Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Extragalactic astronomy from the RIT campus

Michael Richmond
Aug 8, 2012

Believe it or not, it's possible to make some contributions to the field of extragalactic astronomy even from the light-polluted campus of RIT. The opportunities arise only rarely, but with a little luck, and good weather, one can take advantage of them to help other astronomers with much bigger telescopes and much better sites.

In August, 2011, just a little less than one year ago, a supernova was discovered in the nearby galaxy M101.

Not only was this much closer than the typical supernova, but it was also discovered very early in its evolution -- about one day after the explosion began. This gave astronomers a rare chance to measure properties of a Type Ia supernova with very high precision.

Let me tell you about our study of this supernova at RIT. I'll provide some background for those who aren't members of the supernova community.

How Type Ia supernovae are used in cosmology

A supernova explosion involves the titanic destruction of an entire star, producing an extremely luminous lightshow for several weeks or months. There are several different types of supernovae, distinguished by features of their spectra.

Massive stars which run out of fuel in their cores and undergo a catastrophic core collapse can exhibit different types of spectra and very different light curves. Type Ia supernovae, on the other hand, are somewhat homogeneous: they all tend to have similar spectra, similar light curves, and similar overall luminosity. That makes them very useful for the study of cosmology, since we can use them as standard candles*

* Danger, danger Will Robinson! See caveats below.

What exactly causes a Type Ia supernova? The answer isn't known -- or, more accurately, there's no agreement on the answer. One possibility is a single white dwarf accreting material from a nearby companion until its mass exceeds the Chandrasekhar limit. Another model involves the merger of two white dwarfs which are in a very tight binary system, again to create a single object with a super-Chandrasekhar mass. There is evidence for both models and the discussion continues.

The important thing is that if one can use Type Ia supernova as standard candles, then they allow us to measure two very important numbers:

What they don't tell you about Type Ia supernovae

This sounds great -- all type Ia supernovae are exactly the same brightness! We can use them easily for all sorts of cosmological tests.

Right? Right?

Well, no, not really.

It turns out that while almost all type Ia supernovae do fall within a relatively narrow range of limits, and while a decent fraction of them really DO seem to be nearly identical, their properties do span quite a range. For example, look at these absolute magnitudes:

Figure taken from Hamuy et al., AJ 112, 2391 (1996)

Some supernovae are more than 1 magnitude (2.5 times) more luminous than others. Does that sound like a good standard candle? No, of course not.

On the other hand, there does appear to be a relationship between the absolute magnitude of a type Ia supernova and other properties. On the horizontal axis of the graphs above is the parameter called "delta m15", which is just the amount by which the supernova fades in the first 15 days after its maximum light. It seems that

There are other properties which correlate with luminosity, and astronomers have devised several methods which attempt to convert the observed luminosity to a "standard" luminosity. After making such corrections, it seems that the truly random scatter among the peak luminosity of (at least some) type Ia supernovae is only about +/- 15 percent.

Astronomers who wish to use supernovae for cosmological purposes need to answer two questions:

  1. What's the best way to correct the observed light curves?
  2. Once we've corrected the light curves, just what is the absolute luminosity of a type Ia supernova at maximum light?

Observations of SN 2011fe at the RIT Observatory

This is where the RIT Observatory and I come into the picture, in our very minor way. The galaxy M101 is one of the closest big spiral galaxies to our Milky Way, being only about 6.7 Mpc (29.1 mag) away from us.

That means that any supernova in M101 will appear much brighter than the average supernova in our skies. And, indeed, SN 2011fe was the third-brightest supernova visible from Earth in the past 25 years (and perhaps in the past 100 years).

  supernova    peak apparent magnitude       visible with 
   1987A             2.97                    naked eye
   1972E             8.68(*)                 binoculars

   2011fe            9.99                    small telescope
   1993J            10.85                    small telescope
   (*) brightest measured value -- discovered after maximum light

The supernova discovery was announced on Aug 24, 2011, at about 7:50 PM Eastern Daylight Time (that's Aug 25, 2011, in Universal Time), by the Palomar Transient Factory.

The next night, Aug 25, I started observing the supernova from the RIT Observatory with our 12-inch telescope and SBIG ST-8E CCD camera. Our images weren't very pretty, but they did show the supernova and several nearby stars of similar brightness which served as references. You can see a list of our observing sessions by going to the RIT Observatory record and scrolling down to the end.

I measured the brightness of the SN in four optical passbands, using Johnson-Cousins BVRI filters.

At first, the supernova was very blue, compared to the ordinary reference stars. An image in the B filter is at left, in the I filter at right. Note that the camera is a LOT more sensitive to light in the I-band!

I tried to make measurements on every clear night. The bad news (scientifically) is that the skies in Rochester aren't clear all that often. The good news (from a mental health standpoint) is that the skies in Rochester aren't clear all that often. Over the next 180 days, I was able to acquire data on 50 nights. The figure below shows all our data, plus that collected by astronomers at Michigan State University.

As August turned to September, and September turned to October, M101 gradually set earlier and earlier. By mid-October, the galaxy was so low at sunset that I could barely see it. Fortunately, M101 is nearly circumpolar from our latitude, so all I had to do was wait six or seven hours for it to rise again in the east.

In February, 2012, the supernova had faded so much that it was hard to measure it accurately, especially in the I-band. Even after co-adding a number of images, the signal was just too small. So, I stopped the observations after Feb 20, 2012.

One of the worries in this business is that, as a supernova fades, its light becomes contaminated more and more strongly by other stars or nebulosity in its host galaxy. That can lead to a systematic error in the photometry which grows with time. I checked the HST archive for images of M101 and found several pictures taken long before the SN exploded. The closeup below is an image in the I-band equivalent filter F814W. The circles are centered on the position of the supernova with radii 0.5 and 2.5 arcseconds. The two nearest bright objects are the multiple star "P" and the single star "Q", which have apparent magnitudes of I=21.8 and I=22.2, respectively. Since SN 2011fe had an apparent magnitude of I=15.2 when I stopped observing it, it was at least 100 times brighter than these nearby objects, and so my measurements were not affected significantly.

What do these observations tell us?

The story of SN 2011fe is still being told. Astronomers continue to measure its radiation at many wavelengths, and many of the observations have yet to be published. Let me mention just a few things which seem clear at this point.

For more information

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.