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Category Archives: Early Universe

Distant Galaxy Rotation Curves Appear Newtonian

One of the main ways in which dark matter was postulated, primarily in the 1970s, by Vera Rubin (recently deceased) and others, was by looking at the rotation curves for spiral galaxies in their outer regions. Although that was not the first apparent dark matter discovery, which was by Fritz Zwicky from observations of galaxy motion in the Coma cluster of galaxies during the 1930s.

Most investigations of spiral galaxies and star-forming galaxies have been relatively nearby, at low redshift, because of the difficulty in measuring these accurately at high redshift. For what is now a very large sample of hundreds of nearby galaxies, there is a consistent pattern. Galaxy rotation curves flatten out.

M64

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

If there were only ordinary matter one would expect the velocities to drop off as one observes the curve far from a galaxy’s center. This is virtually never seen at low redshifts, the rotation curves consistently flatten out. There are only two possible explanations: dark matter, or modification to the law of gravity at very low accelerations (dark gravity).

Dark matter, unseen matter, would case rotational velocities to be higher than otherwise expected. Dark, or modified gravity, additional gravity beyond Newtonian (or general relativity) would do the same.

Now a team of astronomers (Genzel et al. 2017) have measured the rotation curves of six individual galaxies at moderately high redshifts ranging from about 0.9 to 2.4.

Furthermore, as presented in a companion paper, they have stacked a sample of 97 galaxies with redshifts from 0.6 to 2.6  to derive an average high-redshift rotation curve (P. Lang et al. 2017). While individually they cannot produce sufficiently high quality rotation curves, they are able to produce a mean normalized curve for the sample as a whole with sufficiently good statistics.

In both cases the results show rotation curves that fall off with increasing distance from the galaxy center, and in a manner consistent with little or no dark matter contribution (Keplerian or Newtonian style behavior).

In the paper with rotation curves of 6 galaxies they go on to explain their falling rotation curves as due to “first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius.” 

So in essence they are saying that the central regions of galaxies were relatively more dominated in the past by baryons (ordinary matter), and that since they are measuring Hydrogen alpha emission from gas clouds in this study that they must also take into account the turbulent gas cloud behavior, and this is generally seen to be larger at higher redshifts.

Stacy McGaugh, a Modified Newtonian Dynamics (MOND) proponent, criticizes their work saying that their rotation curves just don’t go far enough out from the galaxy centers to be meaningful. But his criticism of their submission of their first paper to Nature (sometimes considered ‘lightweight’ for astronomy research results) is unfounded since the second paper with the sample of 97 galaxies has been sent to the Astrophysical Journal and is highly detailed in its observational analysis.

The father of MOND, Mordehai Milgrom, takes a more pragmatic view in his commentary. Milgrom calculates that the observed accelerations at the edge of these galaxies are several times higher than the value at which rotation curves should flatten. In addition to this criticism he notes that half of the galaxies have low inclinations, which make the observations less certain, and that the velocity dispersion of gas in galaxies that provides pressure support and allows for lower rotational velocities, is difficult to correct for.

As in MOND, in Erik Verlinde’s emergent gravity there is an extra acceleration (only apparent when the ordinary Newtonian acceleration is very low) of order. This spoofs the behavior of dark matter, but there is no dark matter. The extra ‘dark gravity’ is given by:

g _D = sqrt  {(a_0 \cdot g_B / 6 )}

In this equation a0 = c*H, where H is the Hubble parameter and gB is the usual Newtonian acceleration from the ordinary matter (baryons). Fundamentally, though, Verlinde derives this as the interaction between dark energy, which is an elastic, unequilibrated medium, and baryonic matter.

One could consider that this dark gravity effect might be weaker at high redshifts. One possibility is that density of dark energy evolves with time, although at present no such evolution is observed.

Verlinde assumes a dark energy dominated de Sitter model universe for which the cosmological constant is much larger than the matter contribution and approaches unity, Λ = 1 in units of the critical density. Our universe does not yet fully meet that criteria, but has Λ about 0.68, so it is a reasonable approximation.

At redshifts around z = 1 and 2 this approximation would be much less appropriate. We do not yet have a Verlindean cosmology, so it is not clear how to compute the expected dark gravity in such a case, but it may be less than today, or greater than today. Verlinde’s extra acceleration goes as the square root of the Hubble parameter. That was greater in the past and would imply more dark gravity. But  in reality the effect is due to dark energy, so it may go with the one-fourth power  of an unvarying cosmological constant and not change with time (there is a relationship that goes as H² ∝ Λ in the de Sitter model) or change very slowly.

At very large redshifts matter would completely dominate over the dark energy and the dark gravity effect might be of no consequence, unlike today. As usual we await more observations, both at higher redshifts, and further out from the galaxy centers at moderate redshifts.

References:

R. Genzel et al. 2017, “Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago”, Nature 543, 397–401, http://www.nature.com/nature/journal/v543/n7645/full/nature21685.html

P. Lang et al. 2017, “Falling outer rotation curves of star-forming galaxies at 0.6 < z < 2.6 probed with KMOS^3D and SINS/ZC-SINF” https://arxiv.org/abs/1703.05491

Stacy McGaugh 2017, https://tritonstation.wordpress.com/2017/03/19/declining-rotation-curves-at-high-redshift/

Mordehai Milgrom 2017, “High redshift rotation curves and MOND” https://arxiv.org/abs/1703.06110v2

Erik Verlinde 2016, “Emergent Gravity and the Dark Universe” https;//arXiv.org/abs/1611.02269v1

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Primordial Black Holes as Dark Matter?

LIGO Gravitational Wave Detection Postulated to be Due to Primordial Black Holes

Dark matter remains elusive, with overwhelming evidence for its gravitational effects, but no confirmed direct detection of exotic dark matter particles.

Another possibility which is being re-examined as an explanation for dark matter is that of black holes that formed in the very early universe, which in principle could be of very small mass, or quite large mass. And they may have initially formed at smaller masses and then aggregated gravitationally to form larger black holes.

Recently gravitational waves were discovered for the first time, by both of the LIGO instruments, located in Louisiana and in Washington State. The gravitational wave signal (GW150914) indicates that the source was a pair of black holes, of about 29 and 36 solar masses respectively, spiraling together into a single black hole of about 62 solar masses. A full 3 solar masses’ worth of gravitational energy was radiated way in the merger. Breaking news: LIGO has just this month announced gravitational waves from a second black hole binary of 22 solar masses total. One solar mass of energy was radiated away in the merger.

massrangescompactobjects.jpg

Image credit: NASA/JPL, http://www.nasa.gov/jpl/nustar/pia18842

Most of the black holes that we detect (indirectly, from their accretion disks) are stellar-sized in the range of 10 to 100 solar masses and are believed to be the evolutionary endpoints of massive stars. We detect them when they are surrounded by accretion disks of hot luminous matter outside of their event horizons. The other main category of black holes exceeds a million solar masses and can even be more than a billion solar masses, and are known as supermassive black holes.

It is possible that some of the stellar-sized and even elusive intermediate black holes were formed in the Big Bang. Such black holes are referred to as primordial black holes. There are a variety of theoretical formation mechanisms, such as cosmic strings whose loops in all dimensions are contained within the event horizon radius (Schwarzschild radius). In general such primordial black holes (PBHs) would be distributed in a galaxy’s halo, would interact rarely and not have accretion disks and thus would not be detectable due to electromagnetic radiation. That is, they would behave as dark matter.

Dr. Simon Bird and coauthors have recently proposed that the gravitational wave event (GW150914) could be due to two primordial black holes encountering each other in a galactic halo, radiate enough of their kinetic energy away in gravity waves to become bound to each other and inspiral to a single black hole with a final burst of gravitational radiation. The frequency of events is estimated to be of order a few per year per cubic Gigaparsec (a Gigaparsec is 3.26 billion light years), if the dark matter abundance is dominated by PBHs.

While low-mass PBHs have been ruled out for the most part, except of a window around one one-hundred millionth of a solar mass, the authors suggest a window also remains for PBHs in the range from 20 to 100 solar masses.

Dr. A. Kashlinsky has gone further to suggest that the cosmic infrared background (CIB) of unresolved 2 to 5 micron near-infrared sources is due to PBHs. In this case the PBHs would be the dominant dark matter component in galactic halos and would mediate early star and galaxy formation. Furthermore there is an unresolved soft cosmic X-ray background which appears to be correlated with the CIB.

This would be a trifecta, with PBHs explaining much or most of the dark matter, the CIB and the soft-X-Ray CXB! But at this point it’s all rather speculative.

The LIGO instruments are now upgraded to Advanced LIGO and as more gravitational wave events are detected due to black holes, we can gain further insight into this possible explanation for dark matter, in whole or in part. Improved satellite born experiments to further resolve the CIB and CXB will also help to explore this possibility of PBHs as a major component to dark matter.

References:

S. Bird et al. arXiv:1603.00464v2 “Did LIGO detect Dark Matter”

A. Kashlinksy arXiv:1605.04023v1 “LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies”

http://www.space.com/26857-medium-size-black-hole-discovery-m82.html – “It’s Confirmed! Black Holes Do Come in Medium Sizes”

Video (artist’s representation) of inspiral and merger of binary black hole GW151226 (second gravitational wave detection): https://youtu.be/KwbXxzgAObU

NEW BOOK just released:

S. Perrenod, 2016, 72 Beautiful Galaxies (especially designed for iPad, iOS; ages 12 and up)

Andromeda_galaxy_Galex


Most Distant Galaxy Known: over 95% of the way back to the origin

Recently, a team of astronomers from the U.S., U.K. and The Netherlands have confirmed the most distant galaxy known. This galaxy had previously been estimated to have a redshift of z = 8.57, from photometric methods, that is, from the general shape of the spectrum.

EGSY8p7-a

Image: Hubble Space Telescope, NASA/STScI

More accurate redshifts are obtained by measuring particular emission or absorption lines, which have precisely known laboratory (z = 0) wavelengths.

The team measured Lyman alpha line emission, and have determined the redshift to be z = 8.68, in good agreement with the photometric redshift. The Lyman alpha line is a main transition line in neutral hydrogen that occurs at 1216 Angstroms (.1216 microns) in the rest frame. The authors observed the line in the infrared and centered at 11,776 Angstroms (1.1776 microns) on 2 separate observing nights, detecting the Lyman alpha line each night. The redshift is given by 1 + z = 11,776/1216 = 9.68, thus z for this galaxy is 8.68.

The galaxy image is thought to be somewhat magnified by intervening dark matter gravitational lensing, but less than a factor of 2, and perhaps only around 20%.

The significance here is in the detection of Lyman alpha at such a high redshift, corresponding to a time when the universe was only 600 million years old, less than 5% of its current age. Not only does this result determine the age of this earliest known galaxy, but it also provides insight into the nature of the intergalactic medium.

The cosmic microwave background radiation is the most distant source we can see. It comes from all directions, filling the universe and reflects a time when the universe was only 380,000 years old and transitioned from ionized plasma to neutral hydrogen and helium.

Later on in the universe’s evolution, as the first galaxies and stars form, hot blue stars produce ionizing ultraviolet radiation, and the neutral gas is reionized – electrons are stripped from their atoms. This process has generally thought to have completed by redshift ~ 6, at a time when the universe was around 1 billion years old.

Lyman alpha emission is not expected in a region which is still neutral, that has not yet undergone the reionization process. So the implication here is that the surrounding intergalactic medium in the neighborhood of EGSY8p7 has already been reionized at a significantly higher redshift.

The universe does not become reionized in a uniform way, rather the process would be expected to happen in “bubbles” or regions surrounding energetic galaxies with hot blue stellar populations. Eventually all the ionized regions overlap and the intergalactic medium becomes fully ionized.

This detection helps astronomers to better understand how reionization occurred.

The team’s paper is submitted to the Astrophysical Journal Letters and can be found here:

http://arxiv.org/pdf/1507.02679v2.pdf