Integrating Irregulars into Hubble Sequence ?

[Widdekind]

i understand, from some slides prepared & published by Prof. Yicheng Guo (U.MA), that during early epochs (z~5-1), the fraction of stars residing in well-formed Elliptical galaxies increased from negligible to half; and that by that epoch (z~1), emerging Ellipticals were "born" being about 1Kpc across. At earlier epochs, Irregular galaxies were the most common.

 

i conclude from those facts, that clumpy cloud-like Irregular galaxies (Irr-1) gave rise to both Elliptical, and disky Spiral, galaxies, during early epochs (z~5-1). Those collapsing cloud-like clumps generated compact Elliptical (cE) galaxies, ~1Kpc, i.e. about as big as a Bulge in a disk galaxy. Parsimoniously, if over-simplistically, perhaps (nearly) all clumpy cloudy Irregulars collapsed into Bulge-sized compact Elliptical galaxies, with half becoming Ellipticals, and half becoming Spirals? Perhaps the difference between the former & latter is whether the clump cloud collapsed "dry" (no remaining gas) as Ellipticals, or "wet" (lots of leftover gas) as Spirals?

 

Parsimoniously, perhaps the Hubble sequence can be summarized, with the two kinds of Irregular galaxies being the "on-ramp" and "off-ramp" to & from the sequence:

 

Irr-1

= proto-galactic cloud clump, which collapses into...

 

S0

= Bulge-sized compact Elliptical (cE), ~1Kpc across...

 

dry?

no relic gassy halo => small

Ellipticals

wet?

gassy halo "settles", "disking down" into surrounding swirling disk =>

Spirals

 

tidal disruption?

close encounters distort & disrupt galaxies =>

Irr-2

merger?

mergers (in busy "urban" clusters) generally "dry" the colliding galaxies, shock-heating their gassy halos, into surrounding Intra-Cluster Medium (ICM), and mingling their ellipsoids into single, larger, super-sized cluster-central large

Ellipticals

(cD)

If so, the Irr-1 are collapsing cloud clumps, leading "onto" the Hubble Sequence; whereas Irr-2 are tidally disrupted & distorted systems, undergoing (or emerging from a recent) close encounter, and so exiting "off of" the Hubble Sequence; and larger Ellipticals generally result from full-fledged major mergers.