The radial velocity measure of nearby galaxies

We propose to demonstrate the capacity to measure radial shift of nearby galaxy by using a low cost spectrograph, the Alpy 600 (from Shelyak inst.), and a modest aperture telescope, a 200-mm Newtonian (Takahashi CN212). A perfect educational project for student. The objects concerned are Messier 31, Messier 33 and
NGC 7331.

The observation is made from the Observatoire de Haute-Provence spectroscopy course 2013 - South of France (August 2013). A view of the setup:

The main scientific camera is an Atik460EX model (used in bin.1x1 mode). The Alpy 600 spectrographe is equipped with the optional guide module and optional calibration module. The guiding camera is an Atik314L+. The telescope focal length is 850 mm.

Messier 31 observation

Messier 31, the Andromeda Galaxy, is the closest large galaxy visible to the naked eye in the Northern Hemisphere. It is 2.5 million light-years away and is part of the

Image of observed Messier 31 field taken with Atik314L+ guiding camera (15 seconds exposure). The black is the spectrograph entrance slit (here a special "photometric" slit designed for observation of variables stars - at the upper part the slit width is 23 microns, in the lower part the width is 230 microns).

The Alpy 600 spectrum of M31 nucleus region. Stack of 4 x 600 seconds exposure. The most visible absorption lines are indentified. The lines (or bands) noted Atmo. are from Earth atmosphere molecules, not from the galaxy. The wavelength are in rest value (for a not moving object).

The full calibratred spectrum of M31 central region:

Messier 31 spectrum plot. The spectral power resolution given by the spectrograph is near R = 600 in the red.

Left the observed spectrum of M31. Right, the spectral profil of some stars taken with the same setup. A hot star (HD221756), a medium temperature star (HD225239, spectrum similar to Sun) and a reletively cool star, Aldebaran (alpha Tau). The Messier 31 central region spectrum match more with Aldebaran spectrum. Clearly, this part of the galaxy contain red cool objects (stellar population I).

Now, direct comparaison of M31 and Aldebaran spectra on the same graph:

The black spectral profile is for M31, the red spectral profile is for Aldebaran.

The comparaison is not easy because the difference of continuum shape. All is most simple is the continuum is rectified (the low form is removed), here by using ISIS interactive "Continuum" function:

It is now evident... all the spectral lines of Messier 31 are blue-shifted relative to Aldebaran. Clearly, M31 is is approaching our Milky Way! We now compute the relative radial velocity of Sun and the galaxy.

It is important to take into account the proper velocity of Aldebaran. SIMBAD shows that Aldebaran star away from the Sun at the speed 54 km/s:


We use the cross-corellation tool available into ISIS for compture the relative radial velocity of M31:

The operation of the function is obvious. Simply provide both profile (here limited to the interval 3800 - 6600 A to avoid taking into account the telluric lines). Note also taking into account the Aldebaran speed.

ISIS return the result : V = -305 +/-50 km/s. The error margin +/-50 km/s is from the typical calibration error of Alpy 600 spectrograph (+/- 0.5 A) and correlation precision in presence of noise. Here the value given by SIMBAD:

The agreement between the experimental value and the official value is very good (but with a margin of error of + / -50 km/s, do not forget)!

Messier 33 observation

 The next example concern Messier 33 galaxy in the Local Group. It is a faint surface object. So, for measure the galaxy radial velocity I decided to use the emission lines of a bright gaseous nebulae occupying the arms: NGC 604. Here is a view of the nebula in the Alpy 600 camera guiding system (30 seconds exposure):

The extragalactic nebula NGC 604 centered on the 23 microns entrance slit of the specrtrograph.

2D spectrum of NGC 604 nebulae in Messier 33. Stack of only 2 x 600 seconds exposure with Alpy 600 and 200 mm telescope.

Spectral profile of NGC 604.

The radial shift is measured from hydrogen and oxygen lines:


Rest wavelength (air)

Observed wavelength (air)

Radial velocity


4861.36 A

4857.33 A

-249 km/s


4958.91 A

4954.74 A

-252 km/s


5006.84 A

5002.76 A

-244 km/s


6562.80 A

6557.97 A

-221 km/s




-241 km/s

For the date the Earth's orbital velocity around the Sun is approximately 27 km/sec toward the galaxy:

The mesured heliocentric veolocity of M33 is finaly: -241 + 27 = 214 +/- 40 km/s.

The SIMBAD radial velocity for M33 is -182 km/s. The order of magnitude of the velocity is find. As for M31, the M33 galaxy is moving towards the Milky Way.

The history of radial velocity measurements of M33 (from


But in Rosa & all, A&A, 130, 29-38, 1984, the heliocentric radial velocity of HII region NGC 604 is indicated to -240 km/s, different from the mean galaxy velocity, a value coherent with my typical RV error evaluation (see

NGC 7331 observation

The galaxy NGC 7331 is a spiral galaxy external to the Local Group, at about 40 megalight-years. It is observed during the same session of M31 and M33.

The image of NGC 7331 projected on the entrance slit during spectra acquisition (30 sec. exposure).

2D spectrum of NGC 7331. Exposure time of 3 x 600 sec (30 minutes). Date 10.071/08/2013. OHP

Comparison of NGC7331 spectrum (black plot) and Aldebaran spectrum (red plot). The red shift of NGC7331 is evident (see Na I line for example).

The measured radial velocity for NGC7331 is +728 km/s (I do not consider here the difference in heliocentric velocity between NGC 7331 and Aldebaran because the two objects are close in on the celestial sphere). The SIMBAD value is +818 km/s.

We start by measuring the expanding Universe !