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Galaxy mass, gas vs stars?
I had read and adopted that within the bulge of galaxies (and within
ellipticals) there is very little gas. ie, stars are the bulk of the mass. Please do not include dark mass, I am only interested in baryon mass. Then, in Bullet cluster, supposedly the majority of mass is in the form of gas, something like 4 to 1 gas to stars I think it is. Yet the glowing gas that is there, seems to have two hot spots, rather than a more uniform spherical glow I would expect. And in various accounts, different scientists have said there is little mass in gas, then another says there is most of the mass in gas. So I'm confused. What I've read seems to go back and forth. Is one talking about baryonic mass and the other talking about dark mass? Is there more mass in gas or stars, and how do we know? is this from radio observations, or absorption lines or ??? Does this hold for the MW galaxy? And if so in the MW, where is the gas...ie, are the gas clouds we observe in spiral arms that gas, or, is it farther out at 30 to 50 kpc radii? rt |
#3
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Galaxy mass, gas vs stars?
In article ,
writes: I had read and adopted that within the bulge of galaxies (and within ellipticals) there is very little gas. ie, stars are the bulk of the mass. That's correct at the present cosmic time, though the gas fraction was much higher in the distant past. The gas fraction for the Milky Way is about 10%, for example, mostly neutral hydrogen. For other galaxies, as Phillip wrote, the typical fraction depends on galaxy type and mass (which are correlated), but except for very small galaxies, stars dominate the mass. Then, in Bullet cluster, supposedly the majority of mass is in the form of gas, something like 4 to 1 gas to stars I think it is. In galaxy _clusters_, gas dominates. Most of this gas is outside galaxies in the general "intracluster medium," as it's called. The intracluster gas is mostly ionized hydrogen. Yet the glowing gas that is there, seems to have two hot spots, rather than a more uniform spherical glow I would expect. As you say, the picture shows the _glowing_ gas. The gas emissivity depends on both temperature and density, so the light (X-ray in this case) doesn't directly trace the mass. The (assumed) collision would have created a complicated shock structure, and either the gas has rebounded, or there is gas in the middle that has low emissivity. I don't know whether the details have been modeled. And in various accounts, different scientists have said there is little mass in gas, then another says there is most of the mass in gas. Depends on where you are talking about. Is there more mass in gas or stars, and how do we know? is this from radio observations, or absorption lines or ??? There are a variety of lines of evidence. As you say, it's mainly the 21 cm radio line for neutral hydrogen in the Milky Way and local galaxies. There are carbon monoxide radio lines for molecular gas. Intracluster gas mass is mainly from X-ray observations. Stellar masses come from visible and infrared observations. All of these have uncertainties, but I don't think the basic picture will change. The overall census of baryons comes from Big Bang nucleosynthesis and also from fluctuations in the CMB. These are independent estimates, but they agree. in the MW, where is the gas...ie, are the gas clouds we observe in spiral arms that gas Pretty much in the spiral arm clouds, though the gas doesn't drop off quite as fast with radius as the stellar density. The largest single molecular cloud in the MW, though, is Sgr B2, which is within a few parsecs of the Galactic center. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#4
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Galaxy mass, gas vs stars?
On 10/17/16 2:04 AM, Phillip Helbig (undress to reply) wrote:
In article , writes: I had read and adopted that within the bulge of galaxies (and within ellipticals) there is very little gas. ie, stars are the bulk of the mass. OK. Please do not include dark mass, I am only interested in baryon mass. OK. Then, in Bullet cluster, supposedly the majority of mass is in the form of gas, something like 4 to 1 gas to stars I think it is. Yet the glowing gas that is there, seems to have two hot spots, rather than a more uniform spherical glow I would expect. The interesting thing about the Bullet Cluster is that it is a collision of two galaxies and demonstrates different properties of gas and dark matter. The dark-matter distribution and stellar distribution of each galaxy are similar, while the gas seems to be lagging behind, as if braked by the collision. This makes sense if dark matter and stars are essentially collisionless, but gas is not. This differential lagging 'braking' process makes sense if gas, dark matter and stars have different diameters/mass ratios and are necessarily differentially slowed by an underlying medium. This Stokes' law concept Acceleration = medium viscosity * velocity * object diameter/object mass object position = (1/2) * acceleration * time^2 has been used over the years to experimentally establish this phenomenon in various contexts from rain drops or hailstones falling from clouds to stream bed sand particle size spatial distribution. One can conclude that gas particles have a higher diameter/mass ratio than dark matter and stars. Dark matter and stars are measurably collisionless but cannot be considered absolutely collisionless to underlying medium. The issue of space time viscosity has been addressed in the literature. There are two clumps each of stars, gas, and dark matter, because there are two galaxies. So this is not a good example to learn about the distribution of gas in galaxies in general. And in various accounts, different scientists have said there is little mass in gas, then another says there is most of the mass in gas. So I'm confused. What I've read seems to go back and forth. Is one talking about baryonic mass and the other talking about dark mass? Is there more mass in gas or stars, and how do we know? is this from radio observations, or absorption lines or ??? Does this hold for the MW galaxy? And if so in the MW, where is the gas...ie, are the gas clouds we observe in spiral arms that gas, or, is it farther out at 30 to 50 kpc radii? Googling "gas in galaxies" I find http://www.pages.astronomy.ua.edu/ke...axies/gas.html Here, one can read that the gas fraction increases along the Hubble sequence, with SINGLE galaxies having typical H I (neutral hydrogen) gas masses of a few tens of billions of solar masses, i.e. about 10% of the mass in in stars. But late types such as Sm and Im can have more mass in gas than in the visible stellar population. Of course, there is also molecular hydrogen, and also other types of gas. On the whole, though, in individual galaxies there is more mass in stars than in neutral hydrogen gas. There is also ionized hydrogen gas, known as H II. In clusters, as opposed to individual galaxies, there can be more mass in this than there is in the stars and neutral gas combined. The Wikipedia page for "galaxy cluster" gives mass fractions of 1%, 9%, and 90% for galaxies (i.e. mass in stars), intergalactic gas, and dark matter, respectively. (At this level of precision, it doesn't matter if the galaxy fraction includes just the stars or also gas within the galaxy.) The intracluster medium is mainly ionized hydrogen and helium. If you are interested in individual galaxies, then there is variation from one to another. Even though on average there is more mass in stars, particular galaxies might have more mass in gas than in stars, or be essentially gas-free. There is also the question whether the gas was produced within the galaxy or was accreted from elsewhere. |
#5
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Galaxy mass, gas vs stars?
[[Mod. note -- I apologise for the delay in processing this article,
which the author submitted on Tue, 18 Oct 2016 09:41:00 -0500 (CDT). -- jt]] On 10/18/16 1:21 AM, Steve Willner wrote: In article , writes: I had read and adopted that within the bulge of galaxies (and within ellipticals) there is very little gas. ie, stars are the bulk of the mass. That's correct at the present cosmic time, though the gas fraction was much higher in the distant past. The gas fraction for the Milky Way is about 10%, for example, mostly neutral hydrogen. For other galaxies, as Phillip wrote, the typical fraction depends on galaxy type and mass (which are correlated), but except for very small galaxies, stars dominate the mass. Then, in Bullet cluster, supposedly the majority of mass is in the form of gas, something like 4 to 1 gas to stars I think it is. In galaxy _clusters_, gas dominates. Most of this gas is outside galaxies in the general "intracluster medium," as it's called. The intracluster gas is mostly ionized hydrogen. Yet the glowing gas that is there, seems to have two hot spots, rather than a more uniform spherical glow I would expect. As you say, the picture shows the _glowing_ gas. The gas emissivity depends on both temperature and density, so the light (X-ray in this case) doesn't directly trace the mass. The (assumed) collision would have created a complicated shock structure, and either the gas has rebounded, or there is gas in the middle that has low emissivity. I don't know whether the details have been modeled. And in various accounts, different scientists have said there is little mass in gas, then another says there is most of the mass in gas. Depends on where you are talking about. Is there more mass in gas or stars, and how do we know? is this from radio observations, or absorption lines or ??? There are a variety of lines of evidence. As you say, it's mainly the 21 cm radio line for neutral hydrogen in the Milky Way and local galaxies. There are carbon monoxide radio lines for molecular gas. Intracluster gas mass is mainly from X-ray observations. Stellar masses come from visible and infrared observations. All of these have uncertainties, but I don't think the basic picture will change. The overall census of baryons comes from Big Bang nucleosynthesis and also from fluctuations in the CMB. These are independent estimates, but they agree. These agreements assume all baryons are in gaseous form. What if other baryons are in another phase with equal agreement back to a congruent Big Bang nucleosynthesis? (ex: solid large dense objects not detectable with current instruments)? in the MW, where is the gas...ie, are the gas clouds we observe in spiral arms that gas Pretty much in the spiral arm clouds, though the gas doesn't drop off quite as fast with radius as the stellar density. The largest single molecular cloud in the MW, though, is Sgr B2, which is within a few parsecs of the Galactic center. |
#6
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Galaxy mass, gas vs stars?
In article , "Richard D.
Saam" writes: The overall census of baryons comes from Big Bang nucleosynthesis and also from fluctuations in the CMB. These are independent estimates, but they agree. These agreements assume all baryons are in gaseous form. What if other baryons are in another phase with equal agreement back to a congruent Big Bang nucleosynthesis? (ex: solid large dense objects not detectable with current instruments)? A standard exercise is to demonstrate that even if all dark matter (not just dark baryons) were in bricks which behave optically like real bricks do, they wouldn't be detectable.* As for baryons, we know how many there are. A substantial amount in bricks or whatever also needs a mechanism. Why can't they be copies of the ApJ? Because we know of no mechanism to produce them. Similarly, a valid hypothesis needs more than "one can't yet rule it out"; it needs to be plausible. ------ * Of course, by definition non-baryonic dark matter does not interact electromagnetically. The point is that even that much matter in macroscopic objects would be difficult to detect. |
#7
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Galaxy mass, gas vs stars?
In article ,
"Richard D. Saam" writes: What if other baryons are in another phase with equal agreement back to a congruent Big Bang nucleosynthesis? (ex: solid large dense objects not detectable with current instruments)? The pre-2015 understanding of the universe's baryon census is summarized by Fig 9 at https://arxiv.org/abs/1112.2706 In 2015, X-ray observations http://adsabs.harvard.edu/abs/2015Natur.528..105E suggested that the baryons labeled "missing" have been found. The story is far from over; one example can be seen at http://adsabs.harvard.edu/abs/2015MNRAS.450.2261M An ADS search will give lots more references. As Phillip wrote, not every alternative can be ruled out, but so far the standard model appears consistent with existing data. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#8
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Galaxy mass, gas vs stars?
On Tuesday, October 25, 2016 at 2:17:45 PM UTC-7, Steve Willner wrote:
In article , "Richard D. Saam" writes: What if other baryons are in another phase with equal agreement back to a congruent Big Bang nucleosynthesis? (ex: solid large dense objects not detectable with current instruments)? The pre-2015 understanding of the universe's baryon census is summarized by Fig 9 at https://arxiv.org/abs/1112.2706 Thanks for links / info. On census, I assume "galaxies" means, "stars"? Ross |
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