Astronomical Observatory: Cool Images

Astr212 Galaxy Projects, Spring 2005

M91, a barred spiral galaxy, Andrew Butler

Introduction:

M91 is located 60 million light-years from Earth. It is an object that astronomers classify as a "galaxy."
Galaxies are immense groupings of stars that contain anywhere from millions to hundreds of billions of stars.
In between galaxies, there are almost no stars. The earth itself resides in a galaxy called the Milky Way
Galaxy. All the stars you can see at night reside in the Milky Way, plus a lot more stars you can't see with
your naked eye, either because they're too far away or not bright enough. For the same reason, we can see
only a few galaxies with our naked eyes. However, with large telescopes, astronomers have been able to see billions of other galaxies! Not only this, but astronomers have also found many types of galaxies, with a wide variety of shapes, sizes, and properties. In general, galaxies are divided into three categories: spirals,
ellipticals, and irregulars. Spiral galaxies, as their name suggests, have various spiral arm patterns. They are characterized by the size of their central bulges, which are dense groupings of stars, and how tightly their
arms are wound. Some spirals, like M91, even have a prominent bar going straight through their bulges, from whose ends the arms appear to begin. These kinds of galaxies are called barred spirals and are
characterized in ways similar to those of regular spirals. Two characteristics of barred spirals that make them distinct from regular spirals are that their arms can start either from a ring located around the bulge or from
the bulge itself. Galaxies that appear to be somewhere in between these two characteristics are given their
own designation. M91 is such a galaxy. It has a medium-sized bulge and intermediately wound spiral arms
that appear to start from an area in between the bulge and the ring, so it is given the designation SBb(rs).
The SB stands for the fact that it’s a barred spiral, the b stands for it being in between a and c (large bulge
with tightly wound spiral arms and small bulge with loosely wound spiral arms, respectively), and the rs stands
for it being in between r and s (spiral arms starting from a ring and spiral arms starting at the bulge,
respectively).

Amazingly, it has only been a couple hundred years since humans have known about galaxies. The 18th
century French comet hunter, Charles Messier, discovered many fuzzy, faint objects in the night sky
previously unknown to mankind, a lot of which we now know are galaxies. He catalogued these objects and compiled them into what is currently known as the Messier catalog. On one particular night, March 18, 1781,
he discovered eight nebulous objects in the same region of the sky, which we now know are galaxies in the
Virgo Galaxy Cluster. He catalogued the last object he found as M91 (the M stands for Messier).
Unfortunately, he recorded an incorrect position for it. For many decades, M91 eluded the eyes of
astronomers. Finally, almost 200 years later, in 1969, amateur astronomer William C. Williams found its
correct location.

Today, we know that M91 is moving away from us at a speed of about 400 km/s, which is about 900,000
mph! This particular speed is peculiar because the Virgo Cluster is moving away from us at a speed of 1100
km/s, so M91 must be traveling toward us at 700 km/s relative to the other galaxies in the Virgo Cluster.

Structures and Colors in the image above:

The middle of the galaxy is orange-yellow. This indicates that there are a lot of older, redder, and low-mass
stars near the bulge. There doesn't seem to be many dust lanes in that area, so this means that new stars probably won't be forming very much in that area of the galaxy anymore (dust and gas compress to form
stars). The spiral arms are definitely emitting blue light, which is produced by blue stars. Dust lanes appear
as dark lines blocking the light of these stars. This indicates the presence of a lot more dust than in the
bulge. Since blue stars don't live very long, this means that there has been a lot of recent star formation in
the spiral arms. The other galaxy visible to the lower left of M91 shows these characteristics as well
(orange-yellow bulge, blue spiral arms). Since elliptical galaxies have used up most of their gas and dust,
and thus don't contain many young blue stars, this other galaxy must also be a spiral.

Multi-wavelength Images:

The above image taken at near infrared wavelengths shows the distribution of K and M stars in M91, which
emit most of their light in the near infrared. Since K and M stars live a very long time, M91 must be relatively
old and star formation must have occurred in its early history, not just its recent history.

The above image shows the Doppler shift of the carbon monoxide (CO) emission in M91, which has a
wavelength in the radio part of the electromagnetic spectrum. The CO emission in this image is produced
when a CO molecule drops down into a lower energy level because of a change in rotational inertia. Since molecular clouds emit at this wavelength, the image shows the distribution of molecular clouds in M91. The
darker spots show where the emission is redshifted (moving away from us) relative to the center of the galaxy
and the lighter areas show where it is blueshifted (moving toward us) relative to the center of the galaxy.
Since the top half of the central area is blueshifted and the bottom half is redshifted, M91 must be rotating clockwise.

Light Profile:

A light profile maps the light of the galaxy. In other words, it shows how bright the galaxy appears at various
radii from its center. Using a light profile, the scale length of a spiral galaxy can be determined. The scale
length is the distance over which the light emitted by the galaxy decreases by 1/e (e ≈ 2.7), or approximately
1/3.

For the major (apparently "taller") axis of M91, the natural log fit was:

y = -0.036x + 7.072 (y = mx + b)

where y is the natural log of the brightness, m is the negative inverse of the scale length in pixels, x is the
radius from the center of M91 in pixels, and b is the natural log of the central surface brightness. The radius
was measured in pixels at first because the image dimensions were measured in pixels. The radius will be converted to kpc later (1 kpc = 3260 light-years). The uncertainties on m and b were:

Δm = ± 1.235 × 10^-3 pixels and Δb = ± 0.110

Using the values in the equation, the scale length for the major axis of M91 was about 28.0 ± 0.97 pixels.

For the minor (apparently "shorter") axis, the fit was:

y = -0.052x + 7.715 (y = mx + b)

with uncertainties:

Δm = ± 7.81 × 10^-4 pixels and Δb = ± 0.048

Thus, the scale length for the minor axis of M91 was about 19.3 ± 0.29 pixels.

To find the linear, actual size of the scale length of M91, the scale of the image, 1.18 arcseconds/pixel, and
the distance to M91 had to be used. Approximating M91's distance as 18 Mpc (1 pc = 3.26 light-years), the distance to the Virgo Cluster, the scale lengths came about to be:

Major axis = 2.8 ± 0.1 kpc

Minor axis = 2.0 ± 0.03 kpc

A larger scale length means that the galaxy has a slower decrease in brightness than a galaxy with a
smaller scale length. M91's scale lengths are somewhat average.

Using these quantities, the angle at which the galaxy is inclined relative to the plane perpendicular to our line
of sight can be calculated. For M91, it came out to be:

Inclination angle = 46°

References:

Elmegreen, D.M. 1998, Galaxies and Galactic Structure (New Jersey: Prentice Hall).

Jarrett, T. H. et al. "The 2MASS Large Galaxy Atlas", 2003, Astronomical Journal 125, 525

Knight, J D. Messier Catalog M91 - M100.

Kutner, M. L. 2003, Astronomy: A Physical Perspective, 2nd ed. (Cambridge: Cambridge University Press). Sections 14.5.3, 16.5, 17.1.

Sofue, Yoshiaki et al. "The Virgo High-Resolution CO Survey: I. CO Atlas." Publications of the Astronomical
Society of Japan 55.1 (2003): 17-58.

Students for the Exploration and Development of Space: M 91.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and
Space Administration.