Gas stripping and its Effect on the Stellar Populations of Virgo Cluster Galaxies Hugh H. Crowl UMass with Jeff Kenney (Yale)‏ Jacqueline van Gorkom (Columbia), Aeree Chung (NRAO), Bernd Vollmer (CDS), Jim Rose (UNC:CH)‏ Image by A. Chung

Stellar Populations in Outer Disks “red” outer disk blue outer disk SDSS images from website of D. Hogg Older Stellar Population Stripped longer ago? Younger Stellar population Stripped more recently? The age of the last generation of stars will serve as a chronometer to determine when and, thereby, where in the cluster galaxies are stripped.

Evolution of a Stellar Population As a stellar population ages, two of the most dramatically changing quantities are: –UV color (GALEX)‏ –Balmer lines (Optical Spectroscopy)‏ Bruzual & Charlot (2003)‏ Bruzual and Charlot (2003) single burst stellar population evolutionary model. HH Starburst 99 Models HH HH HH

Observations Observed on the 3.5m WIYN telescope using the SparsePak formatted field spectroscopy array λ = 4000 Å – 6700 Å 5.5 Å FWHM resolution SparsePak Array: 75 fibers in an 80” x 80” field GALEX photometry of the outer disk, combined with optical spectroscopy, allows us to characterize the SF history for the youngest stellar populations.

NGC 4522 Edge-on spiral galaxy in Virgo 3.3° (~1 Mpc) from the cluster core One of the best known cases for ongoing ISM-ICM interaction Kenney et al (2004)‏ Vollmer et al (2004)‏ Polarized Radio Continuum on HI 40% of the HI is extraplanar Polarized Radio Continuum at “leading edge” of interaction Estimates of Ram Pressure from simple models are 10 times too weak to account for observed stripping!

NGC 4522: Spectral Observations SparsePak on RSparsepak on Hα Galaxies observed beyond the gas truncation radius, and still within the stellar disk. Crowl & Kenney (2006)‏

NGC 4522: A Local Analog to k+a Galaxies? Outer disk spectrum suggests recent stripping, and is similar to “k+a” spectra at higher redshift. Dressler et al. (1999)‏ Crowl & Kenney (2006)‏

GALEX photometry constrains the recent star formation FUV-bright outer disk GALEX FUV + NUV Image log(F FUV /F V )‏ log(F FUV /F NUV )‏ Galaxy was stripped in its current location, far from the cluster core!

VIVA: VLA Imaging of Virgo in Atomic gas

Comparison of Stellar Population Ages to Gas Stripping Simulations Agreement of truncation times with timescales from HI simulations implies: Truncation timescales are basically correct Assumed star formation history (i.e. truncation) is reasonable; there are no large bursts of star formation at the time of stripping. Star formation is cut off when HI is stripped

Age vs. Cluster Location If galaxies are only stripped in the center of the cluster, the galaxies with the youngest outer disk ages should be closest to M87 While some disks are consistent with beings stripped in the core, the outer disks of some galaxies are too young to have been stripped there.

Global Color Evolution While we can study galaxies in Virgo in detail, it is much harder to observe the effect of transformation on galaxies in higher redshift clusters –we need to understand the effect of transformation on global properties The global colors of our VIVA galaxies are quickly affected by the quenching of star formation. I3392 N4522 N4405 N4569 N4388 N4424 N4064 N4580 N4419 t q ~100 Myr t q ~ Myr t q >400 Myr Contours: 140K+ SDSS Galaxies (Blanton et al. 2003)‏ Crosses: VIVA Galaxies

I3392 N4522 N4569 t q ~100 Myr t q ~ Myr t q >400 Myr older interaction recent interaction

Summary NGC 4522, an example of ongoing stripping, has a young stellar population in its outer disk despite its location in the outer cluster. –In Virgo, some galaxies are stripped outside the core. It appears that galaxies can have their gas stripped at much larger cluster radii than typically assumed. The three of our sample galaxies that overlap with gas simulations have truncation ages that agree with the simulation estimates. –Star formation ends shortly after the neutral gas is stripped Global galaxy colors are affected on timescales of ~400 Myr. Stellar populations of stripped spirals provide us a unique and useful tool for probing the interaction history of cluster galaxies.