High Redshift Galaxy Rotation Curves

Spiral galaxies rotate, and the velocity of rotation versus distance from the center is known as the rotation curve. Our own Sun takes about 240 million years to rotate about the galaxy’s center.

M64 M64, image credit: NASA, ESA, and the Hubble Heritage Team (AURA/STScI)

Almost always the rotation curves flatten out far from the center. This was not expected. If the mass is concentrated as the light in a galaxy, the velocity should continue to decline as one goes further out, according to Newtonian dynamics.

The flattening can be interpreted as due to the presence of dark matter, or alternatively, due to deviation from Newtonian dynamics and general relativity when accelerations are very low.

Now researchers have looked at a sample of over 100 distant, high-redshift galaxies and they find the rotation curves look much more like one would expect with only Newtonian gravity. So is dark matter not important at higher redshifts? Read more at:

Distant Galaxy Rotation Curves Appear Newtonian

 

The Curiously Tangential Dwarf Galaxies

(Originally posted at darkmatterdarkenergy.com)

There are some 50 or so satellite galaxies around the Milky Way, the most famous of which are the Magellanic Clouds. Somewhat incredibly, half of these have been discovered within the last 2 years, since they are small, faint, and have low surface brightness. The image below shows only the well known ‘classical’ satellites. The satellites are categorized primarily as dwarf spheroidals, and most are low in gas content.

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Image credit: Wikipedia, Richard Powell, Creative Commons Attribution-Share Alike 2.5 Generic

“Satellite galaxies that orbit from 1,000 ly (310 pc) of the edge of the disc of the Milky Way Galaxy to the edge of the dark matter halo of the Milky Way at 980×103 ly (300 kpc) from the center of the galaxy, are generally depleted in hydrogen gas compared to those that orbit more distantly. The reason is the dense hot gas halo of the Milky Way, which strips cold gas from the satellites. Satellites beyond that region still retain copious quantities of gas.” – Wikipedia article

In a recent paper “The tangential velocity excess of the Milky Way satellites“, Marius Cautun and Carlos Frenk find that a sample of satellites (drawn from those known for more than a few years) deviates from the predictions of the canonical Λ – Cold Dark Matter (ΛCDMcosmology. (Λ refers to the cosmological constant, or dark energy).

“We estimate the systemic orbital kinematics of the Milky Way classical satellites and compare them with predictions from the Λ cold dark matter (ΛCDM) model derived from a semi-analytical galaxy formation model applied to high resolution cosmological N-body simulations. We find that the Galactic satellite system is atypical of ΛCDM systems. The subset of 10 Galactic satellites with proper motion measurements has a velocity anisotropy, β = −2.2 ± 0.4, that lies in the 2.9% tail of the ΛCDM distribution. Individually, the Milky Way satellites have radial velocities that are lower than expected for their proper motions, with 9 out of the 10 having at most 20% of their orbital kinetic energy invested in radial motion. Such extreme values are expected in only 1.5% of ΛCDM satellites systems. This tangential motion excess is unrelated to the existence of a Galactic ‘disc of satellites’. We present theoretical predictions for larger satellite samples that may become available as more proper motion measurements are obtained.”

Radial velocities are easy, we get those from redshifts. Tangential velocities are much tougher, but can be obtained from relatively nearby objects by measuring their proper motions. That is, how much do their apparent positions change on the sky after many years have passed. It’s all the more tough when your object is not a point object, but a fuzzy galaxy!

For a ‘random’ distribution of velocities in accordance with ΛCDM cosmology, one would expect the two components of tangential velocity to be each roughly equal on average to the radial component, and thus 2/3 of the kinetic energy would be tangential and 1/3 would be radial. But rather than 33% of the kinetic energy being in radial motion, they find that the Galactic satellites have only about 1/2 that amount in radial, and over 80% of their kinetic energy in tangential motion.

Formally, they find a negative velocity anistropy, β, which as it is defined in practice, should be around zero for a ΛCDM distribution. They find that β differs from zero by 5 standard deviations.

One possible explanation is that the dwarf galaxies are mainly at their perigee or apogee points of their orbits. But why should this be the case? Another explanation: “alternatively indicate that the Galactic satellites have orbits that are, on average, closer to circular than is typical in ΛCDM. This would mean that MW halo mass estimates based on satellite orbits (e.g. Barber et al. 2014) are biased low.” Perhaps the Milky Way halo mass estimate is too low. Or, they also posit, without elaborating, do the excess tangential motions “indicate new physics in the dark sector”?

So one speculation is that the tangential motions are reflective of emergent gravity class of theories, for which dark matter is not required, but for which the gravitational force changes (strengthens) at low accelerations, of order c \cdot H, where H is the Hubble parameter, and the value works out to be around 2 centimeters per second per year. And it does this in a way that ‘spoofs’ the existence and gravitational affect of dark matter. This is also what is argued for in Modified Newtonian Dynamics, which is an empirical observation about galaxy light curves.

In the next article of this series we will look at Erik Verlinde’s emergent gravity proposal, which he has just enhanced, and will attempt to explain it as best we can. If you want to prepare yourself for this challenging adventure, first read his 2011 paper, “On the Origin of Gravity and the Laws of Newton”.

Two Trillion Galaxies

Two trillion! That’s 2,000,000,000,000 galaxies. A trillion is a million times a million.

For a long time astronomers have said that our observable universe has about 100 billion galaxies. Two trillion is 2,000 billion, or 20 times larger.

It’s still smaller than the U.S. national debt at about $19 trillion, but those are just dollars, not galaxies. There are only 7 billion people on Earth, this number is 300 times larger.

Why so many?

Galaxies are not static. They evolve.

So what’s the twist, why were astronomers so wrong for so long? Well, it’s because now we can see galaxies much further away than before, thanks to the Hubble Space Telescope and other advanced telescopes. In fact we are now detecting many galaxies that were formed in the first billion years of the universe’s history.

And the universe is about 13.8 billion years old.

A team of astronomers from the University of Nottingham, the Leiden Observatory, and the University of Edinburgh, have built extremely detailed 3D maps of distant galaxies in order to estimate the density of these. They have used data from the Hubble Space Telescope and various ground-based telescopes.

When we look at distant galaxies we are also looking back into the past, since light travels at a finite speed. The researchers, lead by Prof. Christopher Consilice at the University of Nottingham, found the density of galaxies when the universe was a few billion years old to be about 10 times higher than at present (after correcting for the expansion of the universe). He noted that “we are missing the vast majority of galaxies because they are very faint and far away”.

These earlier galaxies were smaller, less massive, much less so. Large galaxies today like the Milky Way and the Andromeda galaxy have masses of around a trillion times the Sun. These galaxies were much more like the two dozen satellite galaxies found around the Milky Way, such as the Magellanic Clouds.

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The Large Magellanic Cloud has a mass of about 1% of our Milky Way Galaxy. Image credit: NASA (C141 flight)

The main conclusion of the study? We know that galaxies undergo mergers. Apparently there have been many more mergers than previously assumed. Large galaxies such as our Milky Way have been formed by multiple successive mergers.

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NGC 3921 is actually two galaxies in the process of merging. Note the strange and twisted orientation of the spiral arms, and the appearance of two disk like structures.Image credit: ESA/Hubble & NASA

So while there were originally trillions of galaxies in the early universe, in today’s universe the number has been reduced by mergers to order hundreds of billions.

Mergers will continue, in fact the Andromeda Galaxy is headed our way, and it’s twice as big as we are!

Reference

https://www.ras.org.uk/news-and-press/2910-a-universe-of-two-trillion-galaxies – Royal Astronomical Society press release

 

Pisces A and B: Dwarf Galaxies with Young Stars

Note the blue colors of Pisces A and B
Pisces A and Pisces B as seen by Hubble Telescope; Image credit: NASA / ESA / E. Tollerud (STScI)

The galaxies Pisces A and B are dwarf (very small) galaxies outside of our Local Group, but relatively nearby. Note their blue colors, indicating recent young star formation. The Local Group includes Andromeda, the Milky Way and another 50 or so smaller galaxies.

These two dwarfs are in a region of low density at the edge of the Local Void, an area with few galaxies. It appears they have recently moved out of the Local Void into a higher-density region. Their evolution and star formation may have sped up as this occurred.

Pisces A is nearer, around 18 million light-years away, and B is at 29 million light-years distance. Their masses are approximately 10 million and 30 million solar masses, respectively. By comparison, our Milky Way’s mass is 30 to 100 times larger.

 

The Antenna galaxies

This is a pair of colliding galaxies, presenting an absolutely stunning image. Each of the two spiral galaxies is not so different from our Milky Way. They have spent hundreds of millions of years in a violent cosmic dance. Stars are being ripped from the respective galaxies and flung out into an arc between the two.

Gas clouds are in reddish colors, and the star-forming regions are seen in blue. The Antennae are in starburst mode, which is a phase of very rapid star formation. There are also many prominent dust lanes (dark areas).

Antennae Galaxies reloaded
Antenna galaxies

Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgement: B. Whitmore ( Space Telescope Science Institute) and James Long (ESA/Hubble)

This is a composite image based on both visible and near-infrared data from the Hubble Space Telescope.

M31 – Andromeda Galaxy

“Our beautiful neighbor, M31, or the Andromeda Galaxy, is the best known galaxy apart from our Milky Way. M31 is a spiral galaxy, similar to, but larger than, our own galaxy. It has about two or three times the number of stars as the Milky Way.

M31 is visible to the naked eye on Moonless nights. Its existence has been known for over 1,000 years, but its nature as a galaxy separate from our own was not confirmed until around 1920, with the “Great Debate” between Harlow Shapley and Heber Curtis. Curtis was an advocate of many nebulae being external galaxies in their own right, and was proven correct.

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Image credit: Andromeda Galaxy (with h-alpha)” by Adam Evans – M31, the Andromeda Galaxy (now with h-alpha), Uploaded by NotFromUtrecht. Licensed under CC BY 2.0 via Wikimedia Commons

Our galaxy and the Andromeda Galaxy are gravitationally bound together in our Local Group, along with about 50 other galaxies, which are almost all small dwarf galaxies.

The distance to the Andromeda Galaxy is around 2.5 million light-years (thus we are seeing it as it was 2.5 million years ago). The mass is estimated at over a trillion solar masses, and it has a star count of approximately a trillion stars. It is moving toward us at 300 kilometers per second (1/1000 of the speed of light). It[…]”

Excerpt From: “72 Beautiful Galaxies” iBook

M63 – Sunflower Galaxy

“M63 is an Sb spiral in the direction of Canes Venatici. It was discovered in 1779 by Pierre Mechain, a friend of Charles Messier, who composed the Messier catalog. The nickname of this galaxy, the “Sunflower” galaxy is reasonably obvious, since it has a shape and coloration similar to a sunflower and exhibits beautiful yellow colors as well as blue. The latter are regions with recent ongoing star formation. Its spiral arms were first noticed in the mid-19th century.
The Sunflower galaxy is 37 million light-years distant, and is part of a group of galaxies together with M51.”

Hubble-Views-the-Sunflower-Galaxy

Image credit: ESA/Hubble & NASA