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

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.

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.

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

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.

[[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.

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.

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

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