![]() Using a principal-component analysis, we reduce the complexity of our model for large redshift-space separations. Once inserted in the streaming equation, the fit yields an excellent description of redshift-space correlations at all scales that vastly outperforms the Gaussian and exponential approximations. Finally, we introduce a mixture of Gaussians which is known in statistics as the generalized hyperbolic distribution and show that it provides an accurate fit to the PDF. We test the often used Gaussian ansatz for the PDF of pairwise velocities and discuss its limitations. We then discuss several statistical properties of the pairwise velocities for DM particles and haloes by using a suite of high-resolution N-body simulations. In the first case, we recover the classic equation while we demonstrate that modifications are necessary for unordered pairs. Following a kinetic-theory approach, we derive the fundamental equations of the streaming model for ordered and unordered pairs. Its key element is the probability density function (PDF) of line-of-sight pairwise peculiar velocities. If it were possible to travel at the speed of light, it would still take 2.3 million years to reach the Andromeda Galaxy.The streaming model describes the mapping between real and redshift space for 2-point clustering statistics. If we traveled at 17.3 km/s, it would take us 40,000,000,000 (or 40 billion!) years to get to the nearest galaxy that is like our own - the Andromeda Galaxy! This travel time is longer than scientists believe the Universe has been around. Knowing this, we can calculate that, at the distance of 5,000,000 parsecs (which is the radius of this circle), 0.1 arcsecond is about 2.5 parsecs. * 1 arcsecond is a unit of angle that's 1/3600 of a degree. This makes it more difficult to study individual objects! So each pixel covers 2.5 by 2.5 arcseconds (times the 'thickness' of the galaxy) - which is likely to contain several stars at least. If the galaxy is 5,000,000 parsecs away, you can use trigonometry to figure out that 0.1 arcsecond corresponds to ~2.5 parsecs. Let's use something very small, like 0.1 arcsecond as our angle*. How can astronomers figure out how severe this crowding problem is? Each side of a picture element (pixel) defines an angle on the sky, determined by the design of the instrument. It's not just the apparent faintness of stars at such large distances the crowding of stars in a small patch of the sky is a more important problem. ![]() But even with the sharp vision of the Hubble Space Telescope, this can't be done much beyond the Local Group. We can actually study individual objects (ie a specific star) in Local Group galaxies, even at distances as great as 2,000,000 parsecs. Why Are These Distances Important To Astronomers? Other methods that are used to find the distances to galaxies can be found on the ABCs of Distance. Edwin Hubble used just such a star in the Andromeda Galaxy to find that the galaxy was more than a million light years away - much further than the most distant reaches of our own galaxy.įor more information about Cepheids, please read the section on calculating distances in the Milky Way. How Do We Calculate Distances of This Magnitude?Ĭepheid Variable stars are used to calculate distances of this magnitude. ![]() This translates to 2.3 million light years, or 725 kpc. When we talk about the distances to other galaxies, we often use the units of kiloparsec (kpc) and Megaparsec (Mpc). It is also possible that the Local Group may one day merge with the next nearest big galaxy cluster, the Virgo Cluster. The dynamics of the Local Group are changing, and some astronomers speculated that one day the two large spirals in it (M31 and the Milky Way) may collide and merge to form a giant elliptical galaxy. Both M31 and the Milky Way have dwarf galaxies associated with them. M31 and the Milky Way are the most massive members of the Local Group, with M33 being the 3rd largest. There are over 30 galaxies that are considered to be in the local group, and they are spread over a diameter of nearly 10 million light years, with the center of them being somewhere between the Milky Way and M31. It has two small satellite galaxies, M32 and M110.Īlso prominent in the local group is the Triangulum Galaxy (M33), Leo I, and NGC 6822. One of the most prominent members of the Local group is M31, the Andromeda Galaxy.
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