Usual Suspect: Is Distant Galaxy a Milky Way 'Baby'?

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When searching for galaxies to study, it is easiest to pick out the biggest and brightest. This is especially true if you want to look at very distant galaxies; ones that were around when the Universe was very young. It is quite a challenge to pick out a faint, normal, almost “boring” galaxy from a time long ago, but when astronomers used the 10-meter Keck telescope to do just that, it was pretty exciting.

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The galaxy, called DLA2222-0946, was first discovered in absorption, meaning that it was blocking some of the light of a distant quasar behind it in a very specific way. In fact, the first few letters of its name stand for “Damped Lyman-alpha,” a system where the neutral hydrogen in a galaxy absorbs certain frequencies of light from the background quasar. It is thought that these massive reservoirs of neutral gas are the progenitors of today’s Milky Way-like galaxies.

Top: A map of the hydrogen emission from DLA2222-0946. The mapped region, which covers only a portion of the galaxy, is about 5,000 parsecs (or 16,300 light-years) across. The position of the background quasar is marked by a “Q”. Bottom: The corresponding map of the movement of the gas in the galaxy. Red means moving away from us; blue is moving towards us.
R. Jorgenson

This galaxy was then detected by the huge 10-meter telescope Keck I telescope. Not only was it detected, but it was resolved, meaning that astronomers could get some spatial information from it. As you can see above, the pictures are jaw-droppingly amazing, but the information contained in those images is important. In order to get such a clear picture, the team used the Laser Guide Star Adaptive Optics system, or a laser guide “star” for calibration and real-time adjustments to the telescope mirrors to compensate for the turbulence of the atmosphere.

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The astronomers also used the OSIRIS instrument on Keck. This get you a spectrum of every pixel in the image in infrared wavelengths. So, not only could they detect the massive hydrogen reservoirs in the galaxy, the astronomers could see how the gas was moving. Now, DLA2222-0946 is not just known by the small portion of it that blocks the light of a distant quasar, but becomes a whole galaxy to explore. It seems to be a disk galaxy seen edge-on.

We see this galaxy as it exists 10.8 billion years ago, during a prolific epoch of star formation in the Universe. However, this relatively normal galaxy seems to be forming stars at a rate of ten times the mass of the sun per year, or ten times the current star formation rate of the Milky Way today. The galaxy is about 6 million times the mass of the sun, so small in comparison to today’s Milky Way.

The astronomers detected oxygen and nitrogen emission lines in the galaxy, allowing them to estimate the “metallicity” of the galaxy. This comes from the older definition of “metals” from astronomy, anything on the periodic table that’s not hydrogen and helium. We care about metallicity because it tells something about how many epochs of star formation came before, since elements heavier than helium come from the cores of and violent explosions of massive stars. It also tells you something about when planetary systems can be formed, particularly rocky planets, as these are composed almost entirely of NOT hydrogen and helium.

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The estimate of metallicity from the absorption of the background quasar was about one-third that of the sun, but the new measurements of the center of the galaxy put the estimate closer to 75 percent the sun’s metallicity. However, it has been hypothesized that only 10 percent metallicity of the sun is really needed to create a rocky planet, though there are still a lot of unknowns that could affect that.

Is this a picture of what the Milky Way looked like at a young age? Maybe. And that makes this seemingly “normal” galaxy all the more interesting to us. Though we tend to focus on the biggest and weirdest, science needs to look at all the “usual” suspects to form a complete picture.

This work will be published in Astrophysical Journal; and a preprint is available at arXiv.org. 

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