John Robens' Summer 2008 Progress page

What am I doing?

Looking at optical (R-band) images of ALFALFA HI detections without optical counterparts in SDSS

Why am I doing this? The ALFALFA survey found an HI tidal-like tail structure extending to the southwest of the Virgo Cluster pair NGC 4532/ DDO 137. This is the most massive and longest HI tail structure found in the Virgo Cluster so far. Want to know more about how this tail structure was formed. Possible causes of extended tail: Intracluster Medium: pressure of the ICM hot gas strips galaxy's gas (improbable) Gravitational Interaction: produce stellar streams, but they may be faint if interaction was high velocity. Previous imaging shows no obvious optical counterparts at the positions of these HI clumps from 45 minute exposures. My Project I am looking at 100-110 minute R-band exposures of several fields in the tail (shown right), searching for possible faint optical counterparts. These fields were obtained during observing runs in February and May 2008 at the 0.9-m telescope at Cerro Tololo Interamerican Observatory. Imaging faint structures using CCD's requires correction of sky gradients and instrumental effects. Multiple (9-18) "dark sky flats" or "blanks" were obtained and combined to form a flat correction. The blank fields were chosen to contain other ALFALFA HI detections without optical counterparts in SDSS. (possible dark galaxies!)

Integrated Smoothed ALFALFA HI map of the NGC 4532/DDO 137 complex More about this Virgo Pair, from Koopmann, R. A. et al. 2008 (pdf)





Combined image of the 100 minute exposure of field 1 the blue square is the approximate region of the HI detection color code: red = star, green = galaxy, orange = not registered as object (according to the SDSS) More about how I got this image

Looking for two things: counterparts near the HI clump a larger extended stellar tail in the field that may be very faint Nothing found yet.





Examining the blank fields





SDSS image of one of the blank field exposures used for reduction of field 3. (AGC 249284)

Why is this of interest? The galaxy in the image with the green square, UGC 9305, has a velocity of 2370 km/s. The HI detection has a velocity of 2412 km/s, and is about 40 kpc from the galaxy.

So what does this HI field look like?

After zooming in on the field several times, the following image was obtained.



the crosshairs in the image approximately correspond to the field of interest as determined by the SDSS site

though there does not appear to be a counterpart there, this is a work in progress, and further image reduction may reveal fainter objects (smoothing, etc.)





background image from helios.augustana.edu/astronomy/