galaxies in voids

A new paper on astro-ph on void galaxies by Patiri et al confirms the results of Blanton et al and Quintero et al that the environment effects on galaxy formation are directly related to star-formation histories, and all the structural (morphological) relations with environment are products of the structural–star-formation relations and the star-formation–environment relations; ie, the morphology–environment relation is secondary to more fundamental relationships. This implies that galaxy morphologies are set by local processes (though does not prove it).

all talk

Willman gave an intriguing group meeting talk about results from her last run of simulations of a Local-Group-like dark-matter concentration in a cosmological context. There is a hope of constraining aspects of early reionization with the star formation histories of the dwarfs.

Wu showed me that we do have detections of some dwarf galaxies at 8 microns in IRAC, so we ought to have detections in the IRS spectra, unless I did some kind of calculation wrong (disturbingly possible).

Masjedi told me about what he learned from his participation in the Galaxies in the Cosmic Web conference. There is rapidly growing interest in the constraints on galaxy evolution from close pairs, internal stellar densities, and continuity—all the issues we work on here.



A major theme in my talks and research for the last few years is that the Universe we see (ie, an instantaneous picture of our past light cone) is a snapshot of a continuous process in which galaxies are evolving by forming stars, merging, accreting, fading, etc. In principle, if our sample is representative, the entire continuous process ought to be reflected, statistically, in our snapshot. At the very least, the snapshot must be consistent with the continuous process. I imagine, in effect, a kind of continuity equation analysis of the data.

In particular, I think a lot of the trivial ideas thrown into the literature about how galaxies might be evolving are strongly in conflict with this continuity consistency requirement. I worked on that a bit more directly today, by working on my small project (mentioned earlier) on what the galaxy central surface-brightness distribution (ie, the distribution of stellar densities at galaxy centers) has to say about what kinds of processes can be taking place in galaxy evolution.


scalar objective function

One of my soap-box issues (inherited from Roweis) is that whenever you make a measurement with data, you better be optimizing a justified scalar objective function. If you aren't, then you haven't made the best measurement you can, in any sense. Parenthetically, this is one of my objections to the HST drizzle algorithm for making combined images; it has no such scalar.

Marshall and I spent Friday night, past midnight, working on the correct scalar objective function for the automated lens finder. It is not an easy problem; we don't have a final solution, but we learned a lot. Some desiderata: It should be close to zero when lens models don't work well on the source, and it should be close to unity when the lens model under consideration explains most of the image intensity. It should not depend strongly on whether the images have had their foreground and sky subtraction done correctly on large scales, or other details of subtraction of the foreground galaxy light.


faster code, high-redshift stellar mass

[This post is for today.]

Phil Marshall and I sped up our lens-finding code by a factor of two today; we only have five or ten more factors of two to go!

I gave the IPAC talk, on mass build-up at redshift of 0.1 or so.

Haojing Yan (Spitzer) showed me an estimate of the comoving density in stellar mass at a redshift of six!

lens finding, IR spectra

[This post is for yesterday.]

Phil Marshall, Lexi Moustakas, Chris Fassnacht (Davis), and I continued working on the automated lens finding code and paper. In the usual style, Marshall and I are pair coding.

I had an extremely pleasant lunch with Richard Ellis (Caltech) and his large (and productive) group. We discussed many things, including lensing and galaxy evolution and the intersection of the two.

Brent Buckalew (Spitzer) showed me nuclear and extra-nuclear mid-IR spectra of star-forming galaxies so I can see what to expect from our dwarf galaxies. Unfortunately, the low metallicity dwarfs in his sample show very little interesting emission, which bodes ill for us. Some of Buckalew's galaxies have strong metallicity gradients, but none strong enough that there are spectra with PAH emission at the nucleus, but without in the outskirts.


lens finding, ULIRG environments

Phil Marshall (SLAC), Lexi Moustakas (JPL), and I spent all morning working on our nascent paper on automatically finding strong gravitational lenses in high-resolution imaging data. Our beautiful idea is to brute-force consider every possible lens model for every object in an imaging data set, and then use the places where the sky is blank to veto lens models that predict multiple images in the wrong places. This appears to be close to working, so naturally we start writing the paper immediately.

In the afternoon I spoke with Joe Mazarella (IPAC) and collaborators about LIRGs, ULIRGs, and their environments. This is wide-open territory with a lot of likely interesting possible outcomes.

low-luminosity galaxies

Stomach flu got most of my time today, but I did take a look at Wu's visualization of our Spitzer/IRAC 3.6 micron imaging of low-luminosity galaxies. They all seem to be well detected in our 60-s integrations. Will they be detected at 8 microns? Our Spitzer/IRS spectra suggest not.



I didn't do much work today, but I talked to Ranga-Ram Chary (Spitzer) about galaxy evolution, and Tom Soifer (Spitzer) about the post-cryogen future of the Spitzer Space Telescope.


mid-infrared spectra

Not being able to see any flux in any of our Spitzer mid-IR spectra of dwarf galaxies (we didn't necessarily expect to be able to see anything), I added up all of our two-d spectra to see if we get anything on average. Here's what I got:

See that whopping spectrum? No? I see something, but you have to have astronomical eyes. Next week: figuring out what limits we can place.


galaxy environments

I had a long talk with Chuck Steidel (Caltech) today in which we ranged over many topics, but in particular we discussed the best evidence that galaxies can and do interact with their large-scale environments, by, eg, driving massive outflows with star-formation events. Chuck pointed out that there are four times as many metal (in astronomical parlance: elements beyond helium) atoms outside galaxies as inside, so in addition to his direct evidence for outflows, there is a pretty strong indirect argument.

I gave an informal lunch talk to the galaxy-interested denizens of the Spitzer Science Center on what we know about galaxy environments from the Sloan Digital Sky Survey and related data.


galaxy evolution

I discussed galaxy morphologies and evolution in their distribution with redshift with Kartik Sheth (Spitzer). Sheth finds that the bar fraction is indeed evolving with redshift, and thinks it has to do with the mean dynamical state of disk galaxies being hotter at earlier times. I told him that I don't believe in morphology! We also discussed tests of the hypothesis that spiral arms are transients, as I mentioned in some recent post.

Jessica Krick (Spitzer) told me about intracluster stars and cD galaxies. She finds that it is very hard to decide what is a cD and what is intracluster; there may be no real distinction between these at all. She also finds it hard to measure radial gradients in their colors, because the intracluster light is so low in intensity (and she has the best flat-fielded data ever, basically). We discussed galaxy evolution and hierarchical clustering, and we figured out a possible project (based on Masjedi's projects) to see if cD galaxies are plausibly built from galaxies that fall into the clusters (the standard paradigm).

Lin Yan (Spitzer) gave a nice talk on high-redshift sources observed with Spitzer.


reducing Spitzer spectra

Today I took Lee Armus's (Spitzer) suggestions about how to look for faint sources in IRS spectra (by differencing) and turned them into a simple data reduction script. They worked nicely, in that most of the detector artifacts went away, but I still don't see our galaxies! On the other hand, we don't expect the galaxies to be very bright in the mid-infrared, so maybe our observations just put upper limits? My expected brightness and sensitivity calculations (for the proposal) involved some serious guessing.


starburst galaxies in the mid-infrared

George Helou here (Spitzer Science Center, where I will be for the next two weeks) showed me beautiful data on the mid-infrared properties of nearby galaxies, including NGC 1377, which is a luminous lenticular, but it has a tiny knot of star formation that looks like an enormously powerful, low-metallicity starburst (perhaps a bit of gas that was accreted?). The starburst knot shows enormous silicate absorption and effective extinction, but no PAH emission at all. The more normal galaxies he showed me (primarily from the Spitzer SINGS program) generally show PAH (polycyclic aromatic hydrocarbon) emission when there is star formation, with some variation depending on the contribution of AGN to the radiation field. On the other hand, the galaxies show a huge variation in the hot and warm dust underlying the PAH emission, making analysis of the PAH features non-trivial. Fortunately his team is working on some generally useful tools. Although most investigators seem to think that PAH emission is related to metallicity, this is not at all clear from the currently available data.

Lee Armus here agreed to help Ronin Wu and me understand our Spitzer spectra of low-luminosity galaxies, for which the galaxies Helou showed me will serve as context. We might be able to shed light on the PAH relationship to metallicity and other possible things (such as radiation field, dust temperature, and geometry).

central stellar densities

On the plane to LA, I sketched out a short paper on using the central stellar densities of galaxies to show that red galaxies are not faded versions of blue galaxies, that if blue galaxies are to fade into red galaxies, there need to be huge star-formation events, and that we can identify the predecessors of present-day red galaxies by looking for blue galaxies with huge surface brightnesses. Working title: Blue galaxies would rather burn out than fade away.



I spent what little time I had on Friday advising students, as I am on my way to the Spitzer Science Center to be the astronomer in residence for a couple of weeks. It should be a time to get my Spitzer programs understood, which will be a big help to me and to Ronin Wu, who is working on our Spitzer imaging and spectroscopy of low-luminosity galaxies.


galaxy groups

Quintero and I discussed his work on duplicating his clustocentric distance measures for galaxies in a numerical simulation. He has made a group catalog analogously to Berlind's in the simulation, and is now "observing" the clustocentric distances, in order to test our hypothesis that his environmental dependences of color and radial profile will not be matched in the simulation.

Berlind, Blanton, and I discussed the relative merits of auto-correlations and cross-correlations for measuring bias and testing hypotheses. I am a huge believer in cross-correlations with dense populations. Berlind is about to test trends in cluster ages with environment.


dark energy, solar systems, LRG accretion

[I ran out of time for posting yesterday, so this post covers two days.]

Eric Linder (LBL) gave a very nice lunchtime talk on Monday about dark energy, including a classification of most dark energy models (ie, physical models) into freezing and thawing types. He showed that the next generation of experiments (eg, SNAP or JDEM) ought to be able to distinguish these types, although more will probably have to be done to resolve a particular model within the type. In the current experiments, there are large degeneracies still available for the physical properties of the dark matter.

In the afternoon on Monday, Roman Rafikov (CITA) gave a beautiful review of what is known about our own Solar System and other planetary systems, and puzzles about the formation of the planets (and in particular their solid cores). Unfortunately I had to leave before he finished!

Today I basically worked on the big, bright galaxies in SDSS, with conversations with Blanton about sky, and Quintero about presentation, if you will.

In other news, Masjedi has a beautiful result: The mass spectrum of merging events for LRGs. He can show that the mass accretion onto luminous red galaxies is likely dominated by L-star and brighter galaxies, and that LRGs accrete at most tens of percent of their masses since redshift unity. This agrees very nicely with other, less direct measurements, so it is a wonderful and productive result.


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.



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.


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.