APOD: Messier 66 Close Up (2021 Jan 28)

[Ann]

Ram pressure galaxy ESO 137-001, photographed in (I think) visible and ultraviolet light. Credit: NASA,ESA.

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Ram pressure galaxy ESO 137-001, photographed in visible, ultraviolet and X-ray light. Credits: NASA, ESA, CXC.

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Galaxy ESO 137-001 is a perfect example of a ram pressure galaxy.

ESA/Hubble wrote:

This new Hubble image shows spiral galaxy ESO 137-001, framed against a bright background as it moves through the heart of galaxy cluster Abell 3627.

This image not only captures the galaxy and its backdrop in stunning detail, but also something more dramatic – intense blue streaks streaming outwards from the galaxy, seen shining brightly in ultraviolet light.

These streaks are in fact hot, wispy streams of gas that are being torn away from the galaxy by its surroundings as it moves through space. This violent galactic disrobing is due to a process known as ram pressure stripping – a drag force felt by an object moving through a fluid.

I apologize to ESA/Hubble for quoting the whole caption, but it was quite short, and all of it seemed important.

As you saw from the caption, galaxy ESO 137-001 is plunging through large galaxy cluster Abell 3627. Like all large galaxy clusters, Abell 3627 is full of hot gas, and ESO 137-001 feels this gas like a headwind. ESO 137-001 is being stripped of its own gas during its plunge through Abell 3627, but some of its gas is still "attached to" the leeward side of the galactic disk. Here we find dense pockets of gas where new stars are born. These are seen as short threads of blue in the picture at left. The blue threads are elongated clusters of hot stars, which glow in blue and ultraviolet light.

The picture at right shows a dramatic long tail (in purplish blue), which is actually gas that is so hot that it glows in X-rays. This gas, too, has been torn from galaxy ESO 137-001. But the gas is invisible in optical light.

ESO 137-001 is rapidly being stripped of its gas content, and will lose its ability to form new stars in the relatively near future.

This is ram pressure. M66, by contrast, doesn't experience ram pressure, just tidal forces.

Ann

[neufer]

By the way, when a galaxy rams into a cluster and gets shiny and blue, is the blue from blue giants and supergiants formation?

Can it be directly the galaxy gas disk hitting the cluster gas?

Thermal light is blue when it is very hot. Not 6000°K like the Sun but say 20,000°K.

At 300° thermal speed of molecules is 400 m/s (N2) or 1200 m/s (He). I guess it's 1700 m/s (H2) or 2400 m/s (H).

So at 20,000°K it's 70 times the energy or 8 times the speed, 20,000 m/s

OK, I quit. Ramming into a cluster at 3000 m/s is done, at 20,000 m/s is not.

You sort of have the right ideas.

However:

• 1) low density excited gases emit narrow individual spectral lines
  ... NOT continuous black body radiation.
  
  2) colliding neutral gas atoms might well be mostly elastic and, thereby,
  deposit their colliding energy into turbulence & sound waves rather than into ionization.

<<An H II region or HII region is a region of interstellar atomic hydrogen that is ionized.It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm.

Chemically, H II regions consist of about 90% hydrogen. The strongest hydrogen emission line, the H-alpha line at 656.3 nm, gives H II regions their characteristic red colour. (This emission line comes from excited un-ionized hydrogen.)

Typically H II regions reach temperatures of 10,000 K. They are mostly ionised gases with weak magnetic fields with strengths of several nanoteslas. Nevertheless, H II regions are almost always associated with a cold molecular gas, which originated from the same parent GMC.

H II regions are found only in spiral galaxies like the Milky Way and irregular galaxies. They are not seen in elliptical galaxies. In irregular galaxies, they may be dispersed throughout the galaxy, but in spirals they are most abundant within the spiral arms. A large spiral galaxy may contain thousands of H II regions.

The reason H II regions rarely appear in elliptical galaxies is that ellipticals are believed to form through galaxy mergers. In galaxy clusters, such mergers are frequent. When galaxies collide, individual stars almost never collide, but the GMCs and H II regions in the colliding galaxies are severely agitated. Under these conditions, enormous bursts of star formation are triggered, so rapid that most of the gas is converted into stars rather than the normal rate of 10% or less.

Galaxies undergoing such rapid star formation are known as starburst galaxies. The post-merger elliptical galaxy has a very low gas content, and so H II regions can no longer form. Twenty-first century observations have shown that a very small number of H II regions exist outside galaxies altogether. These intergalactic H II regions may be the remnants of tidal disruptions of small galaxies, and in some cases may represent a new generation of stars in a galaxy's most recently accreted gas.>>

[VictorBorun]

• 1) low density excited gases emit narrow individual spectral lines
  ... NOT continuous black body radiation.
  
  2) colliding neutral gas atoms might well be mostly elastic and, thereby,
  deposit their colliding energy into turbulence & sound waves rather than into ionization.

I think colliding atom+ion, or ion+ion, or electron+atom, or electron+ion can convert kinetic energy to photons and emit thermal radiation.
Like CMB, from pristine H/He plasma at 3000°K, in thermal equilibrium with radiation till the recombination era came.
And why would the galaxy gas stay neutral after ramming at 3000 km/s into a cluster?

[neufer]

• 1) low density excited gases emit narrow individual spectral lines
  ... NOT continuous black body radiation.
  
  2) colliding neutral gas atoms might well be mostly elastic and, thereby,
  deposit their colliding energy into turbulence & sound waves rather than into ionization.

I think colliding atom+ion, or ion+ion, or electron+atom, or electron+ion can convert kinetic energy to photons and emit thermal radiation.
Like CMB, from pristine H/He plasma at 3000°K, in thermal equilibrium with radiation till the recombination era came.
And why would the galaxy gas stay neutral after ramming at 3000 km/s into a cluster?

• I stated that you were generally on the right track...but it is not visibly bright and
  you should be thinking more along the lines of a couple of hundred of kilometers per second.

<<Within astronomy and astrophysics, James E. Gunn and J. Richard Gott first suggested that galaxies in a galaxy cluster moving through a hot intracluster medium would experience a pressure of:

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This pressure can strip gas out of the galaxy where, essentially, the gas is gravitationally bound to the galaxy less strongly than the force from the intracluster medium 'wind' due to the ram pressure. Evidence of this ram pressure stripping can be seen in the image of NGC 4402.

The image of NGC 4402 highlights some telltale signs of ram pressure stripping such as the curved, or convex, appearance of the disc of gas and dust, a result of the forces exerted by the heated gas. Light being emitted by the disc backlights the swirling dust that is being swept out by the gas. Studying ram pressure stripping helps astronomers better understand the mechanisms that drive the evolution of galaxies, and how the rate of star formation is suppressed in very dense regions of the Universe like clusters.

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Ram pressure stripping is thought to have profound effects on the evolution of galaxies. As galaxies fall toward the center of a cluster, more and more of their gas is stripped out, including the cool, denser gas that is the source of continued star formation. Spiral galaxies that have fallen at least to the core of both the Virgo and Coma clusters have had their gas (neutral hydrogen) depleted in this way and simulations suggest that this process can happen relatively quickly, with 100% depletion occurring in 100 million years to a more gradual few billion years.

Recent radio observation of carbon monoxide (CO) emission from three galaxies (NGC 4330, NGC 4402, and NGC 4522) in the Virgo cluster point to the molecular gas not being stripped but instead being compressed by the ram pressure. Increased Hα emission, a sign of star formation, corresponds to the compressed CO region, suggesting that star formation may be accelerated, at least temporarily, while ram pressure stripping of neutral hydrogen is ongoing.

NGC 4402 is moving away from Earth at around 232 kilometers per second. It is falling into the Virgo galaxy cluster. Images show evidence that the material it once contained to enable it to form stars has been stripped away in a process known as "ram-pressure stripping". This is due to NGC 4402's cooler gasses being struck by hot x-ray gasses coming from the middle of the Virgo galaxy cluster as it moves toward it.>>

Click to play embedded YouTube video.

<<Messier 86 (also known as M86 or NGC 4406) is an elliptical or lenticular galaxy in the constellation Virgo. It was discovered by Charles Messier in 1781. M86 lies in the heart of the Virgo Cluster of galaxies and forms a most conspicuous group with another large galaxy known as Messier 84. It displays the highest blue shift of all Messier objects, as it is, net of its other vectors of travel, approaching the Milky Way at 244 km/s. This is due to both galaxies falling roughly towards the center of the Virgo cluster from opposing ends.

Messier 86 is linked by several filaments of ionized gas to the severely disrupted spiral galaxy NGC 4438, indicating that M86 may have stripped some gas and interstellar dust from the spiral. It is also suffering ram-pressure stripping as it moves at high speed through Virgo's intracluster medium, losing its interstellar medium and leaving behind a very long trail of X ray-emitting hot gas that has been detected with the help of the Chandra space telescope.>>

[VictorBorun]

I wonder why a galaxy in a cluster should experience a ram pressure if that galaxy is native and has formed within the dark and normal matter currents and clumps.
If the cluster's halo is rotating than all the galaxies shoud be rotating with it, would not they?
If the clusters's halo collapse somewhat (mostly the normal matter fraction) into a cluster-size disk, it could take galaxies along, could not it?