Galaxy Star Formation max @ Scutum,Norma arms ?

[Widdekind]

In the Milky Way Galaxy, Star Formation Rates apparently peak at about 5 kpc from the Galactic Core:

Observations of supernovae remnants indicate that supernovae peak at about 60% of the Sun's distance from the galactic center

[~8.5 kpc]

, where they are about 1.6 times more frequent than at our location

*

.

And, as indicated upon National Geographic's amazing Milky Way Galaxy poster**, this coincides w/ the Scutum & Norma spiral arms. Indeed:

The region where the

Scutum-Crux

arm meets the central bulge of the galaxy is rich in star-forming regions

. In 2006 a large cluster of new stars containing 14 red supergiant stars was discovered there and named RSGC1. In 2007 a cluster of approximately 50,000 newly formed stars named RSGC2 was located only a few hundred light years from RSGC1; it is thought to be

less than 20 million years old

and contains 26 red supergiant stars, the largest grouping of such stars known

***

.

Apparently, the region within the Scutum & Norma arms is basically considered the Galactic Core:

Intriguingly, it is precisely at this point, that the density of [Thin?] Disk gas becomes exceeded, by the density of [Thin?] Disk stars#:

This strongly suggests some sort of connection, between Star Formation Rates, and the relative Disk densities of Gas vs. Stars. And, there are no Spiral Arm structures in the central region, where the Star Formation Rates begin to drop off.

*

Guillermo Gonzalez, Donald Brownlee, & Peter D. Ward.

Refuges for Life in a Hostile Universe

.

Printed in: [scientific American]

Majestic Universe

, pg. 10.

**

http://shop.nationalgeographic.com/product/307/359/173.html

***

http://en.wikipedia.org/wiki/Scutum-Crux_Arm

#

Guillermo Gonzalez,

et al, ibid

.

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From the data depicted in the attached charts (see OP above), it is clear that, at a distance of only 5 kpc from the Galactic Core:

1. Metallicity is some 60% higher
   
2. Disk Gas Density is some 60% higher
   

And, according to the quoted text from the same source:

• Supernovae Rate is some 60% higher
  

This strongly suggests, that — at least outside the Galactic Core regions — Disk Gas Density ([math]\rho_{ISM}[/math]), Supernovae Rates ([math]\Gamma_{SN}[/math]), and Metallicity (Z) are all comparatively closely correlated:

[math]\rho_{ISM} \propto \Gamma_{SN} \propto Z[/math]

 

Moreover, the Supernovae Rate roughly tracks the Star Formation Rate ([math]\Gamma_{SF}[/math]) (especially so for Type II Supernovae):

Type II Supernovae

involve short-lived, massive stars, so their rate closely tracks the

Star-Formation Rate.

The rate of

Type I Supernovae,

on the other hand, depends upon the production of longer-lived, intermediate mass stars, so it responds more slowly to changes in the

Star Formation Rate.

(ibid.,

pg. 8

)

Thus, as a rough rule:

[math]\rho_{ISM} \propto \Gamma_{SF} \propto \Gamma_{SN} \propto Z[/math]

This chain of cause & effect explains the observations, that denser Disk regions are also much more "Metallicitous".

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Cosmologists essentially say that the densest regions of the early Cosmos collapsed to create the biggest Galaxies. And, those biggest Galaxy-regions will tend to have had the most Star Formation, and, hence, have made the most Metals. This corroborates the above conclusion, on the "grand scale".

 

Indeed,

 

The broader Universe looks even less inviting than our Galaxy. About 80% of stars in the local Universe reside in Galaxies that are less luminous than the Milky Way. B/c the average Metallicity of a Galaxy correlates w/ its Luminosity, entire Galaxies could be deficient in Earth-sized planets (ibid., pg. 11).

 

And, the Luminosity of Galaxies tracks their Masses (citation pending)...

 

so, even on the Galactic scale, "Metals track mass" — which is essentially saying the same thing as "denser Disk regions are much more 'Metallicitious'" (where there's more mass, there's more Metals).

 

 

 

ADDENDUM: In the Local Group of Galaxies, apparently, only the Milky Way & Andromeda Galaxies are likely to harbor (in)habited worlds.

Edited August 29, 2009 by Widdekind

[Widdekind]

(1) Disk stars orbit their Galaxy's center, on orbits that primarily pass through, and along, that Galaxy's spiral arms (OP).

 

(2) And, most spiral Galaxy's — especially, apparently, big ones, like the Milky Way — have much straighter arms, that project outwards, "radially", much more rapidly (Wikipedia). To wit, the arms of the Milky Way are much more "limp", and are wound up much more tightly.

 

CONCLUSION: Since the Milky Way's "limp" arms are much more circular (more like the rim of a wheel), disk star orbits, which must pass through & along those arms, can also be much more circular. Conversely, in most other spiral Galaxies, their straighter arms (more like the spokes of a wheel), so that their stars' orbits must be much more elliptical.

 

This could have (potentially profound) implications, upon the possibility of Life, in the Galactic Habitable Zones amidst such straight-armed Galaxies, b/c highly elliptical orbits would take said stars deep into their Galaxies' cores (exposing said Star Systems to copious quantities of collisions & radiations), and then far out towards their Galaxies' rims. To wit, such orbits could take said stars beyond both the inner & outer edges of their Galaxies' Habitable Zones.

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so, even on the Galactic scale, "Metals track mass" — which is essentially saying the same thing as "denser Disk regions are much more 'Metallicitious'" (where there's more mass, there's more Metals)[/color]

 

The following journal article corroborates this "Galactic Mass-Metallicty (M-Z) Relation":

http://arxiv.org/abs/0903.4167

[Widdekind]

According to the Scientific American special issue Majestic Universe:

 

only metal-rich stars are associated with surrounding planetary systems. (Somehow, condensed dust is needed to 'seed' planet formation?) And, metals are steadily accumulating, at the rate of +8% per Gyr, in our neighborhood. And more, more metals were manufactured, more quickly, closer to the core of our galaxy, where disk densities, and star formation rates, are higher. Thus, moving inwards, from our sun's galactic orbit, disk metallicity increases at the rate of +17% per 'coreward' Kpc. Er go, comparing the figures, the galactic core is 'more mature', than our sun's vicinity, by the rate of ~2 Gyr per coreward Kpc.

 

Now, citing the same source, our star formed with an anomalously large amount of metals, ~40% more than nearby stars, of similar age. Thus, our star formed ~5 Gya, with as much metal, then, as nearby stars do, today. However, merely moving coreward 2 Kpc (7 Kly), one would commonly see star systems as old, and as 'metal mature', as our solar system. And, moving even closer to the core, one would witness star systems billions of years more metal mature still.

 

Fig. 1 --

Milky Way galaxy disk metallicity profile.

Our sun is anomalously metal rich, for its remote neighborhood (

+5 Gyr

).

Now, reflecting the relatively recent metallization, of our region of the galactic disk, most exo-planetary systems, which humans have observed to date, are relatively young (<3 Gyr). Yet, they are also nearby (<100s ly). To observe the expected cornucopia of exo-planets, then, human sensing systems will need to peer hundreds of times further afield (10,000s ly) -- something along the lines, of a Pharaonic-scale 'Eye of Horus' super-telescope.