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UA Astronomers Use New Data to Weigh the Milky Way

Steward Grad Student Ekta Patel and Steward Assistant Professor Gurtina Besla are the lead authors of a study to use the best-available observational data and physically conserved quantities to place robust limits on the mass of the Milky Way. Ekta provides the following text:

"Calculating the mass of our galaxy precisely is important for our understanding of galaxy physics, but it is difficult to do since we live inside of the Milky Way. The current widely-accepted theory on the growth of structures in our universe is the cold dark matter theory. This theory predicts that Milky Way mass galaxies should host around 100-200 satellite galaxies, or galaxies that orbit other galaxies. Currently, we know of about 50 satellite galaxies orbiting our own, so one reason a precise mass estimate is useful is to test how well this theory matches our observations in the context of the satellite galaxy population.

Many values of the Milky Way's mass, including some 30-year-old results from Steward astronomers, have been published over the last few decades using different methods. In this work, we present a method that appears to be robust when we use the information available to us and we hope that it will be a way to move forward as the observational data sets continue to grow and as numerical simulations of the universe improve. This method focuses on using the orbital angular momentum of satellite galaxies rather than their position and velocity to construct a mass estimate for the Milky Way. The benefit of using angular momentum is that it is conserved over time, unlike position and velocity. This means that no matter where a satellite galaxy might be in its orbit around its host galaxy (i.e. closest approach or farthest distance), the method still proves to be reliable. The second benefit of our method is that we use the properties of eight or nine Milky Way satellite galaxies simultaneously, which helps in yielding a more precise estimate. As the properties of additional Milky Way satellites are measured, we will be able to revise our current Milky Way mass estimate, which is about 960 billion times the mass of the Sun."

The UA press release can be found HERE.

The cover photo had to be trimmed to fit. To see Yuri Beletsky's full-size photo of the Milky Way with two of its satellite galaxies (the two Magellanic Clouds), click here. To see many more of his photos, go HERE.

left:courtesy M3 Engineering and TMT International Observatory LLC; right: courtesy GMT Mason Media and GMT Project

GMT, TMT, NOAO, and NSF Team Up on Next Generation of Telescopes

On Monday, May 21, 2018, it was announced that the GMT project, the Thirty Meter International Observatory, and NOAO and the NSF have joined forces. HERE is the press release from NOAO from which we quote a few paragraphs (the photos come from TMT and GMT websites):

"Our shared mission is to strengthen scientific leadership by the U.S. community-at-large through access to extremely large telescopes in the Northern and Southern Hemispheres. This two-hemisphere model will provide the U.S. science community with greater and more diverse research opportunities than can be achieved with a single telescope, and hence more opportunities for leadership.

Our immediate task is advocacy for frontier research programs led by U.S community scientists that can achieve exceptional advancements in humanity’s understanding of the cosmos.

Our audience is the U.S. research community as represented by the upcoming Decadal Survey of Astronomy and Astrophysics (an enterprise of the U.S. National Academies).

As an essential part of that immediate task, we will work with the U.S. research community to develop exemplar Key Science Programs (KSPs) within major research areas including the dark universe, first stars & first galaxies, exoplanet atmospheres, the surfaces of satellites and other small bodies throughout Solar System, and/or other topics to be proposed and prioritized by community-based working groups.

Key Science Programs are envisioned to be open collaborations that gather observers, theorists, and data scientists together to exploit significant investments of Thirty Meter Telescope (TMT) and Giant Magellan Telescope (GMT) observing time, from tens to hundreds of nights. Some of these collaborations are expected to be international in nature. If well-justified by KSP plans, we envisage that at least 25% of the observing time at each international observatory will be available for the U.S. community."


Steward Astronomers Speaking at the Flandrau Lecture Series on Exoplanets

Flandrau Science Center and Planetarium is hosting a three-part series on Exoplanets [the first talks were in March].

The second talks will be held Thursday, April 12 at 6pm and feature Dr. Tommi Koskinen of LPL and Dr. Serena Kim of Steward. The third set of talks are on Thursday, May 17 at 6pm, with speakers Dr. Peter Behroozi of Steward and Doctoral Candidate Lauren Schatz of Optical Sciences. To learn more, check the link. There is a $10 cost and tickets are first-come, first-served.


Congratulations to three of our Graduate Students

Three of our graduate students were recognized yesterday for their Excellence in Service, Teaching, and Scholarship.

Recognized in a College of Science Award ceremony yesterday were:

  • Eckhart Spalding - Service Award, Department of Astronomy
  • Melissa Halford - Teaching Award, Department of Astronomy
  • Carolyn Raithel - Scholarship Award, Department of Astronomy and, as announced yesterday, named Outstanding Scholar for the entire College of Science!

Congratulations to them and to those who support our outstanding students! 


2017 Departmental Photo Contest

Please join us in congratulating the winner of Steward Observatory's 2017 Annual Departmental Photo Contest. As Buell Jannuzi, Director of Steward Observatory, announced during his Holiday presentation, Travis Deyoe of the Mt. Lemmon Sky Center won this year's photo contest with his panoramic image of Steward Observatory Telescopes on Mt. Lemmon in Southern Arizona. Please click "More->" to get  to the full article. You can then click on each image below to get enlarged versions. 

The Steward Observatory Mt. Lemmon Sky Center is an exceptional science learning facility located at the summit of the Catalina Mountains. The nightly "SkyNights" program provides the opportunity to explore astronomical wonders throughout the year. Join at

As there were many impressive and beautiful photo submissions, we also chose the following seven images as honorable mentions. Congratulations to those whose photos were chosen and thank you to all who submitted to this year's photo contest, as we expect to use all images submitted throughout the department. 

Alessandro Cavallaro, LBT with the snow

Adam Block, Coronas

Dan Avner, Pomenis Astrograph Commissioning 

Minghao Yue, Milky Way at Glacier National Park 

Travis Deyoe, Rosette


Brian Duffy, APEX Telescope in the Atacama Desert

                        Adam Block, Reflection Nebula and dust in Auriga                       



Gurtina Besla Honored for TIMESTEP

The  Office of Diversity and Inclusive Excellence and the Commission on the Status of Women are honoring several people, including Astronomy/Steward's Assistant Professor Dr Gurtina Besla. Besla is receiving the Edith Sayre Auslander Emerging Vision Award for her work with the TIMESTEP program. You can read about the award and about TIMESTEP HERE and HERE.