GalexView - surveying the Sthn sky with UV eyes

[madbadgalaxyman (Robert)]

One of the most interesting "virtual telescopes" on the internet is GalexView, which enables you to view objects, pan, and zoom, over large areas of the southern & northern sky.

This software "telescope" shows you the ultraviolet sky as seen by the GALEX satellite; it is somewhat similar to an electronic star chart, but with imaging data from the GALEX telescope shown in a large screen display.

The address is:
http://galex.stsci.edu/GR6


When you use it, make sure that you:
(1) Click on "full resolution" in the control panel.
(2) Click on the "full screen" icon, which is a very small dark-coloured icon (about the fourth icon from the left), so as to show a bigger display. The icon seems to have a small galaxy on it.
(3) Activate the "display graphics" tick box, which has the effect of putting a red target ring around the object you have asked the telescope to point at. This can be very important, because sometimes, when you ask the virtual telescope to point at an object, it does not exactly centre on it.
(4) De-activate the "display objects" tick box, to remove a multitude of of overlays from the image.

The only deficiency of the displayed images is that the resolution is unpredictable. The images are sometimes extremely noisy, but at other times they have been cleaned up.
Sometimes, depending on precisely what co-ordinates or object you ask the software to point at, GalexView may point to two different versions of the same imaging data.

So...... what can you see in these GALEX images?

- The colour Yellow in the GALEX images codes for near-ultraviolet (NUV) light, which mainly traces an older stellar population in galaxies. For instance: the bulges of spiral galaxies, the light from Elliptical galaxies, the smooth component of spiral arms(old stars & no star formation).
- The colour Blue in these images codes for
far-ultraviolet (FUV) light. This mostly comes from very young and massive stars, e.g. in spiral arms.

The remarkable thing about GALEX images is that the blue channel (FUV) is extremely sensitive to those areas of the universe where massive and hot stars are forming in the universe, today. Often, this current star formation is shown by GALEX to be where we expect it to be, for instance in spiral arms. But at other times, there are massive surprises;
e.g. many gaseous and/or optically-luminous Tidal Tails show evidence for current star formation, and perhaps even the formation of dwarf galaxies. Also, some field S0 galaxies, supposedly "old and red and dead", show evidence for current star formation in the central part of the galaxy.

Note: It can be a waste of time to point this virtual telescope at an elliptical or an S0 galaxy, as it may hardly show anything; these objects typically emit very little ultraviolet light. In contrast, even very-low surface brightness Irregular Galaxies light up like flares in these ultraviolet images!!

Just for fun, here are the GALEX images of NGC 2841 (see the previous discussion about its anomalous outermost extensions in the Deep Sky Images forum) and NGC 1672.





cheers,
madbadgalaxyman

[Suzy]

Thanks Robert, I'm going to have so much fun with this!

[madbadgalaxyman (Robert)]

Yeah, the GalexView virtual telescope is a lot of fun!
The interesting thing about it is that sometimes there is little to see......yet at other times there are big surprises.

Putting all-sky imaging data into a "virtual sky" interface such as:
//server1.wikisky.org
and
//skyserver.sdss.org/dr8/en/tools/chart

makes it a lot of fun to explore the sky.

I see this technology as potentially enabling amateurs to find peculiarities, anomalies, unusual phenomena, etc., which have escaped the notice of professional astronomers.
(it is already happening with SDSS and the Galaxyzoo collaboration)

cheers,
madbadgalaxyman

[madbadgalaxyman (Robert)]

As mentioned previously, the really beautiful all-sky Virtual Telescope interface that allows us to access Ultraviolet Images from the GALEX satellite, has the major drawback that it sometimes displays two different versions of the same field, depending upon which object you actually ask it to point at.

Here is an example of this problem (sorry, it's northern). This is the interesting and little studied NGC 198 & NGC 182 field in a distant supercluster of galaxies that is nominally 5 times as distant as the Virgo Cluster.

Depending upon whether I use NGC 198 or NGC 182 as the target object, I get either this:



, or this:




These three distant spiral galaxies are small, no more than
1 to 1.5 arcminutes across, so the cleaned version of this field makes all the difference. In particular, NGC 182 has a very fine example of a star-forming ring.

I have not yet found a way to get around the problem that sometimes very poor data is displayed even when better data is available!

cheers,
madbadgalaxyman

[trogers]

madbadgalaxyman,

GALEX contains many surveys of various quality levels (or exposure depth). You can filter your search results to just the deeper surveys by clicking on the Survey Column, then select the desired survey in the histogram to the lower left:

AIS: All Sky Survey (shallowest/lowest quality)
MIS: Medium Imaging Survey
NGS: Nearby Galaxy Survey
GII: Guest Investigator Survey
DIS: Deep Sky Survey (deepest/highest quality)

The exposure time column is also an indication of the quality of the image and can be filtered as well.

You can also adjust the brightness/contrast of the loaded in the lower right imaging controls panel. (shown in the screen snap)

Since so many of the GALEX survey images (tiles) overlap the same region of the sky, identical objects are extracted from each. The AIS survey overlaps itself quite often, since it is carpeting the entire sky with a circular radius. (Think of the Olympic Rings). So when an object is selected in the result grid, the image it was extracted from is loaded into the viewer. This behavior tends to cause confusion.

I attached a screen snap demonstrating the filtering and contrast controls.

I hope this helps,
Tony Rogers
GalexView Developer