ALMA: Milky-Way-Like Galaxies Seen in their Awkward Adolescent Years

[bystander]

Milky-Way-Like Galaxies Seen in their Awkward Adolescent Years
ALMA | NRAO | 2016 Dec 20

[img3="Four Milky-Way-like progenitor galaxies as seen as they would have appeared 9 billion years ago. ALMA observations of carbon monoxide (red) is superimposed on images taken with the Hubble Space Telescope. The carbon monoxide would most likely be suffused throughout the young galaxies.
Credit. ALMA (ESO/NAOJ/NRAO); C. Papovich (TAMU); A. Angelich (NRAO/AUI/NSF); NASA/ESA Hubble Space Telescope"]http://www.almaobservatory.org/images/n ... 1220_1.jpg[/img3][hr][/hr]

Spiral galaxies like our own Milky Way were not always the well-ordered, pinwheel-like structures we see in the universe today. Astronomers believe that about 8-10 billion years ago, progenitors of the Milky Way and similar spiral galaxies were smaller, less organized, but amazingly rich in star-forming material; so much so, that they would have been veritable star factories, churning out new stars faster than at any other point in their lifetimes.

Now, astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found evidence to support this view. By studying four very young versions of galaxies like the Milky Way as they were seen approximately 9 billion years ago, the astronomers discovered that each galaxy was incredibly rich in carbon monoxide gas, a well-known tracer of star-forming gas.

“We used ALMA to detect adolescent versions of the Milky Way and found that such galaxies do indeed have much higher amounts of molecular gas, which would fuel rapid star formation,” said Casey Papovich ... “I liken these galaxies to an adolescent human who consumes prodigious amounts of food to fuel their own growth.”

Though the relative abundance of star-forming gas is extreme in these galaxies, they are not yet fully formed and rather small compared to the Milky Way as we see it today. The new ALMA data indicate that the vast majority of the mass in these galaxies is in cold molecular gas rather than in stars. These observations, the astronomers note, are helping build a complete picture of how matter in Milky-Way-size galaxies evolved and how our own galaxy formed. ...

Study Helps Prove Galaxy Evolution Theory
Texas A&M University | 2016 Dec 19

Large Molecular Gas Reservoirs in Ancestors of Milky Way-Mass Galaxies 9 Billion Years Ago - Casey Papovich et al

• Nature Astronomy 1:0003 (19 Dec 2016) DOI: 10.1038/s41550-016-0003
  arXiv.org > astro-ph > arXiv:1610.05313 > 17 Oct 2016

[bystander]

Young, Thin and Hyperactive: That’s What Outlier Galaxies Look Like
International School for Advanced Studies (SISSA) | 2016 Dec 13

A new model explains the exceptions to the galaxy main sequence

[img3="ALMA probes the Hubble Ultra Deep Field
Credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA/J. Dunlop et al.
and S. Beckwith (STScI) and the HUDF Team"]https://cdn.eso.org/images/screen/eso1633a.jpg[/img3][hr][/hr]

The more massive, or full of stars, a galaxy is, the faster the stars in it are formed. This seems to be the general rule, which is contradicted, however, by some abnormal cases, for example thin (not massive) galaxies that are hyperactive in their star formation. Until now the phenomenon had been explained by catastrophic external events like galaxies colliding and merging, but a new theory offers an alternative explanation, related to an in situ (internal) process of galaxy evolution. The new theory correctly reproduces the behaviour of both normal and abnormal (or outlier) galaxies, and may be further tested by new observations. ...

If we put the galaxies for which we have the relevant data into a graph relating the mass of stars in each galaxy with the star formation rate of that galaxy, most of them would appear as a compact cloud, which could be described by using a simple function. This graph is known as the galaxy main sequence (GMS), a fundamental observational relation for scientists who study galaxies. The picture that emerges is simple: the more massive the galaxy, the faster its star formation process tends to be.

But straightforward as it seems, there’s a problem. There are some exceptions (abnormal or outlier cases) that do not seem to follow the rule. Certain galaxies, in fact, while not containing many stars have very intense rates of star formation. The most accredited hypothesis to explain these abnormal cases invokes collision and merging between two galaxies: these outliers would therefore be nothing but galaxies captured during their collision, a phenomenon that would lead to a sudden, though transient, increase in their star formation rate. ...

The Main Sequences of Starforming Galaxies and Active Galactic Nuclei at High Redshift - Claudia Mancuso et al

• Astrophysical Journal 833(2):152 (20 Dec 2016) DOI: 10.3847/1538-4357/833/2/152
  arXiv.org > astro-ph > arXiv:1610.05910 > 19 Oct 2016