KITP: colourful extragalactic globulars

and we continue our lesson on extragalactic globulars, with a trifectaduet of talks and an emphasis on colour: red or blue, it is like South Central here sometimes

remember, red is metal rich or old
blue is metal poor or young...
except the reds are actually probably younger than the blues
so it is metal dominated, except for really young clusters

Steve first

then Jay

PS: our summary 20 minute talks have an interesting habit of actually lasting 60-90 minutes. I blame that rude chap in the front row who keeps interrupting the speakers with stoopid questions...

as usual the multimedia video and postcasts and slides are online

remember, remember:

Fe peak elements are very uniform within a particular globular
which limits any possible self-enrichment

this is in spite of multiple stellar populations, α-elements and s-process elements, gradients and anti-correlations

famously the colour/metal distribution is bimodal
no mass-metallicity relation seen in Milky Way
there are spatial/kinematic differences - blue are halo population, red are thick disk/bulge

globular clusters are NOT small galaxies (ok. except maybe for ω Cen).
recent HST data suggests there is "blue tilt" in extragalactic systems,
ie that there is a mass-metallicity relation (cf Strader and Brodie et al 2006, also Harris et al) - the more massive blue clusters are redder, and thus more metal rich
Z ~ M^0.5 ?

test with deep HST imaging (Waters et al (2009))
best fit Z ~ M^0.1
use mixture modeling with bimodal cut in V-I which depends on magnitude (weakly)

neither is actually measuring Z but assuming V-I is a metal proxy

difference due to aperture correction?
deep data allows for resolved cluster
Waters et al recover tilt with uniform aperture correction

like this?

only the brightest globulars show the tilt?

so globulars are:
not self-enriched
no dark matter halos
much weaker mass-radius relation
not on M-σ relation (RZ2109 in NGC4472 [ed. hm, I think that is over interpreting - you _could_ have a stellar BH accreting and an IMBH co-existing - arguably more likely to catch a stellar BH accreting in such situation, or not, not trivial])

NGC 4472 - from Hogg's SDSS RC3 selection - pretty pictures galore!

Hm, there's something going on - the galaxies show a mass-metallicity relation

now globulars have Δ Z of ~ 1+ at constant mass
so if the mass-metallicity relation holds, then at best it bifurcates down around few million solar masses, and the blue branch may either go near vertical or it may get stronger depending on who you believe

so the blue globulars are galaxy like, or not
but the red globulars are definitely not.
so there.

which gets us back to formation mechanisms and whether (some) globulars have extended low density dark matter halos, or not

we got late to coffee and cookies, and not only had the string theorists eaten all the good cookies, but they also finished the french roast.
bastards

and we resume...

Jay, he says:

wait, we didn't use V-I, we use g-z and Harris used B-I
see this trend in the blue globs, like Harries...
is it real, or is it memorex?
is the tilt only for the brightest

ie are we seeing the mass-metallicity relation come down upon the little triangle tip of the globular luminosity-colour relation...

shape of the blue tilt, and the mass above which it is seen appears to vary for different galaxies.... hmm
cf Wehner et al 2008 from ground

NGC 3311

we are agnostic about what the heck is going on at low masses, but maybe there really is a tilt in the blue branch at high luminosity (masses >~ 2 million solar, or so, or less)
I am sure there are differing opinions on this.

due to self-enrichment?
maybe.
star formation efficiency varies with cluster mass (current mass natch... does that well trace zero age mass)
or gas retention varies with cluster current mass (as proxy for zero age mass)
or both

this of course has been modeled by Bailin and Harris (2008) - this predicts that the red branch should start tilting at high enough a mass, it is just that we don't see globulars that massive, or if we do we don't call them globulars...

Jay has data for M31 suggesting the metal rich (Z >~ -1) are younger (5-10 Gyr)
and the M/L is lower 1 -- 1.5 rather than 2-3 for old blue ones
different IMF??? (submitted)

but they are old,so the IMF must be crazy near solar mass...
this is screwy