Peculiar Arp87 - tidal effect, peculiar redshift? (1Nov2007)

[craigellachie]

If our Solar system were in this galaxy, say, in the trail between the two, what would the tidal forces effect on Earht? I realise that the time frame is huge, but are there any effects we would notice passing close to another galaxy, long term or short term?

[orin stepanek]

http://antwrp.gsfc.nasa.gov/apod/ap071101.html
I don't feel that there would be much of an effect. Consider that we are in orbit around the galaxy and moving at a great amount of speed as is; also bound by the sun and earth's gravity and no tidal forces are felt. The nearness of the moon does affect the ocean but that is because of nearness to Earth. I'm not an expert but that is my opinion.
Orin

[Qev]

I agree, on the scale of something the size of a galaxy, the tidal forces across a distance as small as that of a solar system would be effectively insignificant. I imagine that close encounters between stars themselves may become somewhat more frequent, though, and that could certainly cause problems for an unlucky planetary system.

[BMAONE23]

but what a sight in the night sky

[redpark]

in the full resolution picture, there are three background galaxies in a line more-or-less tangent to the top of the face-on galaxy. i'd expect more-distant galaxies to be both smaller in apparent size and more red-shifted in colour. why then is the one on the right both bigger and redder than the one in the middle?

i'd guess that the red colour is not the result of red shift. what then explains it?

thanks, ed

[astro_uk]

Which ones exactly do you mean?

There are some objects that are probably just red stars in the milky way, there is one small very red spiral that is visible between the two galaxies, this is probably just an S0 galaxy, a spiral which has stopped forming young blue stars and now only has the old red stars left.

Part of the issue is also probably just due to the scaling chosen to represent the colours, this is essentially arbitrary because it is impossible to exactly match the response of the filters used to take the images to the colour response of the human eye, so often people make choices that look pretty but don't necessarily represent the colours as the would appear to us.

[Nereid]

More on apparent colour: this HST webpage gives the following info:

Color:
This image is a composite of many separate exposures made by the WFPC2 instrument on the Hubble Space Telescope. Three filters were used to sample broad wavelength ranges, while one filter was used to sample a narrow wavelength band. The color results from assigning different hues (colors) to each monochromatic image. In this case, the assigned colors are:
F450W (B) blue
F555W (V) green
F656N (Halpha + [N II]) and F814 (I) red

Among other things, note that the I filter is an infrared one; if you looked at an object through a filter which allowed through only light in this broad band, you'd see nothing at all, because the neurons associated with rods in human eyes do not fire (at a rate which the brain interprets as light).

[redpark]

thanks for the very helpful replies.

for the record, the three objects i referred to are located at:

(-207, 761)
(-818, 954)
(-962, 876)

where the coordinates are measured in pixels from the lower right corner of the full resolution (2302 x 1176) image.

[Chris Peterson]

in the full resolution picture, there are three background galaxies in a line more-or-less tangent to the top of the face-on galaxy. i'd expect more-distant galaxies to be both smaller in apparent size and more red-shifted in colour. why then is the one on the right both bigger and redder than the one in the middle?

Redshifted objects do not appear redder. Their actual appearance depends on what wavelengths are dominant, and where they end up after they are shifted. Don't forget that short wavelengths are shifted up into the visible range, and long ones are shifted out of the visible range. A highly redshifted object might look blue if it was a strong ultraviolet source.

[Chris Peterson]

but what a sight in the night sky

I think most of what is going on would be invisible from either galaxy, just as most of our own galaxy is invisible to us. You'd certainly see nothing in the sky brighter than the Milky Way, although from the inner edge of the galaxy on the right, it looks like you'd have a nice face on view of the other galaxy. You would have a Milky Way on one side of the sky, and a Milky Disc on the other.