View Full Version : How will we see the other side of the galaxy

2004-Nov-13, 04:09 AM
How will we see the other side of the galaxy?

Our imagination has been there we have maps (http://www.coffeenebula.com/gazetteer/layout.php) where TV mini series have taken us there.

Richard Powell (http://www.anzwers.org/free/universe/milkyway.html) has some great links to the evidence we have.

But how can we see of the hill? Over the Galactic bulge?

LBV 1806-20 (http://www.solstation.com/x-objects/1806-20.htm) has brought with it some interesting information about the GMCs between us.

Solstation (http://www.solstation.com/x-objects.htm) has some other Far disk objects.

Is finding and studying thing like the Sagitarius Dwarf (http://www.solstation.com/x-objects/sag-deg.htm) and LBV 1806-20 going to be the way we 'see' through to the other side?

Will we launch probes north and south of the galactic plane and work through the dust and stars with infared and radio?

Are we going to learn to use microlensing of stars to catch glimpses of it?

Or are we just goiing to have to wait till we find a message from an alien civilisation that is polite enough to include a veiw from the top of the hill (Bulge) ?

Is it just not interesting? Or are there two many things that are more interesting?

What do you think?

2004-Nov-13, 07:09 AM
The satellite method won't work... by the time the satelittes were in view of the other side they would be so far away any signal would be so weak we wouldn't even notice it. Among other reasons...

It would require a very specific and unlikely arrangement of bodies to microlense the other side of the galaxy to this side, let alone us being able to see it.

It would be much easier to just look at similar galaxies to know if the grass on the other side of the hill IS greener.

2004-Nov-13, 02:22 PM
We may find a section of the electromagnetic spectrum by which we can observe what's going on over on the other side.

2004-Nov-15, 04:33 AM

It is a tough one. Any image we get of, the other side, will be over 26 000 years old anyway, Unless we find tachyons.

I think of the SIRTF and other infered facilities like KECK, an a wider base interferometry based in satelites.

I Wonder about gravity waves, if they are on the EM spectrum, and a gravity telescopes. Radio has provided most of the evidence on the total structure.

thanks guys :)

2004-Nov-15, 10:55 PM
Increase SETI funding in hopes that an alien civilation has e-mailed us a picture. :P

Massimo Marengo
2004-Nov-26, 07:04 AM
Hello, I am an astronomer working for the InfraRed Array camera of the Spitzer Space Telescope.

Having a detailed map of the structure and the dynamics of the Galaxy would be scientifically very valuable, because it would help answering a lot of questions about the formation and evolution of the Universe, and general physics questions about how gravity works. The quantity of matter that we can see in the Galaxy is not enough to explain its dynamics, and we have to assume the existence of "dark matter" to have enough gravitational pull to keep the Galaxy together (the same is true for other galaxies too). Given the importance of the subject, there are many projects to try to give the best possible answer.

We know that the Galaxy has a spiral structure, with a bar in its center (you can see a schematic map based on our current knowledge here (http://www.ras.ucalgary.ca/CGPS/where/plan/plan_basic_big.gif)). This general structure gives us a basic idea of what we would find on the "other side" of the Galaxy, even though we cannot have a direct look. The spiral arms are filled by diffuse interstellar matter (cosmic dust and molecular gas), that is very opaque to visible radiation. For this reason optical surveys are not very good in providing a map of the Galaxy, especially of the "other side of the Galaxy" which is hidden behind the bulge. The interstellar medium, however, is not so opaque in the infrared and radio, and surveys have a better success at those wavelengths (see maps of the Milky Way at different wavelengths here (http://adc.gsfc.nasa.gov/mw/mmw_sci.html#maps). Even those surveys, however, cannot penetrate the densest regions at the center of the Galaxy. With Spitzer, for example, we are conducting a survey of the whole galactic plane (GLIMPSE (http://www.astro.wisc.edu/sirtf/glimpse-about.html)) that however is excluding the very central region (we have other observations that focus on the galactic center, but they won't see through it).

Microlensing would not work because the light deflection of a microlensing event is minuscole, and certainly not enough to allow the light to pass far enough from the galactic center where it would not be absorbed. Gravitational waves? We still haven't detected them, let alone make images with them. The galactic center may be a powerful source of gravitational waves itself, so any source behind it would likely be very difficult to detect.

The best way for having a good look of the other side of the Galaxy would of course be to travel outside the galactic plane and take an image from above or below. This is however impossible with the current technology, because the time necessary to travel that far is too long for such a project to be practical (the Pioneer 10 (http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNStat.html) probe, which is the most remote object ever made by the human kind, after more than 30 years has still to leave the Solar System, and will need other 2 million years to reach the star Aldebaran which is only 68 light years away). To do such a feat we would need some kind of super-luminal drive (http://en.wikipedia.org/wiki/Alcubierre_drive), which is unpractical for the foreseeable future.

More sensitive radio and infrared telescopes may be able to penetrate the screen of dust and gas of the very dense central regions of our Galaxy. In the meantime we have to content ourselves by extrapolating the general structure of the other side from what we can currently map from this side.

Massimo Marengo

2004-Nov-29, 05:02 AM
Thanks Massimo,

I think the most successful modelling of the other-side is H2 (http://cfa-www.harvard.edu/mmw/background.html) mapping of the Millimeter-Wave Group. By charting the motion of HII star forming regions they came up with this velocity map


Milky Way in Molecular Clouds: (http://cfa-www.harvard.edu/mmw/MilkyWayinMolClouds.html) A New Complete CO Survey
Dame, Hartmann, & Thaddeus (2001)

Hopefully we will develop technology to create a spectro-velocity map of stars based on their spectra.

Maybe the Gaia (http://sci.esa.int/science-e/www/area/index.cfm?fareaid=26) mission will be a huge step in this direction.


Massimo Marengo
2004-Nov-29, 10:01 PM
Hello again.

The map above (which has been made by folks down the hall from my office) is in fact tracing CO and not molecular Hydrogen (H2). Even though molecular Hydrogen is the most common molecule in the Interstellar Medium, it is usually very difficult to detect (in fact, with Spitzer we can observe direct H2 emission, but only in particular conditions, at mid-IR wavelengths). Fortunately, Carbon and Oxigen are the two other most abundant elements that forms molecules, and tend to be locked into CO molecules that are very easy to observe. So observing CO one can accurately trace the structure and cinematic of the molecular clouds in the Galaxy, which are mainly distributed along the spiral arms.

Getting accurate velocity measurements of stars from their spectra is already possible: in fact we can have measurements so accurate to detect the wobble of stars caused by orbiting planets. The only problem with this is that one needs very high spectral resolution and high signal to noise ratio, which means it can be done only with nearby stars that are bright enough. It is unlike we can use this technique to map accurately the Galaxy, especially the stars in or behind the obscured central regions.

Note that spectral techniques measures only radial velocities (the component of the velocity along the line of sight, e.g. how much the star is approaching or receding us). There is no way of measuring the velocity on the plane of the sky from the spectra of the star. Gaia will solve this problem because it is an astrometric mission, which accurately measures the position of stars in the sky (and many of them , about 1/100 of the total stars in the Galaxy). This way Gaia will map with unprecedented accuracy the position and distance (using parallaxes (http://en.wikipedia.org/wiki/Parallax)) of a good portion of the stars in our Galaxy, including their three-dimensional motion (the proper motion on the plane of the sky by repeating observation at different times, to detect the movement of the stars, and the radial velocities by taking simultaneous spectra and measuring the doppler shift of their CaII absorption lines). Gaia will indeed be a huge step in determining the structure of our galaxy and for studying stellar populations.

Massimo Marengo

2004-Dec-08, 03:48 AM
Thanks again Massimo,

Can you say hi to Tom Dame for me?

I had read about the CO being easier to detect, and have seen another radial velocity map on another site, now gone :(

The impression I have of Gaia is that it want be observing further that the galactic center, I will be glad if I am wrong!

Thanks for clearing up the spectral issue for me.