit different behaviors? What are the actual effects of gas infall, supernovae, and small-scale motions? What is the velocity profile of these rotation curves, and can they teach us anything about the interplay of radiation, normal matter, and dark matter?
While we hope to learn these answers, we can now ask these questions sensibly in the aftermath of having measured the movement and internal motions of a galaxy so far away. At least for the first two, they rotate very similarly to their much older cousins, a quite unexpected result. Thanks to ALMA, we’re taking those coveted next steps into the final frontier.”
It wasn’t supposed to be this way. When you form galaxies in the very young Universe, it’s supposed to be a chaotic, turbulent place. Sure, you have gravitation, pulling matter in and creating a pancake-like shape. But then you form stars, and everything goes haywire. Supernovae go off, gas falls in, protogalaxies merge and get swallowed, motions get stirred up, and turbulence should permeate the galaxy. It ought to take billions of years for them to quiet down into a Milky Way-like whirlpool. Well, for the first time, owing to ALMA and Renske Smit’s team, the internal motions of galaxies less than a billion years old were measured, and – surprise! – their movement is smooth and not chaotic at all.