Review: Demographics of Bulge Types within 11 Mpc and Implications for Galaxy Evolution

For the fifth meeting of the astronomy twitter journal club we decided to try a new time – Sunday at 10am GMT – which was friendlier for eastern timezones. The paper chosen for discussion was Demographics of Bulge Types within 11 Mpc and Implications for Galaxy Evolution by Fisher & Drory (2011). (There’s a preview of the paper and an unedited transcript of all the tweets if you want more details.)

The authors of the paper carry out an inventory of bulge types (ellipticals, classical bulges, pseudobulges or bulgeless) in local galaxies (within 11 Mpc). They find that “…whether counting by number, star formation rate, or stellar mass, the dominant galaxy type in the local universe has pure disk characteristics (either hosting a pseudobulge or being bulgeless).” Their results are potentially a problem for galaxy formation models, which predict more ‘classical’ bulge types than they see in their sample.

The discussion initially focused on what the results might mean for theorists:

The gist of the paper is that current observations don’t match the models. i.e. the models aren’t working. Correct? – @kashfarooq

the implication is that there have way fewer mergers than models predict. assuming pseudobulges are not produced by mergers – @astropixie

They differentiate between pseudo-bulges and classical bulges by their Sérsic index. 2 = classical but…classical bulges can have Sérsic indices less than 2. So, some of their pseudo-bulges may be classical. – @kashfarooq

i dont know how well the models distinguish between classical and pseudo-bulges? – @astropixie

Yes, the point I was trying to make – but for their observations. Could they have classed something as pseudobulge when it is classical – @kashfarooq

There were some ideas about how the work could be extended/improved:

And: it’s common to use 24um+opt+UV to measure SFR but would like to see FIR (or radio). A Herschel or ALMA project? – @StephenSerjeant

Maybe something for LOFAR? – @AdrianusV

Sounds like something that could be done pretty easily with the large Herschel surveys (i.e. ATLAS)… – @astronomyjc

any followup for SINGS or THINGS (kennicutt) surveys to do this? THINGS is HI survey i think – @astropixie

Yes I was just wondering if any of their targets are in JCMT NGS, THINGS, SINGS, Herschel Ref Survey etc. – @StephenSerjeant

In conclusion:

So what’s the evolution: no bulge>pseudo>classic or reverse? Or no evolution at all?- @AdrianusV

I think the paper’s suggesting 2 modes of evolution: (1) no bulge –> classical bulge & (2) no bulge –> ps-bulge – @astronomyjc

i think the paper is suggesting there is less merger-driven evolution than models assume. more secular evolution…but observations are always most sensitive to high mass gals which are ellipticals or have classical bulges. – @astropixie

challenge when comparing observations to models is to measure galaxy structure the same way. this is a good step i think – @astropixie

There was also some discussion of the time and day of the meeting. The overall feeling was that most people prefer Thursday:

There don’t seem to be as many people taking part today as normal. Is Thursday eve a better time? – @astronomyjc

i know many aussie colleagues are traveling right now 😦 was happy to participate but maybe thurs is better…? or maybe the paper wasnt controversial enough to stir up the crowd!?! – @astropixie

Thursday is definitely better, can’t justify sitting here while family is buzzing around:) – @khanzadyan

Though there was at least one fan:

Quite enjoying the combination of #radio3, duvet, coffee and #astrojc – @StephenSerjeant

Overall the paper was well received. I’ll leave the last word to @StephenSerjeant who nicely sums up the difficulties in getting accurate galaxy formation models given the wide range of data:

And to think I’ve heard theorists say there are only 2 types of galaxy. Some even put the number at 1. As (I think?) @JimFraeErskine said, it’s kinda like saying there is only 1 or at most 2 types of tree… – @StephenSerjeant

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