Why Does Infrared Astronomy Matter?

Simply put, the reason why infrared astronomy is so important is because

1) it enables us to peer through the veil of interstellar dust, which blocks light in the visible wavelengths of the electromagnetic spectrum. It is able to do this because light in the infrared regions pass right through interstellar dust, completely unaffected, thus enabling us to see objects whose light would normally be blocked from our view (e.g. Herbig-Haro objects, stars orbiting the supermassive black hole of our Milky Way, Sagittarius A*, etc.) and

2) it enables us to see objects at extreme cosmological distances, such as galaxies in the very early universe, the light of which is so red-shifted that they are only visible in the infrared. The new infrared detector aboard the newly refurbished Hubble Space Telescope (HST) is able to do this to some degree. I will explain further what I mean by “to some degree” later.

In talking about the infrared regions of the electromagnetic spectrum we’re talking about wavelengths that range between the visible and radio bands of the electromagnetic spectrum. We are talking about a range of wavelengths from 0.8 micrometers (μm)–1 micrometer (or micron) being one millionth of a meter–to about 1,000 μm. Further, infrared radiation has been broken down into smaller and somewhat arbitrary regions, i.e., near-infrared, mid-infrared and far-infrared. The wavelength range of each breaks down as follows:

near-infrared:  0.8 to 8.0 μm
mid-infrared:   8.0 to 30 μm
far-infrared:    30 to 300 μm

It should be noted that some astronomers regard the far-infrared as beginning at ~8-13 μm. But for our purposes will will stick to the table above. Anything above the 300 μm wavelength is considered submillimeter radiation.

The Various Types of Infrared Sources

There are many types of objects, elements, ions and chemical compounds in the universe that emit infrared radiation. Proto-stars such as T Tauri stars (proto-stars which will evolve into stars like our sun but are burning the isotope deuterium because they haven’t become hot enough yet for hydrogen fusion to ignite, so they remain above the main sequence) and the dust clouds in which they are enshrouded are great emitters of infrared radiation. Cool molecular clouds have also been known to be emitters of infrared radiation at wavelengths ranging from 60-200 μm.

Of the elements and ions that emit in the infrared, atomic hydrogen can emit in the near infrared in what is called the Brackett and Paschen lines. Neutral and ionized states of iron, sulfur, nitrogen, oxygen and neon will emit in the mid- and far-infrared region of the spectrum. Doubly ionized oxygen (OIII) and nitrogen (NIII) will emit at specific wavelengths in what is referred to as the forbidden lines of the infrared spectrum.

Peering Through Interstellar Dust

One of the many things in astronomy that has fascinated me for years are Herbig-Haro (HH) objects. These are small peculiar bright nebulae that owe there luminosity to the flux of radiation and/or the shock waves from gas-flows–sometimes bipolar, leading to, in some cases, a string of HH objects–from T Tauri stars or other protostellar object. Ae and Be, a.k.a. Herbig Ae and Be stars (the ‘e’ symbolizing ’emission lines’), which have emission lines in their spectrum because of a shell of surrounding gas and dust, are closely related to Herbig-Haro objects. It has been learned that these peculiar bright nebulae are actually a small part of very large, dark and dense clouds of gas and dust.

A few years ago the Spitzer Space Telescope took an image of a Herbig Haro object cataloged as HH (Herbig-Haro) 46/47. Below is the Spitzer image with an inset image of HH (Herbig-Haro) 46/47 in visible light. I think the difference is quite apparent.

Credit: A. Noriega-Crespo (SSC/Caltech) et al., JPL, Caltech, NASA (Inset: Digital Sky Survey)

Spitzer sees in a range of the infrared that goes from a wavelength of 3 μm in the mid-infrared to 180 μm in the far-infrared. It sees further into the far-infrared than any other space-based or land-based telescope. But since Spitzer’s primary mirror is only 85 centimeters in diameter, the telescope’s resolution at great distance–such as that of the Hubble Deep Field images–is limited. Spitzer simply cannot resolve images of galaxies at that great of a distance. Now the Hubble has greater aperture (2.4 meters) but is not as cool as Spitzer. Spitzer uses liquid helium to stay colder than -262°C. The future James Webb Space Telescope (JWST) will have a segmented primary mirror with an aperture of 6.5 meters and will be much cooler than Hubble, with a passively cooled primary mirror at -225°C. JWST will have a spectral range of viewing from 0.6 μm in the visible (i.e. in the red to orange part of the visible spectrum) to 28.5 μm in the mid-infrared. So, it promises to deliver some breath-taking images as well as some very important data for astronomers.

Closer to home, ground-based observatories such as VISTA, which is part of the European  Southern Observatory (ESO) complex, has produced some remarkable images, such as the one pictured below of the Flame nebula in Orion.

Credit: European Southern Observatory (ESO)

Infrared Astronomy and Cosmology

As mentioned earlier, the ability to see in the infrared is imperative when dealing with vast cosmological distances and viewing objects at those distances. The reason for this is because our universe has been expanding for the last 13.7 billion years and thus the space within it has been being stretched all of that time. Consequently, the wavelengths of the visible light given off by galaxies in the early universe ~13 billion years ago has stretched so much that it is now in the infrared part of the spectrum. Now even though Hubble’s new infrared detector the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) can see in the near-infrared down to a wavelength of 2.5 μm, it still is not able to see as far into the infrared as the future James Webb Space Telescope will.  Below is an illustration of Hubble’s viewing capabilities.

Image Credit: (NASA/ESA/STScI)

But despite Hubble’s limitation as far as seeing into the infrared, it still, in conjunction with Spitzer and other telescopes, ground-based or otherwise, has managed to do some incredible science and obtain some remarkable images as the one shown below. The encircled galaxy below has a redshift of 6.5. This galaxy was already in existence only 850 million years after the Big Bang.

Image Credit: (NASA/ESA/STScI)

I would now like to talk a little bit about the airborne infrared observatory, NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) and then towards the end I would like to conclude with a discussion on how infrared astronomy in conjunction with adaptive optics was used to map out the stars orbiting Sagittarius A* (the asterisk denoting ‘star’, i.e., Sagittarius A-star), thus pinpointing the exact location of the supermassive black hole, Sagittarius A* at the center of our own galaxy.

Stratospheric Observatory for Infrared Astronomy (SOFIA)

NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) is a modified 747 jumbo jet with a German-built Cassegrain reflector mounted in its rear section that is exposed during flight. The primary mirror of SOFIA has a diameter of 2.7 meters and has a very fast focal ratio or f-ratio of 1.28. It is capable of seeing at wavelengths in a range between 0.3 to 1600 μm.

The intention behind the construction of SOFIA was to compliment the Hubble, Spitzer, Herschel and James Webb space telescopes in exploring the universe in the infrared regions of the electromagnetic spectrum. Flying high at altitudes of 39,000 to 45,000 feet, SOFIA will be above 99% of the water vapor in the lower atmosphere. This will allow it greater access to more of the infrared and sub-millimeter spectral range than any ground-based observatories. SOFIA’s capabilities will far exceed those of its predecessor, the Kuiper Airborne Observatory.  Below is an image of SOFIA aboard NASA’s modified 747 jumbo jet.

Credit: (NASA Photo / Carla Thomas)

Credit: (NASA Photo / Carla Thomas)

Sagittarius A* – The Supermassive Black Hole at the Heart of the Milky Way

Eric Becklin

Over forty years ago American astrophysicist, Eric Becklin (pictured above) located a very powerful radio source which was dubbed Sagittarius A. Even though Becklin suspected that these were radio emissions from a supermassive black hole, he had no way of proving it at the time. Knowing that the only way he and the Caltech team to which he belonged could peer through the opaque interstellar dust between them and the radio source was by using an infrared detector. So they bought one from a military contractor and attached to the end of a telescope. And on August of 1966 they went up to Mount Wilson observatory and through the 24 inch telescope they were able to see stars through the interstellar dust. As they continued observing they noticed that the closer they got to the radio source Sagittarius A, the more stars they saw. It was as though some giant force was drawing all of these stars to one central region. Becklin knew then that he was looking at the center of our galaxy. But there was a problem.

One of the biggest obstacles for astronomers is the turbulence in our own atmosphere. It’s what causes stars to appear like they’re twinkling. It’s quite romantic if you’re a young couple out in the country looking up and gazing at the stars. But it is a serious problem when you’re an astronomer and you are trying to capture clear images of celestial objects. So what was Becklin going to do?

Andrea Ghez

Andrea Ghez

Enter the bright and upcoming star of the astronomical community, Andrea Ghez (pictured above) of UCLA. Becklin knew of her work in the then pioneering technology of adaptive optics and solicited her help. So armed with the technology of infrared detectors and adaptive optics, Ghez was able to resolve stars with amazing clarity at the center of the Milky Way.

Pinpointing the Location of Sagittarius A*

So how does one pinpoint the exact location of an invisible supermassive black hole at the center of our galaxy? Very painstakingly! What Andrea Ghez did was take snapshots of stars over a period of many years, thus observing movement of any particular star by comparing a recent image with an older one. Over the course of twelve years using the Keck 10-meter telescopes, Ghez was able to obtain enough astrometric data (1995-2007) and radial velocity measurements made by using Doppler shifts of the stars (2000-2007) to build an accurate model of the orbital trajectories of many stars orbiting Sagittarius A*.

Some remarkable things were revealed in the course of this investigation. For example, stars that came very close to Sagittarius A* at periastron would be traveling away from the supermassive black hole at velocities around ten million miles per hour! Another thing that was discovered in the course of the research is that contrary to the conventional wisdom, stars close to a supermassive black hole are not only not necessarily destroyed but actually are created. Through her Herculean efforts over the years, Andrea Ghez was able to accurately pinpoint the exact location of the supermassive black hole Sagittarius A* with great precision using the orbital trajectories of stars around Sagittarius A*.

To read more about her work and those of her team, you can read the arXiv version of the actual paper they published on their work in PDF format by clicking on the link below:

MEASURING DISTANCE AND PROPERTIES OF THE MILKY WAY’S CENTRAL SUPERMASSIVE BLACKHOLE WITH STELLAR ORBITS

Below is an actual infrared image of the stars around Sagittarius A* and below that are two illustrations of the orbital trajectories of some of those stars as plotted by Andrea Ghez et al.

Sagittarius A*

A view of the center of the Milky Way from a ground-based telescope, with arrows pinpointing the location of Sgr A*. [European Southern Observatory]

This diagram shows the motions of several stars around Sgr A*.

Below is an even more complete and later illustration of the stellar trajectories plotted by Andrea Ghez and her team.

Sagittarius A*

Black Hole at the Center of the Milky Way (from Andrea Ghez at astro.ucla.edu)

So, I hope I have to some degree impressed upon you the importance of infrared astronomy in the investigation of the universe in which we all live.

Copyright © 2010 Eric F. Diaz

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