Description: In this Apogee podcast, Cosmic discusses the discovery of the 34th satellite galaxy to the giant spiral and our closest galactic neighbor M31.
Bio: Cosmic is a self- and crowd-funded independent research astronomer and space musician.
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Hello! This is Cosmic, and welcome to the Apogee podcast! The format of this podcast is myself describing, discussing, and critiquing an article of my choice from an astronomical journal. Sometimes topics within these articles send me off on temporary tangents, but they’re always relevant to the overall topic. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.
If you have any articles to suggest for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT gmail DOT com.
The music you hear in the background are my own compositions, which can be downloaded from my soundcloud channel ‘cosmiclettuce’. I hope you enjoy listening to space music as much as I do.
The apogee for this podcast took place yesterday, 15 November 2014, at 01:57 UTC. The lunar distance at that time was 404336 km, which is 561 km closer than last apogee on 18 October, and 247 km closer than the next apogee on 12 December. This is the 2nd closest apogee of 2014, with the closest one of this year having occurred on 6 May.
Ok, on with the paper!
Just like last apogee, I’m taking a look at another discovery paper, this time from the Astrophysical Journal Letters of October 2013 entitled “PERSEUS I: A DISTANT SATELLITE DWARF GALAXY OF ANDROMEDA”. In this case, it’s the discovery of a new — the 34th — satellite galaxy of the Great Andromeda Galaxy also known as M31. The 22 authors describe how this dwarf galaxy was detected, and then how they went about determining various physical parameters: structure, distance, and brightness. The unique characteristic of this new galaxy is how far away it appears to us from M31: a whopping 27.9 degrees! It’s so far away from M31 that it actually appears in the constellation of Perseus. Because of this, it has been given the name Perseus I. But because it’s also associated with the Andromeda Galaxy, it also has the name Andromeda XXXIII.
This discovery is brought to us from the Panoramic Survey and Rapid Response System (Pan-STARRS1), which is using a 1.8m telescope to image the entire northern sky in five different wavelengths (colors) down to a visual magnitude of about 22. Sky coverage allows Pan-STARRS1 to obtain images of the same patch of sky about four times a year.
No one really expected to find satellite galaxies of M31 to appear so far away from their host, but when two other dwarf galaxies associated with M31 were discovered also using Pan-STARRS1 images 20-plus degrees away, the hunt was on for more. This led to the discovery of Perseus I. A great graphical summary of the known satellite galaxies of M31 is shown in Figure 1 of the paper. I think that would make a great poster.
So how do you go about finding a faint, dwarf galaxy? Well, you need a reasonably concentrated group of stars bright enough to stand out against the background so you can detect them. Once these are detected, compare the colors of the stars to see if they are all similar. Astronomers use a plot called a ‘color-magnitude diagram’ to compare the colors of different stars and to see how they relate to one another. You can see the color- magnitude diagram for Perseus I in Figure 2 of the paper. Similar colors mean similar temperatures which can mean similar masses and ages. So if all of these stars are in the same place in the sky and are all the same age, it’s reasonable to assume that they are all part of the same group.
So once it’s established that this is indeed a large association of stars (a dwarf galaxy) the most important parameters in determining if it’s a satellite galaxy of M31 is its distance and motion. Once an objects’ distance is known, it’s size and relation to other objects can be calculated. Measuring a distance to any astronomical object is very difficult, but it’s also fundamental to our understanding. It’s a little easier when an object is fairly close because parallax can be used. But at intergalactic distances, parallax doesn’t work. What is commonly used instead are “standard candles”. This concept is faily easy to understand with the following example: Imagine that you have a 100 watt light bulb 10 feet away from you. You measure the brightness of the light bulb. You then move the light bulb 10 more feet away (twice as far away) from you and measure the brightness again. What you’ll notice is that the brightness you measure is 1/4th of what it was. This is the famous “one over r-squared law” that you may have heard of. Since the brightness of the bulb hasn’t actually changed (it’s still 100 watts), the difference in brightness has to do with how far away the bulb is. We now have a relationship between distance and brightness. The same concept can be applied to astronomical objects. Stars that are at a particular temperature, mass, and age, will all have about the same intrinsic brightness (like a 100 watt light bulb). We measure the apparent brightness of these stars and compare that to how bright they would be at a known distance. The difference in brightness is directly related to the difference in distance by the one over r-squared law. This brightness difference with standard candles is known as a “distance modulus”.
Making these measurements and calculations, it was determined that Perseus I has a heliocentric distance of 785 kiloparsecs (about 2.54 million light years), which is almost exactly the heliocentric distance to M31 itself (about 780 kiloparsecs)! The authors of this paper conclude that indeed this dwarf galaxy is a bound satellite of M31 with a separation between the two galaxies of 364 kiloparsecs (1.19 million light years).
The only missing part is the motion of Perseus I. How fast and in what direction? This has yet to be determined, but as of the writing of this paper those measurements were being made. Most likely they’ll find its motion in agreement with expectations to make this indeed a bound satellite of M31.
The authors went on the measure the size and shape of Perseus I using a statistical method called “maximum likelihood”. This is an iterative process by which you gently tweak various model parameters (in this case: centroid, radius, major axis position angle, ellipticity, and number of stars) until a model using those parameters best fits the data. Using this method, they found the galaxy to be 3.4 arcminutes in diameter, which at the calculated distance would make this dwarf galaxy about 400 parsecs (about 1300 light years) in diameter. Its shape is fairly elliptical along a nearly north-south major axis. Combine this with its size and it is indeed similar to other dwarf elliptical galaxies.
Why is a discovery like this important and exciting? First, it’s a sign that our technology is improving. We’re seeing things now that would have been impossible to see even five years ago. Second, the existence of this galaxy allows us to get a sense of how things are in that part of the universe. For example, understandning the motion of this dwarf galaxy will allow us to better constrain the mass of M31. With all of the satellite galaxies now known to be associated with M31, the authors of this paper conclude that M31 is “significantly more massive” than the Milky Way galaxy. This not only has implications on how the M31 system is moving and evolving, but also how the Milky Way is doing the same given our close proximity.
Until next apogee — I bid you Peace.
End of podcast:
365 Days of Astronomy
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