Galaxy Evolution I: Galaxy spectra, and how we know what we know part I

To understand how we've built up our picture of galaxy evolution, let's have a look at what galaxy spectra look like.

The coninuum part of a galaxy's spectrum is the sum of the billions of stars' black body spectra:

The 400nm (4000 Angstom) break pointed out by the arrow is caused by (i) the absorption of small wavelength (high energy) photons by metals (non-H, He) in the atmospheres of stars and a lack of hot, blue stars. From what we know, the 400nm break should be weak in spiral galaxies (lots of luminous, hot blue stars) and strong in elliptical galaxies (very few blue stars). But we're getting ahead of ourselves...

Lets's look at some actual spectra of galaxies:

From top left to bottom right, here are the optical images of these galaxies:

NGC4889:

NGC 2775

 

 NGC 6181

NGC 4449

The first two are elliptical galaxies, the third a spiral, and the fourth an irregular galaxy. Notice that the elliptical spectra have notcieable absorption lines and the spiral and irregular emission lines. Absorption lines indicate metals in stellar atmospheres or cold gas in the interstellar medium. Emission lines indicate hot gas, likely heated by hot, blue stars. We can conclude that ellipticals have very few hot, blue stars, leaving only an older population of stars. Spirals and irregulars, on the other hand, appear to have many young, blue stars.

Stay tuned...

Collisionless Nature of Dark Matter

As I was researching articles to review to post to the blog, I came across a title that caught my curiosity. "Quantifying The Collisionless Nature Of Dark Matter And Galaxies In A 1689";(Doron Lemze, Yoel Rephaeli, Rennan Barkana, Tom Broadhurst, Rick Wagner & Mike L. Norman). I found this while searching Worldwidescience.org on topics of surface profile of galaxies and densities.

The word "collisionless" caught my attention because when we think of collisions we think of two or more masses making contact and either combining into one mass or bouncing off each other into different directions from their original trajectory. So I quickly assumed that collisionless refers to the lack of contact between the masses indicated as dark matter.

The authors state that despite a recent collision of two massive clusters, the spatial distribution of dark matter and galaxies are quite similar. A conical shaped shock front is visible indicating two clusters have passed through each other with an obvious collisionally merged gas distribution, but the galaxies and the lensing mass are largely intact, implying straightforwardly that dark matter and galaxies are collisionless (Markevitch et al. 2002; clowe et al. 204; Bradac de al, 2006). It implied that once hydrostatic equilibrium is reached, dark matter and galaxies should have the same mean specific kinectic energy. The total specific kinectic energy of dark matter is proportional to that of the gas which was evident through X-ray and optical observations of groups and clusters using mean emission-weighted gas temperature scales.

The paper was very technical in nature and hard to follow as to their methodologies (probably due to my limited understanding of the processes used). It goes in detail in the use of calculated velocities of dark matter and galaxies as well as galaxies surface number densities and projected velocities dispersions.The paper is a continuing comprehensive study of the dynamical properties of dark matter, galaxies and the hydrodynamics in cluster A 1689. It assumes that if dark matter and galaxies are fully collisionless, they should have the same average specific kinectic energy.

The one thing that the paper does not do is exam the electromagnetic aspect of a "collisionless" particle. that is attraction versus repulsion forces. In my simple mind, I would expect the simple nature of chemistry and physics to come into play. If the gas particles are ionic in nature, would these tend to be more positive thus repulsive as they approach each other therefore, never actually colliding? I guess that is why I am taking this course turning my simple understanding into more highly complex confusion.

Modified Newtonian Dynamics as an Alternative to Dark Matter

MODIFIED NEWTONIAN DYNAMICS AS AN
ALTERNATIVE TO DARK MATTER
Robert H. Sanders1 & Stacy S. McGaugh2

1Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
2Department of Astronomy, University of Maryland, College Park, MD, USA
KEYWORDS: dark matter, galaxy dynamics, gravitational theory, cosmology


MODIFIED NEWTONIAN DYNAMICS AS AN
ALTERNATIVE TO DARK MATTER
Robert H. Sanders1 & Stacy S. McGaugh2
1Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
2Department of Astronomy, University of Maryland, College Park, MD, USA

KEYWORDS: dark matter, galaxy dynamics, gravitational theory, cosmology

ABSTRACT:
Modified Newtonian dynamics (MOND) is an empirically motivated modification of Newtonian gravity or inertia suggested by Milgrom as an alternative to cosmic dark matter. The basic idea is that at accelerations below ao _ 10−8 cm/s2 _ cHo/6 the effective gravitational attraction approaches pgnao where gn is the usual Newtonian acceleration. This simple algorithm yields flat rotation curves for spiral galaxies and amass-rotation velocity relation of the form M / V 4 that forms the basis for the observed luminosity-rotation velocity relation– the Tully-Fisher law. We review the phenomenological success of MOND on scales ranging from dwarf spheroidal galaxies to superclusters, and demonstrate that the evidence for dark matter can be
equally well interpreted as evidence for MOND. We discuss the possible physical basis for an acceleration based modification of Newtonian dynamics as well as the extension of MOND to cosmology and structure
formation.

These two authors discuss and compare math solutions and their accuracy using observed data in our known universe. The incomplete MOND theory cannot be extended to problems of cosmology and structure formation. However by making reasonable assumptions one may speculate on the form of a MOND cosmology.

The authors show equations relating the force of gravity to the force of charge particles as:

F = (GM/ r^2 ) x f (r/ro)

Scientist Milgrom (1983a) realized that this length in kpc law was incompatible with observed TF law, L is proportional to v^4. Moreover any modification of a length scale would imply that larger galaxies should exhibit a larger discrepancy. (Sanders 1986) This is contrary to observed data.

A most interesting question for me concerning gravity and dark matter was investigated directly by “dark halos” with an acceleration scale can the phenomenology of MOND be reproduced by dark halos– specifically the kind of dark halos that emerge from cosmological N-body simulations with an initial fluctuation spectrum given by cold dark matter (CDM)? This is an important question because the phenomenological success of MOND may be telling us something about the universal distribution of dark matter rather than anything about the law of gravity or inertia at low accelerations.
This question was first considered by Begeman et al. (1991) who attempted to devise disk-halo coupling rules that could yield a one-parameter fit to rotation curves (M/L of the visible disk) similar to modified Newtonian dynamics (MOND). Without any physical justification the core radius and asymptotic circular velocity of an isothermal halo were adjusted to the scale length and maximum rotation velocity of the disk to yield a characteristic acceleration. With such coupling rules, the fits to galaxy rotation curves were of lower quality than the MOND fits (particularly for the dwarf systems), and there were numerous ambiguities (e.g., in gas dominated galaxies, what is the proper disk length scale?). Similar ad hoc coupling rules between visible and dark components have also been considered by Giraud (2000).

While this disagrement between scientist has been intense for over thirty years this is a short period for science history. The disagreement leads scientist to follow their own intuitive judgment about how to apply science money and methods. I do not know of any other group better qualified to make such decisions. ( Like maybe politicians, actors, or news anchors??)

Advanced Newtonian Gravity

The equations for advanced Newtonian gravity are discussed in the article by A Worsley entitled, “An advanced dynamic adaptation of Newtonian equations of gravity. The adaptation of Newtonian dynamics.” (Physics Essays 21, 3 (2008) pg. 222-227)

Worsley starts the article by briefly discussing the history of general relativity (GTR) and it's successes and failures, namely it's tendency to breakdown mathematically when dealing with black holes. In addition, Worsely points out, the equations become increasingly complex: a definite downside.

What is interesting is the approach that Worsley uses to arrive at his advanced dynamic adaptation of Newtonian gravity. He postulates that by reversing Einstein's process of arriving at GTR, he can reformulate the equations for gravity into a force. This method provides many advantages listed by the author. First, the method is much easier to use with far less complicated computations, it obviates the infinitely dense black hole singularity predicted by full GTR, and restores the principle of simultaneity. Worsley even goes as far as to say that the new equations of adapted Newtonian gravity can be used to shed light on the “dark matter” of the universe.

Basically, Worsley takes the curvature of space-time equations for GTR and modifies them mathematically by defining the equations in terms of geodesics. He then translates these new curvature equations back into the equations for an advanced Newtonian force of gravity. Although the actual derivations of Worsley's equations are not included in the paper, his resulting equations are.

To prove that the new equations are valid, the paper works several examples... namely the general relativistic estimate for the perihelion advance of Mercury. The results show that the advanced Newtonian equations can give results that agree almost perfectly with GTR and observed data. The author then uses his equations in higher mass density objects (like binary pulsars & black holes) with great success; the equations do not collapse.

In full GTR equations, the Schwartzchild radius seem to be the limit of the calculations... but by using the advanced Newtonian force of gravity equations, Worsley is able to estimate the forces exerted by the Schwarzschild radius and inside the black hole. Additionally, the Schwarzschild radius can now be used to define the radius for the escape velocity of light. This is important, because modern physics tells us that the speed of space-time is allowed to exceed the speed of light! (I didn't know this.. prior to the paper!)

In the end, Worsley states that advanced Newtonian gravity achieves a far greater accuracy than Newton's laws of gravity because of the use of the higher order terms. Since the equations prove valid (that is, they agree with calculations from GTR on low and medium density bodies), the author claims that they must also be valid for higher mass density objects like black holes... and by using these calculations for the force of gravity, even at the Schwarzschild radius, we can make calculations of the missing dark matter of the universe.

Lack of WIMPS in Italy

The XENON project in Italy has been looking for WIMPS for over a year now. The data for the first six months was publicly released and it detected zero Weakly Interacting Massive Particles. These particles are believed to be "dark matter" and the lack of evidence shows that there are a great number of holes in the CDM hypothesis. The inability to detect this matter is an epistemological problem for the CDM theory. The article is at http://physicsworld.com/cws/article/news/45697

Modified Newtonian Dynamics

The review article Modified Newtonian Dynamics: A Falsification of Cold Dark Matter was written by R.H. Sanders of the Kapteyn Astronomical Institute at the University of Gronigen. This article was obtained by the EBSCO Academic Search complete and was originally published in Advances in Astronomy Volume 2009. I have the original article and would be happy to email it to anyone interested in it.

This review article is written by someone who is clearly a proponent of MOND (Modified Newtonian Dynamics) instead of CDM (Cold Dark Matter). The author begins the article by laying out the problem with galactic rotation curve and the problem with Keplerian motion accounting for observed evidence. The author mentions the lack of evidence for dark matter, excluding the DAMA project which he labels as controversial, in the standard model. The author then mentions the origin of MOND as a result of rotation curves being asymptotically flat and the Tully-Fisher law of mass and luminosity relationship.

The author states that changing the the gravitational acceleration in "low gravity" areas can be modeled to fit the observational data. The gravitational acceleration is modified the term Mu(g/a0) where a0 is a new parameter and Mu is an assymptotic function which has yet to be finalized but there are three main candidates (1/x^2, 1/x, e^-x). In areas of low acceleration this allows the velocity relationship to be:

v^4 = GMa0; a0 is currently valued at 1.2 X 10^-10 m/s^2

The asymptotic nature of Mu allows the modification of Newtonian gravity to approach zero the stronger the gravitational field and therefore match the observational evidence for the inner part of galactic rotation curves. The MOND theory even accounts for pressure supported systems showing that they are finite with density falling rapidly as a function of 1/r^4

The impressive part about the MOND theory is its ability to predict the galactic rotation curve based solely on observable evidence in the near infrared region of the spectrum. This is not just the general averaged rotational velocity but very specific values based on the clumping of observable matter. The MOND theory even accounts for variation in the M/L relationship of the T-F relation based on the color of galaxies.

The most important and the most emphasized point in the paper is that every variation of surface brightness of the galaxy has a corresponding variation in the MOND curve and the observed rotational curve.

The author then lays out the current problems associated with the MOND theory. Clusters of galaxies do not match the observed rotational values with the MOND curves with MOND curves predicting 2-3 times the mass that is observed in clusters. It is suggested that we may be unable to view all of the matter and we recently discovered hot gas emitting xrays that were previously neglected. The missing mass is also theorized to be "soft bosons", unaccounted for neutrinos, and even "dark matter."

MOND began as a simple theory that has progressed over the years. The initial relationship was shown to violate conservation of angular momentum and conservation linear momentum and was modified accordingly with no loss in fitting the model. The MOND theory has also been shown to work with general relativity and is free of anomalies, the first attempt had wave propagation faster than light. MOND says there can be "dark matter" but nowhere near the amounts that are used in the current model of 20% observable matter and 80% dark matter.

After reading this paper I find it difficult to argue with the author. The MOND theory has less unknown variables, is easily modifiable to match observations, the flaws with observation can be explained, uses only one new variable (as opposed to a min of 3 in CDM) and it does not require the creation of a new form of matter. The MOND theory is also similar in structure to Maxwell's equations demonstrating that it can match existing laws of the universe. Above all the MOND theory is more scientific at this time. It is clearly falsifiable, if the mass discrepancy of the galactic clusters is unaccounted for then the theory must be modified or thrown out. CDM is impossible to test because you can not prove that something does NOT exist. From my perspective the CDM proponents seem similar to the ether proponents of the late nineteenth century (although my knowledge is limited). In the absence of other evidence I believe that the simplest explanation is the best and in my opinion that is the MOND theory.

Relativity of Wrong

Here is a link to the portion I referenced in class of the essay by Isaac Asimov. This is in response to the amount of discrepancy between observation and theory as it relates to our suppositions and the extrapolation of errors.


http://chem.tufts.edu/AnswersInScience/RelativityofWrong.htm

-David Downing

A couple of explanatory resources: Pulsars, and Relativity

Here are a couple of good educational resources that have come up in the last couple of classes. First, here's a link to the Jodrell Bank site where pulsar signals are translated into sound:

http://www.jb.man.ac.uk/~pulsar/Education/Sounds/sounds.html

And secondly, some great presentations of Einstein's arguments for Relativity, together with the counter-arguments of the time from proponents of the ether, courtesy of philosopher of science John Norton:

http://www.pitt.edu/~jdnorton/Goodies/index.html