Star formation rate (sfr) in the galaxy M51

Liceo Scientifico “Eugenio Curiel”, Padova (PD)

ABSTRACT

The purpose of this experience is to calculate the Star Formation Rate, which is the number of solar masses that annually form, in the galaxy M51, through the acquired spectra from the telescope. It has also been calculated how many ionizing photons are emitted every second, and the theoretical number of stars necessary to warm the whole quantity if them all belong to spectral class O5 or B1.

I. INTRODUCTION

Figure 1: The Galaxy M51, or Whirlpool galaxy. Image from National Optical Astronomy Observatory, NOAO.

In the night of 23^rd February 2011, the spectra have been acquired by a composition of the Pennar Telescope 122cm, that has a Newton-Cassegrain configuration, the B&C spectrograph, and the CCD camera, in two different ways: one was horizontal and rotated 90 degrees while the other one was rotated 45 degrees; both spectra were rotated counterclockwise.

Astronomic coordinates of the galaxy are: right ascension 13h 29m 54s, declination 47° 11' 60''; the Whirlpool galaxy is in the costellation Canes Venatici.

Figure 2: The spectrum of the Galaxy M51, rotated 45 degrees counter clockwise. Image elaborated by ds9, originated from Pennar telescope

Figure 3: The Galaxy M51, or Whirlpool galaxy. Image elaborated by ds9, originated from Pennar telescope.

The program IRAF (Image Reduction and Analysis Facility) has been used to analyse the images.

The Hα line has to be identified among the emission lines to individuate the regions to be studied, which in this galaxy represent the arms and the bulge.

The bulge is the highest peak, while the two regions next to it, the westernmost region and the easternmost one are the four arms of M51.

The second peak from West might represent a star located nearby the galaxy.

Figure 4: Emissions from West to East in Hα line. The peaks are the regions of the Whirlpool galaxy.

Figure 5: Correlation of the two spectra by ds9 and IRAF, and the image of the galaxy. Spectra rotated 90 degrees counterclockwise.

For each region it is necessary to calculate the flux of Hα and Hβ lines.

Through the ratio of the two fluxes it is possible to obtain the reddening, that is the absorption of electromagnetic radiation by the atmosphere of our planet and the gases of the Milky Way.

in which:

The mean reddening is Av=1,419; and it is used to find out the real Hα flux valour.

The redshift is calculated only for the bulge, as it should be the same for the whole galaxy.

For λ_observed valour has been used the central valour of bulge’s Hα line.(λ_theoretical = 6563Å)

From here it can be evaluated the distance thanks to the Hubble's law; where Hubble’s constant is considered 75 km·s^-1Mpc^-1

The distance, combined with the Hα intensity, is used to derive the luminosity of the Hα line.

For each region the area is appossimated to a rectangle, in which the base is the slit’s width and the height is the region's breadth, that is the number of pixels in arcsec, converted in kiloparsec with the two formulas where 1 arcsec=1 pixel.

; k = 4,25 arcsec

; px = region’s width in arcsec

Kpc²

Total area of the galaxy is approssimated to a circle.
Kpc²
Finally the total SFR is figured out by multiplying the total area with the mean density of SFR.
Mass_ʘ/year
In the second part, to calculate how many ionizing photons are emitted every second, it has been used this relation:

This valour permits to give estimation of how many class O5 stars would be necessary to heat the ionized gas;

IV. RESULTS

This experience had the aim to estimate the Star Formation Rate, number of solar masses annually formed, of the galaxy M51, through two acquired spectra from the telescope. Another purpose was to calculate how many ionizing photons are emitted every second, and the theoretical number of stars necessary to heat the ionized gas if all the stars belong to spectral class O5 or B1.
The Star Formation Rate of the galaxy results 7 solar masses per year, that means that in the warmest regions of the arms and in the bulge, every year could be created on average 7 stars dimensioned as our Sun.

The mean SFR in spiral galaxy is about 5 solar masses per year; although the datum found out can be accepted, because the error might be due to the imprecision of the distance value, in fact there is still a difference of about 1 Mpc between the closest and the furthest arm of the galaxy.

The number of stars which belong to O5 spectral class, needed to warm all the ionized gas is nearly 12 stars.

In the same way the number of stars which belong to B1 spectral class, needed to warm all the ionized gas is nearly 2·10⁵ stars.

The second valour is higher than the first because B1 spectral class’ stars are cooler (12000K < T < 25000K) and smaller than O5 class’ stars (25000K < T < 50000K); so more stars B1 than O5 are needed to warm the same quantity of gas.

-the redshift of the galaxy’s centre is 0,001545;

-the M51’s distance is 6,1794 Mpc equivalent to about 20 millions light year (1pc = 3,26 ly);

-the mean valour of the reddening is 1,419.

To find out the bulge’s emission spectrum, the star's continuum spectrum has to be subtracted from the rilevated spectrum. Without the subtraction, the Hα line’s intensity results very lower than the theoretical valour, because the emission lines and the absorption lines are summed up.

Figure 6: Bulge spectrum in which emission lines and absorption lines are summed up.

Figure 7: Bulge spectrum without the stars’ continuum spectrum; the emission lines of the bulge are here higher.

It could be the emission track of a star which is located near the galaxy. However it has been considered in the calculations.

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VI. BIBLIOGRAPHY

-http://seds.org/messier/more/m051_noao.html

-Report on a stage at the Astrophysical Observatory in Asiago, M. Lazzari, M. Rocchetto, I. Vidal, "Misura della Star Formation Rate nelle galassie NGC 1569, NGC 2798 e NGC 3227", 2006/2007 Edition;

-http://dipastro.pd.astro.it/osservatorio/telescopio.html