Deep Washington Photometry of the HST M31-halo field Edward Olszewski, Abi Saha, and Andy Dolphin Many thanks to: MMT staff, SAO Instrument builders, Steward technical staff, Especially… Brian McLeod Dennis Smith Mo Conroy Nelson Caldwell Dan Fabricant Tim Pickering MMT telescope operators David Dean (Steward) Steward Machine Shop

Background M31 and the Milky Way are [approximately] twins, yet M31 has a large bulge M31’s halo is [much] more metal-rich than ours M31 has evidence for massive streams of destroyed galaxies. We have evidence for streams, but not as extreme. Our halo is old and metal poor.

Streams in our Milky Way and the future of the Milky Way The halo is “polluted” with remnants of “visitors from outer space” The most impressive example is Sagittarius (Sgr). Sgr has been almost completely destroyed and now has a “comet tail” of stars tidally ripped from it that wraps more than once around its orbit. It is injecting both metal-poor and metal-rich stars into our halo, and is injecting globulars. In a few billion years, the Milky Way will swallow and destroy the Magellanic Clouds, and the Clouds will inject many relatively metal-rich stars into our halo, in fact, more stars than are there now. And not too long after that, we and M31 will swallow each other, possibly creating a single elliptical galaxy.

M31, its bulge, its halo The relatively huge bulge of M31 and the more impressive streams in M31’s halo point to more impressive galaxy cannibalism. In 2003, Tom Brown et al published their HST H-R diagram of a tiny piece of the M31 hal hours in a broad yellow filter (F606W) and 45.4 hours in a broad red filter (F814W) These are among the deepest images ever taken, and reach 50% completeness at 30 th mag. They, unfortunately, have not released the data products nor the photometry catalogs. But, here is their H-R diagram…

You’ll see the image in a movie in a while… They conclude that the typical M31 halo star is relatively “intermediate age”, 6-10 Gyr old, and relatively metal rich, in contrast to a typical halo star in our Milky Way. Their analysis is fine as far as it goes. But ground-based data can offer value added. They only know brightnesses and one color. Stellar evolution of stars of different ages (masses) and abundances can make stars of a specific color. So they made a not-unreasonable assumption that younger stars are more metal rich, and let a stellar populations computer code tell us the mixture of ages and abundances. But wouldn’t it be nice to KNOW the mixture of abundances, and force the code to mimic that empirical mixture? We WWe W We can measure abundances of the red giants.

Properties of red giants and of the Washington System Red giants in M31 range in brightness from V=22 or so, to V=25 or so. Unfortunately, there are lots of faint galaxies that can mimic stars In ordinary seeing. Washington system designed to measure abundances of red giants using broad band photometry. Broad-band = more light = ability to work on fainter stars. Specifically, the Washington C filter, in the near UV, with central wavelength 3860A, when coupled with filters that give you the stellar temperature (say R-I), measures abundances. Given good enough data, we get abundances of all of the red giants In the HST field at once, without using spectroscopy. Getting abundances Of 24 th mag giants with spectroscopy is hard/impossible.

The data We used minicam in 2003, and megacam in 2004 and 2005, and WIYN’s minimo in 2005/2006. Despite bad weather and telescope problems, we collected sec at MMT in Washington C, and 3600 sec in R and 1800 sec in I. We are in the early stages of the final reductions, and have some gorgeous less than ideally calibrated data (thin clouds). Today we show megacam 2005 data, roughly sec and roughly 0.8 arcsec seeing of the stack. We also show WIYN R data at 0.5 arcsec seeing. Here is a movie (I think we need to step out of powerpoint), of HST data at 0.05 arcsec seeing, WIYN R at 0.5, and Washington C at 0.8…

The movie 0.8 arcsec seeing in “U” is pretty darn good, especially for extended periods. But imagine 0.5, et’s do what it takes to get there…

Histogram of C magnitudes As we said, the calibration is less than ideal right now because of clouds. But we’re close

26 th mag at C is pretty darn impressive… Using the WIYN data, here are color-magnitude diagrams… First, R versus R-I, then C versus C-R

Finally, here is the diagnostic diagram

R-I gives temperature, and C-R gives abundance. BUT, C-R has temperature sensitivity, so you need exquisite R-I. We don’t have that. We therefore must re-reduce a subset of the HST data to get good-enough temperature. We must also co-add the 2004 and 2005 data, and somehow average in the minicam C and the WIYN C, and we need to get improved calibration. We will re-reduce the data using HST positions forced upon the ground-based data, which will improve the photometry.

Final philosophical comments We used specialized filters, but never got enough clear weather. It’s easy to parcel out data in numbers of nights that pretty much guarantee that the data will not be calibrated or competitive. It’s a nontrivial problem to solve this telescope-time dilemma.