all talk

I didn't get much identifiable work done on Thursday, but that's because I spent all day arguing about galaxy evolution with the huge galaxy evolution group here, including especially Somerville, van den Bosch, Bell, and visitors Trager, Hopkins, and Davé.

bulge-dominated galaxies and starbursts

[for yesterday:] I was pleased to find Cox et al on the construction of bulge-dominated galaxies from disk-dominated galaxies in merger simulations. They find that they cannot reproduce the observed properties of bulge-dominated galaxies without allowing for dissipational gas dynamics and rapid central star formation. I have been saying this in a hand-waving way for years, and I am very happy to see it all done quantitatively. This really does mean that we can observe the creation of new bulges by looking for starbursts and post-starburst galaxies, as we have been arguing since Quintero et al 2004.

BHB stars

[for Tuesday:] I worked on the possibility of measuring the 3-d velocity of Galactic substructure using blue horizontal branch stars, which all have luminosities in a narrow range (so their magnitudes tell you their distances), and which have radial velocities (for some) and proper motions (for all) in the SDSS.

agn luminosity functions

Phil Hopkins (Harvard) spoke here today about systematically fitting all AGN luminosity functions at all wavelengths by convolving a bolometric luminosity function with the observed range (different at each luminosity) of spectral energy distributions and dust attenuations. His fits look great, and make many predictions. In an aside, he noted that the faint end of the AGN luminosity function at high redshift may well be set by the fading power-law of the AGN after its high-luminosity event. If so, this would be an application of the idea in this obscure paper.

figure captions

Who knew that Quintero and I could spend a full hour working on a single figure caption? And it was just revising it. We worked through all the figure captions and plan to finish the discussion of his results on Monday.

galaxy environments, thick disk

Quintero and I worked on focusing his latest paper onto the processes that control the morphology–density and star-formation–density relations.

Rix and I found that we could get an intelligible proper motion distribution as a function of stellar magnitude for what are plausibly turn-off stars in the halo and in the thick disk, inspired by Girard et al. The relative velocity of a typical thick-disk star relative to the Sun increases with distance, but the proper motion involves an inverse distance (because it is an angular measure of motion), so the distribution for the thick disk is very simple.

high-redshift star formation

Davé's students Finlator and Oppenheimer gave nice talks today about high-redshift star formation in numerical models of structure and galaxy formation. Finlator showed that the star-formation histories of galaxies at early times are very constrained (if the star-formation prescriptions in the models are close to correct); this will either aid strongly the fitting of high-redshift spectral energy distributions of galaxies, or else show that there are issues with their numerical simulation of cosmic star formation. Oppenheimer showed that the long-debated early enrichment of the IGM, which has been suggested because CIV (triply ionized carbon) is seen at all redshifts, is something of a coincidence; the carbon abundance is rising, but the fraction of the carbon in CIV form is falling. Though it looks like the IGM was polluted early and only once, in fact pollution has been a continuous process; there is no requirement of any Pop III stars to enrich the IGM at early times.

Rix, Bell, and a cast of thousands argued this afternoon about how the black hole masses know about their host bulge masses. Some predictions were made.

mergers and the red sequence

[Sorry for the long gap between posts; I was hiking in the Alps!]

Nice lunchtime talk with Sandy Faber (UCSC) and Frank van den Bosch (MPIA) and Davé and Bell regarding the build-up of mass on the red sequence. I pushed my position that starbursts have to be involved. Faber continued the discussion with her talk at the University; she showed that downsizing—the evolution of the mass scale of assembly to lower masses with time—can be seen in all measures of merging and activity.

supernovae and enrichment

Dani Maoz (Tel Aviv) gave a great talk today about the problem that (a) galaxy clusters contain enormous amounts of mass and enormous numbers of stars and are strongly gravitationally bound, and (b) they contain a large amount of iron (a few parts per thousand by mass), and yet (c) they show very low type Ia supernova rates. Either the supernovae go off very early in the lifetime of a stellar population, or else the metals are put out there some other way. The latter explanation—since there are no other ways of expelling metals into the intergalactic medium—is not plausible. But if the type Ia supernovae are prompt, there are issues in understanding supernova rates in the field. It is an extremely rich subject area that is only getting richer as supernova projects evolve, and Maoz is doing his statistics right.

ps: Maoz does most of his observing with a 1-meter telescope!

future of cosmological observations

Rix and I argued today (while walking up from the Bismarckplatz to MPIA) about the future of observational cosmology, in imaging, spectroscopy, radio, optical, and simulations. I made my pitch for constrained realizations, which Rix thought was crazy. It is, of course.

star formation and cosmology

There was a one-day mini-symposium here (at MPIA, Heidelberg) on star formation in a cosmological context. There were good, long talks all day. A few randomly recalled notes:

Though Grebel (Basel) introduced her work entirely in the context of the substructure problem, I think I agreed more with Springel (MPA Garching) who, under questioning, admitted that the substructure problem may be a sub-problem of the luminosity function problem (the problem that the distribution of galaxy luminosities or stellar masses looks nothing like the CDM-predicted distribution of dark-matter halo masses). Grebel showed that the nearby dwarfs—ie, those close to their host galaxies—are not only older in stellar populations than more distant dwarfs, but also more metal-rich. Springel convinced me that cosmic rays might be an important dynamical component of the ISM and affect star formation in galaxies, particularly low-mass galaxies.

Abel (Stanford) showed amazingly high dynamic range (10^14.5) adaptive-mesh simulation results. He argued that the details of ultra-high redshift supernovae were unimportant for initial pollution of the IGM with metals; the distribution of the metals into the IGM is done by a combination of SNe, winds, and radiation (which makes low-density regions that are easy to expand into). Gallagher (Wisconsin) showed details of star formation in starbursts in the local Universe; he convinced me that Perseus A is the most incredible galaxy in the Universe. I am inspired to find SDSS analogs.

statistical subtractions

I worked on the weekend on statistical subtraction of color-magnitude diagrams and proper motion distributions in and out of globular clusters observed by SDSS. I am trying to assess the possibility of measuring proper motions of clusters of stars statistically. The challenge is to subtract the foreground correctly; since in general the foreground is populous, the results depend strongly on how you subtract it.

mergers, agn, starbursts, red sequence

Lots of conversations with Rachel Somerville (MPIA) and Davé today about the idea that mergers trigger starbursts and AGN, and the AGN shuts off star formation so that the fading remnant galaxy can join the red sequence. This picture is observationally tractable, but it is difficult to robustly constrain the fraction of AGN that are merger-triggered, the fraction of mergers that trigger AGN, and all the relevant timescales. We came up with some ideas for Quintero to test when he arrives here next week.

proper motions

Yesterday and today, Rix and I found what proper motions exist in the SDSS data and tried to assess their usefulness for constraining halo properties. It looks a bit dicey.

I gave a few-minute talk today that expanded into a half-hour of discussion about the merging of smaller galaxies into LRGs (Masjedi's new results). The low accretion rate we get implies that either the accretion rate is a very strong function of cosmic time, or else that LRGs formed very early. Romeel Davé (Arizona), who is also visiting, noted that early formation of LRGs is not necessarily unreasonable from a theoretical perspective, because cold mode accretion (which can rapidly build bulges, in principle) is much more common at earlier times.

proper motions

Yesterday and today, Rix and I found what proper motions exist in the SDSS data and tried to assess their usefulness for constraining halo properties. It looks a bit dicey.

I gave a few-minute talk today that expanded into a half-hour of discussion about the merging of smaller galaxies into LRGs (Masjedi's new results). The low accretion rate we get implies that either the accretion rate is a very strong function of cosmic time, or else that LRGs formed very early. Romeel Davé (Arizona), who is also visiting, noted that early formation of LRGs is not necessarily unreasonable from a theoretical perspective, because cold mode accretion (which can rapidly build bulges, in principle) is much more common at earlier times.

merger rates, stellar mass maps

Eric Bell (MPIA) pointed out in conversations about merger rates today that since the LRG mass function is very steep, there is a big difference between the merger rate considered as the probability that a given LRG will merge in the next Gyr and the probability that a given LRG had a major merger in its past. These differ because in the latter question you are dividing by the number of LRGs, and in the former you are dividing by the number of twice-as-massive LRGs. Of course Bell is interested in asking the latter question and we (Masjedi and I) are asking the former. I would say that we are asking the right question and Bell is asking the wrong one, because close pairs tell you about the future, not the past, but then again the evolution of the Universe is continuous, as I often argue.

James Pizagno (OSU) is also here visiting Rix, and we spent some time discussing the project of turning multi-wavelength images of galaxies, which show starlight, into stellar mass maps. Pizagno has some great code, and some very believable mass maps. In some spiral galaxies, the spiral arms disappear entirely when he goes from starlight to stellar mass. We agreed to test his code on some of my bright galaxies.

SDSS SQL

I spent the day reminding myself how TROTW (the rest of the world) accesses the SDSS data—through a public SQL server setup. We at NYU, of course, keep and spin our own copy of the entire data set down to raw (almost), so I haven't had to use the public SQL stuff for more than five years. It works beautifully and it is fast, so I think this will be fun. I am in Heidelberg for the month working on Milky Way substructure with Hans-Walter Rix (MPIA) and his group.

atmospheric extinction

[This post is for Friday.] All I did all day is work on the atmospheric extinction; I was trying to use five years of SDSS spectroscopic data to infer it. Unfortunately, seeing is correlated with airmass, so my results have a grey term due to fiber losses. Not sure what do to at this point, because there is very little we trust in the atmospheric extinction literature and folklore.