Friday, November 20, 2015

NGC 1266 as a Local Candidate for Rapid Cessation of Star Formation

Uncovering the mystery tied into an unusual new phenomenon is getting as good of a handle on the timeline of events that led to this phenomenon as possible. This is particularly true in the case of NGC 1266, which from the outside appears to have been caught at this exact “special” moment in time. The impossibility that we caught this one galaxy in this one time frame is less than a 1% chance. These things do happen, but you want to make sure that what you are seeing is a special time in this galaxy’s life, rather than that your instrumentational capabilities have just gained more ground against nature.

The Herzsprung-Russell diagram of stars, with the spectral classification of Annie Jump Cannon overlaid. The sizes and temperatures of stars are related to the masses and ages of the stars, and different stars have different spectral signatures.

To look at this question, I turned to clues that could be sussed from wavelengths that I had previously not delved deeply into, namely, optical spectra and ultraviolet light. We can figure out the age of stars based on their spectral signatures and the light that they give off. None other than Annie Jump Cannon invented the O-B-A-F-G-K-M stellar classification scheme we use to this day. Because we can look at the elements that are absorbing parts of the stellar light, we can actually age-date stars. When we look at a galaxy’s integrated spectrum, the different stellar populations will contribute to the total spectral light. All we need to do is use good stellar templates and we can figure out the relative contributions of different sorts of stars! This works best with a spectrum. Using this knowledge, we set out to determine the “age” of NGC 1266.

The ultraviolet light in NGC 1266 as seen by the Swift satellite in blue with the current site of the molecular gas overlaid in yellow contours. The UV light comes from a region 2 kpc in diameter, compared to the molecular gas, which is coming from a much smaller region (0.3 kpc). This means there was more molecular gas further out than there currently is.
To find the age of the stars in NGC 1266, we (1) fit templates to its integrated spectrum, (2) derived the age from the 3-dimensional data cubes from SAURON (a spectrum for each pixel in the galaxy), and (3) look at the location of the ultraviolet light from the Swift satellite. For the first 2, the model fits to the single spectrum as well as spatially resolved spectrum showed a ~10% fraction of 500 million year old stars by mass (that is, 10% of the stellar mass in NGC 1266 was coming from youngish stars, old stars are over 10 billion years old). This analysis shows that the stars in NGC 1266 were young, but not infant (10 million years), which seemed to say that whatever stopped star formation stopped it 500 million years ago, not recently (compared to the 2 millions years or so that the AGN had been driving out the molecular gas).

The ultradeep imaging of NGC 1266 taken with the
CFHT. The faint whisp to the South of NGC 1266 is
all the evidence that exists that there might have been
an interaction in this system.
With the age of the stars basically determined, we used the ultraviolet light (which mainly comes from youngish stars) to tell us where the stars used to be. That showed that youngish stars were found out at 2 kiloparsecs, almost 7 times the diameter of the current site of the molecular/star-forming gas. So, now not only did we know that something happened 500 million years ago, it impacted a much larger portion of NGC 1266 than the current site of the molecular gas in the nucleus. Could it be that the event that killed star formation in the system also moved the gas into the center? For this we turned to two more telescopes: the CFHT and Hubble. The CFHT was able to probe deeply into the outskirts of NGC 1266, searching for signs of tidal tails: a key indicator of stellar interactions, and near-infrared imaging of the nucleus by Hubble revealed faint spiral structure. These two things pointed to the possibility that a minor merger (that is, a small galaxy impacted NGC 1266) could have driven the gas into the center and quenched star formation in one fell swoop, 500 million years ago.  This throws a monkey wrench into the story of NGC 1266 and this “special” time, since 500 million years is a lot longer than 2 million years. How can the gas sitting in the center have remained that way? We speculated in this paper, but the answer lies in the next paper in my NGC 1266 blog my paper series.

The 3-color image of the stars in NGC 1266 taken by the Hubble Space telescope.  The near-infrared imaging shows that the stars have the slightest spiral structure visible, which can be excited from a gravitational interaction between NGC 1266 and something smaller, and are long-lived!
Finally, NGC 1266 taught us that we were going about searching for quenching galaxies in a way that missed a huge swath of possible quenchers. NGC 1266 has plenty of ionized gas that is not associated with star formation (see Tim Davis’s observations of this here), and a poststarburst stellar population. Most searches won’t accept galaxies with too much Hα emission (usually from stellar nurseries), but in the case of NGC 1266, this is from shocks. These observations and this work inspired me to form the Shocked POststarburst Galaxy Survey (SPOGS), which will be blogged about in upcoming blogs as well.

The official published version can be found on NASA ADS.
To get a PDF version made by me, you can download it here.

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