My God, it's full of galaxies
May 28, 2011 5:38 PM Subscribe
"The 2MASS Redshift Survey (2MRS) has catalogued more than 43,000 galaxies within 380 million light-years from Earth. In this projection, the plane of the Milky Way runs horizontally across the center of the image. 2MRS is notable for extending closer to the Galactic plane than previous surveys - a region that's generally obscured by dust." Hires image.
Following a few links on the pages, I found this image helped with perspective, but brought up some questions that perhaps someone educated in this could answer?
The explanation of the linked picture says "The image is derived from the 2MASS Extended Source Catalog (XSC)--more than 1.5 million galaxies...". What does "catalogued" mean in this case? Named and tracked in the results of the map? Previously known and now mapped in relation to the rest of the galaxies now seen?
The equation in the legend - I'm assuming "V" is velocity, and "c" is the speed of light, and I'm not sure what the subscript "H" is for - zero/violet is moving the same speed as us and is closer, and 0.07+/deep red is moving faster, thus they're further away. Correct? Or am I combining two separate measurements (distance and speed) into one?
Are there any theories on the Superclusters? They all seem to have very similar speeds in their respective groups which leads me to believe that they're from a similar source. Is this where the "dark matter" theory comes in, holding them all together, or could they have come from smaller explosions after the big bang?
My brain: it itches now. I was going to say that this is a pretty amazing picture, but I have to keep reminding myself that it is not amazing, it is awesome in the true definition of the word - the dots are individual galaxies consisting of tens of millions of stars. This place is huge and magnificent. I hope they're proud of the map, because I sure would be.
posted by Zack_Replica at 7:59 PM on May 28, 2011
The explanation of the linked picture says "The image is derived from the 2MASS Extended Source Catalog (XSC)--more than 1.5 million galaxies...". What does "catalogued" mean in this case? Named and tracked in the results of the map? Previously known and now mapped in relation to the rest of the galaxies now seen?
The equation in the legend - I'm assuming "V" is velocity, and "c" is the speed of light, and I'm not sure what the subscript "H" is for - zero/violet is moving the same speed as us and is closer, and 0.07+/deep red is moving faster, thus they're further away. Correct? Or am I combining two separate measurements (distance and speed) into one?
Are there any theories on the Superclusters? They all seem to have very similar speeds in their respective groups which leads me to believe that they're from a similar source. Is this where the "dark matter" theory comes in, holding them all together, or could they have come from smaller explosions after the big bang?
My brain: it itches now. I was going to say that this is a pretty amazing picture, but I have to keep reminding myself that it is not amazing, it is awesome in the true definition of the word - the dots are individual galaxies consisting of tens of millions of stars. This place is huge and magnificent. I hope they're proud of the map, because I sure would be.
posted by Zack_Replica at 7:59 PM on May 28, 2011
"that they're from a similar source." Of course, I know that they're all from the same source - the Big Bang. What I should have said was "that they're from similar sources." being smaller explosions after the Big Bang. ...or does our single-point-of-perspective just make it look like this is happening? Too. Many. Questions!
posted by Zack_Replica at 8:11 PM on May 28, 2011
posted by Zack_Replica at 8:11 PM on May 28, 2011
The equation in the legend - I'm assuming "V" is velocity, and "c" is the speed of light, and I'm not sure what the subscript "H" is for - zero/violet is moving the same speed as us and is closer, and 0.07+/deep red is moving faster, thus they're further away. Correct? Or am I combining two separate measurements (distance and speed) into one?
The notation is concerning Hubble's Law. As we don't exactly know the value of Hubble's constant (although anyone who says it's anything drastically different from 71 km/s/Mpc is behind the times), we can't give an exact distance based on red shift. If a galaxy is moving away from us at 700 km/s, then, via D = v/H0, it is 10 Mpc away if H0 is 70 km/s/Mpc. If H0 is, say, 50 km/s/Mpc, then it's going to be 14 Mpc. Now, while universal expansion means the galaxies are moving from us (and this is something technically inaccurate but that astronomers say all the time -- the galaxies aren't moving away from us, space is just expanding between us...it's just annoying to say that all the time), they are also moving because of gravitational interactions with other bodies (e.g. superclusters). After enough distance, however, the extra peculiar motion comes out in the wash (but it's why they denote the H in the subscript to specify the Hubble (i.e. expansion) velocity). So, what you have is a distance measurement that is independent of the exact value of Hubble's constant, but can still give you relative distances.
I have admittedly played a little fast and loose with the actual science going on here, but the general point is that they are converting speed into distance. Blue things are nearby, red things are far away.
posted by miguelcervantes at 9:04 PM on May 28, 2011
The notation is concerning Hubble's Law. As we don't exactly know the value of Hubble's constant (although anyone who says it's anything drastically different from 71 km/s/Mpc is behind the times), we can't give an exact distance based on red shift. If a galaxy is moving away from us at 700 km/s, then, via D = v/H0, it is 10 Mpc away if H0 is 70 km/s/Mpc. If H0 is, say, 50 km/s/Mpc, then it's going to be 14 Mpc. Now, while universal expansion means the galaxies are moving from us (and this is something technically inaccurate but that astronomers say all the time -- the galaxies aren't moving away from us, space is just expanding between us...it's just annoying to say that all the time), they are also moving because of gravitational interactions with other bodies (e.g. superclusters). After enough distance, however, the extra peculiar motion comes out in the wash (but it's why they denote the H in the subscript to specify the Hubble (i.e. expansion) velocity). So, what you have is a distance measurement that is independent of the exact value of Hubble's constant, but can still give you relative distances.
I have admittedly played a little fast and loose with the actual science going on here, but the general point is that they are converting speed into distance. Blue things are nearby, red things are far away.
posted by miguelcervantes at 9:04 PM on May 28, 2011
Zack_Replica: I'll try to answer the remaining questions.
An astronomical catalog is just a list of objects with their positions and any other measured properties. The 2MASS Extended Source Catalog is just the list of all the extended objects (i.e. those with measurable size rather than appearing starlike) observed by the 2MASS project, which obtained images of the entire sky in infrared light. Nearby galaxies will appear extended (or fuzzy), faraway galaxies may be too small for 2MASS to distinguish them from stars.
For most of the 2MASS galaxies, we just know how bright they are (in some cases we might know the redshift, and hence distance, from other projects). 2MRS systematically measured redshifts (and hence distances) to 43k galaxies out of that full sample of 1.5 million, and those 43k are the objects shown in the press release images.
In most cases, the galaxies seen in 2MASS images would have been already known (for those, infrared observations help us to measure the total amount of stars a galaxy has formed over its lifetime, as the coolest, most long-lived stars are brightest in the IR). The exception is when you start to look through the plane of our galaxy; in that case, interstellar dust grains (similar in size and composition to soot) block most of the visible light. See here for a nice image of the Milky Way lined up in the same way as the 2MRS press release image - the brownish clouds are regions where dust obscures our view most strongly. 2MRS is the first survey to uniformly get redshifts for galaxies well into the 'zone of avoidance' near the Galactic plane.
Superclusters are very well understood at this point. There are no explosions involved (that was once a popular theory for why we see large regions empty of galaxies, commonly called voids, but there's a lot of evidence against that picture). Instead, matter tends to be attracted towards regions that were a little denser than average after the Big Bang; those regions therefore tend to collect more and more mass over time. These overdense regions (the biggest of which become massive clusters of more than a thousand galaxies) tend to be found near each other. The superclusters are just associations of clusters in the same general region; unlike clusters, they are not held together by gravity. A decent analogy is mountain peaks: the highest mountains (compared to sea level) tend to be found near each other, in part because they are all starting from a higher base. You will find many more high peaks in the Tibetan Plateau, where the average elevation is about 5 km, then in Australia; similarly you will find many more galaxies (and clusters of galaxies) around a supercluster than in a randomly chosen region.
The second set of movies on this page show the process of structure growth occurring in a massive supercomputer simulation of the behavior of dark matter. It appears that we can model this process quite accurately; the general pattern of dark matter clustering predicted in these simulations matches the patterns of galaxy clustering we observe very well.
posted by janewman at 9:56 PM on May 28, 2011 [1 favorite]
An astronomical catalog is just a list of objects with their positions and any other measured properties. The 2MASS Extended Source Catalog is just the list of all the extended objects (i.e. those with measurable size rather than appearing starlike) observed by the 2MASS project, which obtained images of the entire sky in infrared light. Nearby galaxies will appear extended (or fuzzy), faraway galaxies may be too small for 2MASS to distinguish them from stars.
For most of the 2MASS galaxies, we just know how bright they are (in some cases we might know the redshift, and hence distance, from other projects). 2MRS systematically measured redshifts (and hence distances) to 43k galaxies out of that full sample of 1.5 million, and those 43k are the objects shown in the press release images.
In most cases, the galaxies seen in 2MASS images would have been already known (for those, infrared observations help us to measure the total amount of stars a galaxy has formed over its lifetime, as the coolest, most long-lived stars are brightest in the IR). The exception is when you start to look through the plane of our galaxy; in that case, interstellar dust grains (similar in size and composition to soot) block most of the visible light. See here for a nice image of the Milky Way lined up in the same way as the 2MRS press release image - the brownish clouds are regions where dust obscures our view most strongly. 2MRS is the first survey to uniformly get redshifts for galaxies well into the 'zone of avoidance' near the Galactic plane.
Superclusters are very well understood at this point. There are no explosions involved (that was once a popular theory for why we see large regions empty of galaxies, commonly called voids, but there's a lot of evidence against that picture). Instead, matter tends to be attracted towards regions that were a little denser than average after the Big Bang; those regions therefore tend to collect more and more mass over time. These overdense regions (the biggest of which become massive clusters of more than a thousand galaxies) tend to be found near each other. The superclusters are just associations of clusters in the same general region; unlike clusters, they are not held together by gravity. A decent analogy is mountain peaks: the highest mountains (compared to sea level) tend to be found near each other, in part because they are all starting from a higher base. You will find many more high peaks in the Tibetan Plateau, where the average elevation is about 5 km, then in Australia; similarly you will find many more galaxies (and clusters of galaxies) around a supercluster than in a randomly chosen region.
The second set of movies on this page show the process of structure growth occurring in a massive supercomputer simulation of the behavior of dark matter. It appears that we can model this process quite accurately; the general pattern of dark matter clustering predicted in these simulations matches the patterns of galaxy clustering we observe very well.
posted by janewman at 9:56 PM on May 28, 2011 [1 favorite]
The H subscript indicates, I believe, heliocentric velocity - the motion of the Earth around the Sun has been subtracted off in other words.
Karen Masters - mentioned in the article - is a friend and colleague. I'll point her over here in case she has more answers to questions, or wants to correct me on the above.
posted by edd at 3:15 AM on May 29, 2011
Karen Masters - mentioned in the article - is a friend and colleague. I'll point her over here in case she has more answers to questions, or wants to correct me on the above.
posted by edd at 3:15 AM on May 29, 2011
and bwg - Stephen Hawking's initials are in the cosmic microwave background too.
posted by edd at 3:23 AM on May 29, 2011
posted by edd at 3:23 AM on May 29, 2011
edd: "and bwg - Stephen Hawking's initials are in the cosmic microwave background too."
Well they'd better be.
posted by bwg at 3:44 AM on May 29, 2011
Well they'd better be.
posted by bwg at 3:44 AM on May 29, 2011
Brian Greene: Darkness on the Edge of the Universe - "when future astronomers look to the sky, they will no longer witness the past. The past will have drifted beyond the cliffs of space. Observations will reveal nothing but an endless stretch of inky black stillness."
posted by kliuless at 9:00 AM on May 29, 2011
posted by kliuless at 9:00 AM on May 29, 2011
Thanks for the explanations, everyone, they helped a lot. The mountain peaks/supercluster analogy makes sense too, and has led to me spending the last half-hour looking up dark matter theories. And feeling very very tiny.
posted by Zack_Replica at 11:35 AM on May 29, 2011
posted by Zack_Replica at 11:35 AM on May 29, 2011
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posted by bwg at 5:45 PM on May 28, 2011