New Milky Way Observations, Impact on Old and New Theories

[William]

New Milky Way Observations, Impact on Old and New Theories

As a result of recent multi-wave length surveys with large star counts, there is now comprehensive data for the Milky Way. I thought the new papers that discuss this data would be interesting to discuss, as any theory must explain the observations. The observations show clear evidence of structures that indicates there are fundamental mechanisms involved, to form the structures, rather than random mergers.

This thread starts with a brief outline of the formation of the galaxy using the old abrupt cloud collapse theory (see below for details) that has been discussed before in the forum along with some known observation vs theory problem for the abrupt cloud collapse model.

The galactic formation model that has been mentioned a few times in the forum is the “Collapse Model”, which is also called the ELS model, named after the model founders Eggen, Lynden-Bell, and Sandage. The ELS model was developed when there was limited data concerning the Milky Way, as noted below and subsequent papers, new data shows the ELS model, is fundamental flawed. As the ELS model is a dark matter based model the data in question also fundamentally challenges the existence of dark matter. The dark matter theory has bounds, as to how the hypothesized dark matter can affect real matter. (See Dark Matter Puzzle thread in Q&A for additional details.)

Excerpt from “An Introduction to Astrophysics” 2nd Editon, published 2007 by Bradley Carrol & Dale Ostlie, Page 1016, Ch #26 Galactic Evolution

There work (my comment the work of Eggen, Lynden-Bell, and Sandage) was based on observed correlations between the metallicity of stars in the solar neighbourhood, and their (my comment: the stars in question) orbital eccentricity and orbit angular momentum. Eggen, Lynden-Bell, and Sandage noted that most metal-poor stars tend to have the highest eccentricities, the largest w components of their peculiar moments, and the lowest angular momenta about the rotational axis of the Galaxy. On the other hand, metal-rich stars tend to exist in nearly circular orbits and are confined to regions near the plane of the Galaxy...

...To explain the kinematic and chemical properties of the stars in the solar neighbourhood ELS proposed a model where the Milky Way formed from the sudden collapse (roughly 200 million years) of a large prototype galaxy.

The oldest halo stars formed early in the collapse process while still on nearly radial trajectories, resulting in their highly elliptical orbits above and below the Galactic plane. As a further consequence of their rapid formation, the model predicts that the halo stars are naturally very metal-poor (Population II) since the interstellar medium had not yet had time to become enriched by the by-products of stellar nucleoysnthesis. ...

Problems with ELS Model (Same source but paraphrased for length.)
1. Roughly half of the stars in the Milky Way’s halo have retrograde orbits, so the net angular momentum of the halo is 0 km/m^2. On the other hand stars in the inner halo appear to have a small net rotation velocity.

2. A second problem with the ELS model is an age variance between the global clusters and halos stars of approximately 2 billion years which would indicate the collapse took an order of magnitude longer the 200 million years predicted. The ELS model also does not explain the difference in age of the narrow region of the Milky Way’s galactic disk which is roughly 8 billion years old and the thick portion of the disk which is 10 billion years old.

3. A third age difficulty is that clusters near the Galactic core are generally the most metal rich and the oldest, while clusters in the outer halo exhibit a wider range in metallicity and tend to be younger.

[William]

The paper is interesting concerning the finding of ultra dense quiescent galaxies at z~2.3, in the early universe. The massive early galaxies appear to no longer be forming new stars. As noted by the authors a mechanism is required to expand these massive compact galaxies, which are roughly 40 times smaller than a similar mass nearby galaxy. As a comparison the mass of the Milky Way is 5.8 x 10^11 solar masses and the Milky Way disk has a diameter of around 45 kpc.

http://arxiv.org/abs/0802.4094v1

“Confirmation of the remarkable compactness of massive quiescent galaxies at z~2.3: early-type galaxies did not form in a simple monolithic collapse” by P. G. Van Dokkum , M. Franx, M. Kriek, & et al.

Using deep near-infrared spectroscopy Kriek et al. (2006) found that ~45% of massive galaxies at z~2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star-assisted Keck/AO. Considering that their median stellar mass is 1.7x10^11 Solar masses the galaxies are remarkably small, with a median effective radius of 0.9 kpc. Galaxies of similar mass in the nearby Universe have sizes of ~5 kpc and average stellar densities which are two orders of magnitude lower than the z~2.3 galaxies. These results extend earlier work at z~1.5 and confirm previous studies at z>2 which lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most ~10% of the population of K-selected quiescent galaxies at z~2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z~2.3, through dry mergers or other processes, consistent with predictions from hierarchical models.

Also, the smallest galaxies are likely to merge with larger galaxies, and even a merger between two small galaxies will (obviously) reduce their number. Each of these mechanisms could plausibly alter the size – mass relation by a factor of 1.5–2, but not a factor of ∼ 6. This means that some combination of effects is required to bring the compact z ∼ 2.3 galaxies to the local relations— or that we have not yet identified the main mechanism.

http://www.sciencedaily.com/releases...0429095054.htm

[William]

As noted in the first comment, the Milky Way’s halo is comprised of two components, an inner halo that rotates in the same direction as the Milky Way’s disk and an outer halo that rotates in the opposite direction of the Milky Way disk. The inner halo has a metallicity that is roughly 1/3 greater than the outer halo. This is the paper that provides the data to support that statement.

http://arxiv.org/abs/0706.3005

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, which can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for a dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components -- an inner and an outer halo – that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium).

We find that the inner-halo component of the Milky Way dominates the population of halo stars found at distances up to 10-15 kpc from the Galactic Centre (including the Solar neighbourhood). An outer-halo component dominates in the regions beyond 15-20 kpc. We show the inner halo to be a population of stars that are non-spherically distributed about the centre of the Galaxy, with an inferred axial ratio on the order of ~ 0.6. Inner-halo stars possess generally high orbital eccentricities, and exhibit a modest prograde rotation (my comment rotates in the same direction of as the Milky Way Galaxy) (between 0 and 50 km s-1) around the centre of the Galaxy (see Supplementary Table 1). The distribution of metallicities for stars in the inner halo peaks at [Fe/H] = −1.6, with tails extending to higher and lower metallicities.

Outer-halo stars cover a wide range of orbital eccentricities, including many with lower eccentricity orbits than found for most stars associated with the inner halo, and exhibit a clear retrograde net rotation (between –40 and –70 km s-1) about the centre of the Galaxy. The metallicity distribution function (MDF) of the outer halo peaks at lower metallicity than that of the inner halo, around [Fe/H] = −2.2, and includes a larger fraction of low-metallicity stars than does the MDF of the inner-halo population.

The following is a quote from the above paper which states the current standard hypothesis concerning the mechanism to explain the evolution of elements in the intergalactic/extragalactic gas clouds and in the stars and quasars that are formed from the gas clouds.

Metallicity is taken by astronomers to represent the abundances of the elements heavier than helium, which are only created by nucleosynthesis in stars – either internally via nuclear burning in their cores or externally during explosive nucleosynthesis at the end of their lives. The earliest generations of stars have the lowest metallicities, because the gas from which they formed had not been enriched in heavy elements created by previous stars and distributed throughout the primordial interstellar medium by stellar winds and supernovae.

Comment:
It is suggested, to explain metallicity anomalies such as super solar metallicity in high redshift quasars, the Milky Way regional variances in metallicity and in rotation, a second non-nucleosynthesis mechanism is required to explain metallicity variance. Supporting the need for a non-nucleosynthesis explanation for metallicity variance, is the need to explain the mechanism for star burst galaxies and the paradox of youth stars in the galactic core.

I am looking at the data and analysis concerning metallicity and stellar formation. I will start a separate thread later.

[GOURDHEAD]

Roughly half of the stars in the Milky Way’s halo have retrograde orbits, so the net angular momentum of the halo is 0 km/m^2. On the other hand stars in the inner halo appear to have a small net rotation velocity.

For us slow learners does this mean that the galactic orbital velocities of approximately half the stars are opposite in direction to the other half, but within each half the range of velocities subtends a wide spectrum as do the orbital ellipticities? For this to be credible data (analytical conclusion), what fraction of stars' velocities have been measured?

[trinitree88]

William. I think a simpler explanation for the differences in the metallicities of the two regions is that they formed in different galaxies. A merger of a small halo galaxy with the cannabilistic tendencies of the giant Milky Way can lead to an outer halo of stars with a distinct difference in metallicities from the inner Milky Way, and a velocity distribution that is counter to the inner Milky Way too.
Postulating a completely different mechanism in the two spatially separated regions is phenomenologically unnecessary as galactic mergers are clearly transpiring...(Arp's Catalogue of Peculiar Galaxies). pete


see;http://members.aol.com/arpgalaxy/

[William]

In reply to GOURDHEAD's

.. does this mean that the galactic orbital velocities of approximately half the stars are opposite in direction to the other half, but within each half the range of velocities subtends a wide spectrum as do the orbital ellipticities? For this to be credible data (analytical conclusion), what fraction of stars' velocities have been measured?

Yes the angular momentum of the halo is zero but it consists of two separate components with the inner halo rotating with the galaxy and the outer component of the halo rotating opposite to the galaxy's rotation.

The velocities and metallicity of 20,000 stars were measured. Which the authors note is sufficient to confirm at high degree of certainty, their conclusions.

[RussT]

In reply to GOURDHEAD's


Yes the angular momentum of the halo is zero but it consists of two separate components with the inner halo rotating with the galaxy and the outer component of the halo rotating opposite to the galaxy's rotation.

The velocities and metallicity of 20,000 stars were measured. Which the authors note is sufficient to confirm at high degree of certainty, their conclusions.

It appears that you should have considered trinitree88's answer more carefully...

AND, "you" have to be very careful with your usage of 'Retrograde' orbits!

http://en.wikipedia.org/wiki/Retrogr..._direct_motion

[William]

In reply to trinitree88's comment:

I think a simpler explanation for the differences in the metallicities of the two regions is that they formed in different galaxies. A merger of a small halo galaxy with the cannibalistic tendencies of the giant Milky Way can lead to an outer halo of stars with a distinct difference in metallicities from the inner Milky Way, and a velocity distribution that is counter to the inner Milky Way too.

The hypothesis you provide seems reasonable in terms of what happens when a small galaxy mergers with a larger galaxy.

Galaxy Morphology and Evolution
The halo is what would be expected from a dry merger. From a morphological standpoint, would not the merger of two large spiral galaxies produce a elliptical (spheroid). Would a dry major merger produce an elliptical galaxy that would as the halo does show some evidence of the merger history, but have roughly 0 angular momentum. The paper I quoted in the Why do spiral galaxies have angular momentum and elliptical galaxies have zero net angular momentum, noted that theoretically, early formed galaxies have had 4 or 5 major mergers.

The questions are 1) why do elliptical galaxies form in some cases and spiral in others? 2) Why does the percentage of elliptical galaxy (18%) to spiral (75%, there are irregular galaxies also) remain constant, rather than gradually becoming higher in favour of elliptical.

Metallicity
I agree an explanation for metallicity variance in the halo need not be more than the hypothesis that by chance one merger type had relatively higher metallicity stars (inner halo) and the outer halo by chance had relatively lower metallicity stars. That explanation is reasonable except if all spiral galaxies had the same structure, then a different mechanism would be required. For the spiral disk, the metallicity and velocity differences probably require a different mechanism.

There are other specific metallicity problems such as the G dwarf problem which it appears is a problem for all galaxies and all galaxy components or the quasar lack of metallicity evolution with redshift except at high redshift at which point some quasar's show evidence of super solar metallicity, that require an explanation.

I am looking for a possible answer to metallicity evolution and lack of evolution by looking at observations and data concerning Wolf-Rayet galaxies and star burst galaxies. The observation appears to be similar to the "Paradox of Youth" stars (a very large number of stars in a small region of space near the galaxy's in question core) found near the Milky Way's core, but on a larger scale. The Wolf-Rayet stars are outgassing, but I suspect the out gassed matter has not be created from nucleosynthesis.