Authors: Margaret Lazzarini, Benjamin F. Williams, Meredith J. Durbin, Julianne J. Dalcanton, Adam Smercina, Eric F. Bell, Yumi Choi, Andrew Dolphin, Karoline Gilbert, Puragra Guhathakurta, Erik Rosolowsky, Evan Skillman, O. Grace Telford, Daniel Weisz
First Author’s Institution: California Institute of Technology, 1200 E California Blvd., Pasadena, CA 91125, USA
Status: Accepted to the Astrophysical Journal, in press.
The Panchromatic Hubble Andromeda Treasury (PHAT) team has already done the impossible. Led by Professor Julianne Dalcanton (read our interview with her from #AAS233 here!), PHAT completely revolutionized observational astronomy by imaging over 117 million stars in the Andromeda galaxy’s (M31) disk. Imaging M31 took two weeks of Hubble Space Telescope time, an unprecedented achievement considering many observational astronomers are lucky to get even a few precious hours on Hubble!
Now, the PHAT team is ready for round two. They have moved on to Andromeda’s neighbor and the third most massive galaxy in our Local Group: the Triangulum Galaxy, or M33. And of course, this observing program wouldn’t be complete without a new, catchy acronym: the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region, or “PHATTER.” Studying M33, in addition to M31, is beneficial because M33 has had overall higher amounts of star formation and can therefore provide more insight into a new parameter space previously unexplored in M31. M33 also has a lower stellar surface density (AKA lower star-to-area ratio), so resolving individual stars is much easier in M33 than in M31. The PHATTER team has generously made their data publicly available, providing photometry (i.e., the measured flux from astronomical objects) for over 22 million stars covering 38 kpc2 of M33.
This paper, the second in the PHATTER series (where the first described the observations and photometry), measured the star formation history of M33. Measuring the star formation history of a galaxy can provide crucial information about the astrophysical phenomena that shape galaxy formation; for example, how the structure of a galaxy changes over time.
To measure star formation rates of galaxies, astronomers have historically used two different methods. The first method involves studying UV emission from massive, young stars. Because young stars primarily emit in the UV wavelengths, UV flux is often used as a tracer for recent star formation within the last 200 Myr. The second method involves studying H-alpha emission, a Balmer series emission line that occurs when hydrogen electrons fall from their third to second lowest energy level. H-alpha emission often indicates hydrogen is being ionized, usually by young O stars. Therefore, H-alpha emission traces recent star formation within the last 5 Myr. However, both these techniques are limited by dust extinction, which can be difficult to correct for.
The authors in this paper use a third, novel method to measure the star formation history, referred to as “CMD-based modeling.” The basic premise of this technique is that if you have high-accuracy photometry, you can use color-magnitude diagrams (CMDs, the observer’s version of the H-R diagram, where instead of Luminosity vs. Temperature you have Magnitude vs. Color) to infer the star formation rates throughout history that would have produced a given observed population. For example, younger stars spend less time in a given color-magnitude diagram zone than older, red giant branch stars. Another useful benefit of the CMD-modeling technique is that it simultaneously fits for the dust extinction, unlike the UV/H-alpha methods.
To measure the star formation history in bins across the face of M33, the authors split their photometry into ~2,000 regions, each which contained 4,000 stars on average. To measure the star formation history, they fit color-magnitude diagrams in each region using the MATCH software, which finds the combination of stellar populations that best produces the observed color-magnitude diagram. Using this software, the authors were able to reconstruct M33’s star formation history by measuring the star formation rate in ~50 Myr bins, up until 630 Myrs ago. While the CMD-based method requires high-resolution photometry, you can study the star formation rate throughout history, whereas the UV/H-alpha techniques only measure recent star formation.
The Structure of M33
One of the things you can use detailed star formation histories for is measuring how a galaxy’s stellar structure has changed over time. M33 has typically been characterized as a flocculent spiral galaxy, meaning its spiral arms are less defined than that of a grand design spiral galaxy like M101 (see Figure 1 for a comparison of the two). However, by studying the star formation rate throughout M33’s history (as opposed to just the recent star formation), the authors were able to reconstruct the evolution of M33’s spiral structure using the measured star formation rate in ~50 Myr time bins. They found that while M33 does indeed have flocculent spiral structure which formed about 79 Myrs ago, before then, M33 had two distinct spiral arms. In short, the younger stellar populations (younger than 80 Myr) present as a flocculent spiral structure and the older stellar populations are primarily present in two distinct spiral arms. In Figure 2, you can clearly see the split between these two stellar populations. The authors also clearly detect a bar in M33 that is older than about 79 Myr, which is significant because there has been a lot of recent debate in the literature about whether M33 has a bar. The detection of bars in galaxies has strong implications for the galaxy formation history; bars force a lot of gas towards the galaxy’s center, fueling new star formation, building central bulges of stars, and feeding massive black holes. In particular for M33, a small bar could explain discrepancies between models and observed gas velocities in the inner disk. The authors suggest more modeling should be done to explain why the younger stellar populations did not form in a bar, whereas the older stellar populations did.
Finally, the authors compared their global star formation rate (which has units of M⊙/year and measures the total mass of stars being added to the galaxy each year) to that measured by the conventional methods using UV and H-alpha emission. The author found their measured value was about 1.6 times larger than the UV/H-alpha measurement, indicating UV/H-alpha measurements may not capture the full star formation rate of a galaxy. In the future, the authors plan to extend this analysis by focusing on measuring the age gradient of M33’s spiral arms and bar.
Featured image credit: NASA, edited by Abby Lee
Astrobite edited by Isabella Trierweiler