spiral structure in galaxies

I worked on various things relating to big, bright galaxies in the SDSS and other surveys. I also emailed the following to Hans-Walter Rix (MPIA) in preparation for my trip to Heidelberg this summer. It was partly inspired by this post by Rob Knop.

Imagine that spiral arms are created by tidal events which are then "wound up" by differential rotation. Then we can use kinematic information to "unwind" the arms and measure a "time interval" since the last tidal event. Compare this unwinding time to various possible timescales, such as those in star formation histories or in the kinematics of companions and check all simple hypotheses. Now that's an insane project; only the optimistic could embark on that one...!

Weinberg, Atlas

David Weinberg (OSU) gave a great talk today on galaxy evolution in a cosmological context with an emphasis on disk galaxy formation. He was able to match the properties of the distribution of galaxies in disk size, stellar mass, and rotation velocity with a model with cosmological inputs and some by-hand inputs. He predicts that high surface-brightness disks have a larger mass ratio of disk mass to dark-matter halo mass than low surface-brightness disks. In general, my main concern with these kinds of analyses is that they do not allow the disk properties to be set by late gas accretion (which seems likely to me). However, his model makes testable predictions and we can test them!

I officially posted to the SDSS collaboration the Gunn Atlas of Galaxies project, outlining its main features. I will post more about this soon.

discussions

Not much real work today. Lots of discussions about galaxy evolution with David Weinberg (OSU) who is in town. Also discussions with Blanton about estimation of sky level in SDSS images, which, for me, is on the critical path for the oft-mentioned Gunn Atlas.

galaxy images

More work on the Gunn Atlas today.

future data

On Sunday I worked on analysis related to NYU's future in observational astrophysics projects. We will join some kind of big project, but we haven't decided which one. There are a lot of issues, involving science, personnel, and money.

I am at Brown today, giving a talk.

Princeton

I gave the Princeton Gravity Group talk today, about galaxy merger rates as observed in the nearby Universe.

Afterwards, I talked to Padmanabhan and Finkbeiner about SDSS-III, PANSTARRS, PRIMUS, and other big, new survey ideas. Finkbeiner argued that we should forget all that and look for direct evidence for dark matter (by annihilation to gamma rays or any other means available). Padmanabhan was skeptical.

I spoke with Strauss and Ho about high-redshift post-starburst galaxies found in the quasar target selection sample. It looks like there are enough (hundreds out to redshift 0.8 or so!) that we can ask about evolution and environments, in comparison with Quintero's sample.

I spoke with Gunn about the nascent galaxy atlas with his name on it. We are a long way off, but Blanton and I have made huge progress in the last few weeks.

Hester and I continued our discussion of ram-pressure stripping and how the observations in the optical speak to that.

the Gunn atlas

I worked on material for The Gunn Atlas of galaxies from SDSS data.

ram-pressure stripping, surveys, neutrinos, talks

On Monday, Janice Hester (Princeton) showed us at group meeting theoretical and observational results on ram-pressure stripping of gas from galaxies as they fall into clusters or groups. At lunch, Uros Seljak (Trieste) showed us that the dark matter can't (basically) be sterile neutrinos, using the clustering of Lyman alpha forest. In the afternoon, Blanton and I had long discussions of future surveys, with the long-term goal of deciding what we should get ourselves into.

Today I gave two talks at Eastern Illinois University, including a well-attended public talk. Thanks to my hosts for a great time.

talks

On the chinatown bus from Boston to NYC, I worked on the two Harlow Shapley lectures I am giving this week at Eastern Illinois University, one technical and one non-technical.

PRIMUS spectral smoothing, NED mosaics

With Burles I worked on making smoothed versions of the PRIMUS spectra, with a smoothing kernel that is constant in km/s despite the fact that the spectra have very strongly non-linear dispersion.

I also worked on details of the NED SDSS mosaics; they are nearly ready for prime-time.

NED imaging

I worked on subroutines for comprehensive SDSS imaging of NASA Extragalactic Database sources. The idea is that we ought to be able to turn any NED object query output into the instructions for creating an imaging mosaic of SDSS data. This is all in preparation for the nascent Gunn Atlas, discussion of which heated up this week.

environment indicators

I worked on galaxy envionment indicators today.

If you are measuring galaxy environments, you can choose from some options: You can choose a measure of local density that either (1) has a fixed physical length scale (radius) or else (2) is high in signal-to-noise. The reason for this dichotomy is that the former are done, basically, by counting the (necessarily small) number of galaxies in the relevant local volume, while the latter uses something like the Nth closest or a clustocentric distance or tesselation, where the scale is set by what gives good signal-to-noise (and is therefore different in different environments). We have shown that a fixed scale of 1 Mpc is about the best possible scale, but you can't measure it at high signal-to-noise.

After this, you can choose a measure that either (a) uses redshift information or else (b) works with imaging data alone. The latter allows you to go fainter into the luminosity function, and work with cheaper (and more readily available) data, but it costs you a lot in signal-to-noise, because you have (even with good photometric redshifts) a lot of projected galaxies (either foreground or background). Purely imaging indicators also have the problem that they almost never have systematics that are constant with redshift, since foreground/background and photometric redshifts all vary strongly with redshift for reasonable (read: the usual) choices that can be made.

prism spectra

Spent time with Burles looking through prism spectra taken as part of the PRIMUS project. Most of the spectra look nothing like what a spectroscopist is used to looking at, since the dispersion is totally non-linear, and we are (deliberately) not dividing out the system sensitivity (we are doing all fitting/modeling in the observed frame, so we multiply the models, not divide the data; there is a difference). But we found a few clear broad-lined quasars, one of which turned out to be a z=1.6 quasar with double-peaked emission lines incorrectly given a lower redshift in the DEEP2 redshift survey. Apparently PRIMUS is finding lots of redshift errors in the various large surveys, including DEEP2, VVDS, and COMBO-17. Not to worry, though, the vast majority of published redshifts are correct! We formulated a dumb project for me: Find all the SNe among the 18,000 or so extant PRIMUS spectra. I'm on it!

photometry, spectroscopy, projects, inflation

In the late weekend (after taxes), I worked on fundamental photometric calibration, analyzing the contribution by Stubbs and Tonry. I don't think a great deal of what they say is truly new, but it is certainly about time someone wrote it down and got the conversation rolling. It is boring as heck, but it is required if precision cosmology is to move forward. My own contributions on this subject are here (my most-cited first-author paper) and here (my least).

This afternoon, Burles, Coil (Arizona), and I discussed the current and near-future steps with PRIMUS. Coil, Blanton, and Eisenstein are working on sample selection, mechanical collisions, and masks. Cool (Arizona) is working on extractions. Burles is working on getting the wavelength solutions right (his arc-fitting software is incredible). Masjedi is working on redshifts. What's next? Science. Note how I am not working on anything. Very clever, no?

At group meeting, Burles told us about the gravitational lenses he and Bolton (Harvard) have been finding in copious numbers (about 40?) from looking for two-redshift objects in the SDSS spectroscopy. They have one of the largest statistical collections of gravitational lenses in existence, and they have many uses for lensing, cosmology, and galaxy astrophysics.

After Burles, Coil told us about the clustering of quasars (from SDSS) with galaxies (from DEEP2) at redshift of unity. She finds that quasars are clustered like the galaxies, and have similar bias. She finds this at great signal-to-noise by using a cross-correlation (rather than auto-correlation). Indeed, as Eisenstein, I, and others have been arguing for many years, the cross-correlation is much higher in signal-to-noise than the auto-correlation function for rare populations, and you should almost never use the latter when you can use the former. Coil's result is a great advertisement for this fact, because the cross-correlation of DEEP2 with 17 (yes, 17) SDSS quasars has a higher signal-to-noise measurement of clustering at Mpc scales than the entire 2dF QSO survey auto-correlation function!

After Coil, Marla Geha (OCIW) told us about the gas fractions of dwarf galaxies, which show an enormous range, but one that is a very strong function of environment. Dwarf galaxies with very low gas fractions are almost always close to (ie, within hundreds of kpc of) more luminous galaxies. This effect has never been seen before because prior to Blanton et al (2005), there has not been a dwarf sample selected without regard to environment! The very nice thing is that Geha's results (with Blanton and Masjedi) rule out many ideas about dwarf galaxy evolution and support others—a rare thing in the world of galaxy astrophysics, filled as it is with soft predictions.

After group meeting, Mukhanov (Munich) gave a wonderful informal talk about what inflation naively and straightforwardly predicts (and what it does not). Nice!

galaxies and the UV radiation budget

[No posts for a while as I have been out sick. I was only in for two hours since Tuesday, to see this talk:]

Alice Shapley (Princeton) gave a data-packed talk about galaxies at redshift around 2 and 3. She reminded us that there is no more redshift desert around 2, which was a major issue in my graduate thesis work. She spent most of her talk estimating the contribution of high-redshift galaxies to the ionizing ultraviolet radation, which we know is there from, eg, the lyman-alpha forest, and which at an earlier epoch reionized the Universe (but substantially after the Universe recombined from the primordial electron-positron plasma). Though there are huge uncertainties, and no sample of galaxies is large enough and well-enough studied to give a definitive answer, it does appear that the high-redshift galaxies release enough ultraviolet light into intergalactic space to make up most of the ionizing background radiation. In other news, she showed some reasonable (though admittedly not air-tight) evidence that the mass–metallicity relation is much less metal-rich at redshift 3 than at the present day (or than at redshift 1, which Moustakas, a few months ago, showed us was also less metal-rich than the present day).