- #1
fab13
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Homework Statement
I have the following questions as homeworks and I would like to get help.
Here's some informations given to help us to answer :
Photometry :
U=11.60
B=11.16
V=10.20
Redshift : z= 0.00780
Central velocity dispersion : ##\sigma_{v}## = 210 km/s
Introduction :
The elliptical galaxy above has been extracted from a survey in the local universe. The table
presents the magnitudes in three different filters, the redshift and the measured central velocity
dispersion.
3. Questions and attempts to solution
Question 1:
Knowing that the Faber-Jackson relation in the B-band filter can be written as: ##log \sigma_{v} = -0.1 M_{B} + 0.2##
and the absolute magnitude of the Sun in this band is ##M_{B} (Sun)= +5.48##, compute the distance to this galaxy (in Mpc) and its luminosity in the B-band (in solar units).
My element of answer : If ##\sigma_{v}## is given, then we infer ##M_{B}##, then I apply the relation of absolute magnitude :
##M = -2.5 log_{10} (\dfrac{L}{4\pi (10pc)^2}) + C##
So I put ##M_{B}## instead of ##M## in above expression and $L$ the luminosity of galaxy.
Can I determine ##C## constant by setting : ##M_{B,sun} = 5.48## and ##L=L_{sun}=10^{26}## Watt ?
Once I have ##C## constant, How can I get the distance of the galaxy ?
Question 2: Estimate the total mass of this galaxy inside a radius R=20 kpc, and the M/L_{B} ratio (in solar
units). You can use the following values for
G = 1.327 1011 km3 s-2 M_Sun -1 and 1 kpc = 3.086 1016 km
Answer 2: Can I use here the formula sor SIS model, i.e :
##M = \dfrac{2\sigma_{v}^2\,R}{G}##
in order to get the total mass of Galaxy ?
and what about the value of ##L_{B}## luminosity ?
Question 3: Suppose that all stars in this galaxy were born in a single major-merger burst event about 10 Gyr ago. From this original burst, what is the fraction of stellar mass still surviving as stars in the main sequence ? Use a Salpeter IMF, and a star formation range between 0.1 and 120 solar
masses.
I saw that Salpeter IMF is equal to : ##\xi(M) = \xi_{0}M^{-2.35}##
but how to deal with the fraction of stellar mass still surviving as stars in the main sequence ?
Question 4: If the luminosity in the B band is dominated by stars of in the RG branch, with masses ##m ~ 1M_{sun}##
(within ~10%) and average luminosities ##~1000 L_{sun}##, estimate the total stellar mass of this galaxy using the same assumptions as in question 3 regarding the IMF. Comment the result
Elements of answer : it seems that I have to take into account, for the total stellar mass, the stars mass of main sequence and the red-giant mass, doesn't it ?
How to perform this calculus ?
Homework Equations
: none equations are provided for this homework[/B]
Sorry if the answers are simple but I am a beginner in galactic and extra-galactic matters.
Even short answers or suggestions could help me to solve these different questions. Regards