KITP: dwarf globulars

final stretch and we contemplate big stellar clusters in small galaxies

in particular, if you plot the observed number of globular clusters as a function of galaxy magnitude you find the specific incidence, the number of globulars per unit light, is high for large ellipticals and small for Milky Way like spirals, but there is some evidence the specific incidence if also high for low luminosity dwarf galaxies

or is it...?



Fornax dSph from NS at Uppsala

Paul "finally I know how to pronounce his last name" Goudfrooji tutors us on globular clusters in late type dwarf galaxies
ADS link

Is the Milky Way really a minimum-globular cluster galaxy?
If so, why?
We're talking "real" globulars here, the old, metal poor, bound ones - though what is bound is clearly a function of galaxy mass and morphology...



NGC 1705 from HST

Is there a physical effect, like low mass dwarfs being inefficient at destroying globulars
and high mass galaxies eating lots of dwarfs?
Except dwarf irregulars seem to not have old low mass clusters, even though we ought to be seeing them.

Or is it observational bias - we see some globulars in some dwarfs and count those, but we don't count the null results of dwarfs with zero globulars, or those not observed well enough?

Comments?
Comments...?

interesting point - the EHB clusters are massive and compact - include M54 and ω Cen
Are the EHB clusters all nuclei of stripped dwarf galaxies?

that would be very very interesting if true



NGC 59 from SALT (Bonita DeSwardt UCT)

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If anything low mass galaxies should be a bit more effective at destroying clusters.

The apparent rise in specific incidence (or frequency) at the low mass end is mostly/all observational, or more accurately statistical, bias.

The expected no of GCs for dwarf galaxies is so low that just the Poisson noise scatters a bunch of them to high S_N. The error bars are of course not symmetric either for these despite how they are often plotted. Oh and systems that formally have -ve S_N (after background subtraction) are often set to 0 exacerbating the apparent rise.

Arunav;
I am curious...just how many of these formally -ve S_N dwarfs are there? I am interested to know if this is really enough to account for any bias. Are you suggesting that there is no such rise at all in S_N as you go to lower L dwarfs, and that S_N stays (more or less) constant for all dwarf galaxies? Just curious. I would think it might be more difficult *not* having a wide range in S_N for dwarfs; especially at the low-L end (since some dwarfs may well lose some luminosity, making S_N rise).

By Pat Durrell (not verified) on 08 Apr 2009 #permalink

Oh there certainly a wider range of S_N at lower L. That wouldn't be surprising just from stochasticity. The question is whether the mean efficiency turns up (much) at all. Calculating average efficiencies already reduces the apparent sharp upturns in S_N plots.

Beyond that it depends on how people measure things and when they set things to zero etc. etc. Note the inordinate number of galaxies that have exactly 0 GCs and hence S_N=0 here.

Arunav, not every conclusion in all of astronomy is a statistical effect or observational bias.

I agree that it's still uncertain whether the mean efficiency goes up by much at low luminosities but it's simply not true to argue that the increased spread in S_N is just due to shot noise.

A bright dE at M_V~-17.5 should have around 15 GCs if it has the "normal" S_N of ~ 1.5. This means that systems with S_N > 3 should be rare; yet there are lots around S_N ~ 5. A fainter dE with M_V ~ -16.5 should have about 6 GCs, yet some dEs of this luminosity have 30-50 GCs.

This ain't Poisson noise.

Jay
There when certain grand trends change direction due to the very act of measuring in a different way (and not primarily because of shot noise) laws of uncertainly or plain sense suggest they ought not to be believed.

Back to the issue at hand. Note that I was careful not to make a blanket statement that it is necessarily completely flat. But the broader issue is that it doesn't go up by much when average efficiencies are computed (e.g. Fig 2).

Have you looked at the scatter in S_N of "normal" galaxies before selecting 1.5 as the number to consider? And it's curious that you stop at M_V=-16.5 when some of the most dramatic rise is from galaxies that go all the way down to -12. Fig 3 in the Peng paper even has an interesting line that shows the calculated S_N if there is exactly one object per galaxy.

The scatter in S_N for L* ish galaxies is actually pretty small.

Depends on what you definition of "ish" is ...

Random aside: the L*(ish) galaxies always seem shortchanged in the global globular cluster study sense. There is always someone studying the big guys, and the featherweights fit almost completely in the fields of view of common instruments. The extrapolations involved for the middleweights, e.g. arbitrarily assuming that the GCs follow a Sersic profile, are rather unappetizing.