A new technique for estimating the mass of galaxies promises more reliable results, especially when applied to large datasets generated by current and future surveys, according to a research team led by Ekta Patel at the University of Arizona. Published in the Astrophysical Journal the study is the first to combine the observed full three-dimensional movements in several of the Winter Street satellite galaxies with extensive computer simulations to get a high accurate estimate of the mass of our home galaxy.
Determining the mass of galaxies plays a crucial role in unraveling the basic mysteries of the universe's architecture. According to today's cosmological models, a galaxy shows visible material, such as stars, gas and dust, for just 15 percent of its mass. The remaining 85 percent are believed to be living in dark matter, a mysterious component that has never been observed and whose physical properties remain largely unknown. The vast majority of a galaxy mass (mostly dark matter) lies in its halo, a large environment containing few, if any, stars and whose form is largely unknown.
In a widely accepted cosmological model, dark matter filament spans the entire universe and draws light ("common") matter with them. Where they cross, gas and dust accumulate and accumulate in galaxies. For a few billion, small galaxies join together to form larger ones, and as they grow in size and their gravity goes further into space, they attract a zoo of other small galaxies, which then become satellite galaxies. Their paths are determined by their host galaxy, just as the gravity of the sun controls the control of planets and bodies in the solar system.
"We now know that the universe is expanding," said Patel, a fourth-year doctoral student at the UA Department of Astronomy and Steward Observatory. "But when two galaxies come close enough, their mutual attraction is greater than the influence of the expanding universe, so they start crossing each other around a common center, like our Milky Way and our closest neighbor, Andromeda Galaxy."  This movie shows the orbital paths of several Milky Way satellites over the last 2 billion years, derived from this study. Credit: True Patel / University of Arizona
Although Andromeda approaches the Algarve 110 kilometers per second, the two will not merge until about 4.5 billion years from now. According to Patel, tracing of Andromeda's movement is "equivalent to seeing a human hair grow on the moon's distance".
Because it's impossible to "weigh" a galaxy simply by looking at it-much less when the observer happens to be Inside, as is the case with our Milky Way scientist, a galaxy masses by studying movements of celestial objects as they dance around the host galaxy, led by its gravity pressure. Such objects – also called tractors, as they trace their masses of host galaxy – can be satellite galaxies or streams of stars created from the spread of past galaxies that came too close to remain intact.
Unlike previous methods commonly used to estimate a galaxy mass, like measuring its traces and positions, Patel and her co-authors use their angular momentum, giving more reliable results as it does not change over time. The angular momentum of a body in space depends on both distance and velocity. Since satellite galaxies tend to move around the Milky Way in elliptical lanes, their speeds increase as they approach our galaxy and decrease as they go further away. Since the angular momentum is a product of both position and speed, there is no net change regardless of whether the tracker is in its closest or longest position in its orbit.
"Think of a sculptor who makes a pirouette," says Patel. "As she pulls her arms, she spins faster. In other words, her speed is changing, but her angle of angulation keeps her throughout her duration."
The study, presented by Patel on Thursday, June 7th at the 232rd meeting with the American Astronomical Society in Denver, is the first to look at the full three-dimensional movements of nine of the Winter Park's 50 famous satellite galaxies at the same time and compare their angular momentum measurements to one simulated universe containing a total of 20,000 host galaxies similar to our own galaxy. Together, the simulated galaxies host 90,000 satellite galaxies.
Patel's team lowered the mass of Walloon at 0.96 trillion of sunbeds. Earlier estimates had placed our galaxy mass between 700 billion and 2 trillion solar masses. The results also reinforce estimates that indicate that the Andromeda Galaxy (M31) is more massive than our Milky Way.
The authors hope to apply their method to the ever-expanding data as they become available through current and future galactic surveys such as Gaia Space Observatory and LSST, The Large Synoptic Survey Telescope. According to co-author Gurtina Besla, a deputy professor of astronomy at UA, restrictions on the mass of the Milky Way will be improved as new observations are obtained that watch more satellite galaxies speed and next generation simulations will provide higher resolution so that researchers can get better statistics for the smallest mass strings, the so-called ultra-violet galaxies.
"Our method allows us to utilize measurements of the speed of several satellite galaxies at the same time to get answers to what cold dark matter theory would predict the mass of the Milky Way halo in a robust way," says Besla. "It's perfect for take advantage of the current rapid growth in both observation data and numerical abilities. "
Milk Road vs Andromeda (for now we have a chance)
"Calculation of the mass of liquid using the ensemble of classic satellite galaxies", Astrophysical Journal (2018).