MASS DISTRIBUTION OF A
SAMPLE OF 23 GALAXIES
Olivia Garrido
(Observatoire de Paris-Meudon)
C. Carignan, P. Amram, C. Balkowski, M. Marcelin
THE DARK MATTER PROBLEM
At the end of the seventies, thanks to the radioastronomy :
→The rotation curves are constant at large radius oppositely
to the luminous profile (Freeman, 1970; Bosma, 1978; van Albada,
1985; Begeman, 1987).
Problem : 90% of the total mass of a galaxy is not visible and only
detectable by its gravitational effects.
3 solutions : →MOND (modification of the Newton law at the scale
of galaxies; Milgrom, 1983).
→ Magnetic model (Nelson, 1988; Battaner et al.,
1992).
→ Assumption of the presence of dark matter.
How to study the distribution of the dark matter?
We need to know : - Light distribution (stellar mass).
- Gas distribution (gas mass).
- Rotation curve (total mass).
Hybrid rotation curves (RC) combining Hα+HI data
are necessary :
-HI RC are affected by beam-smearing & low spatial
resolution (around 15' or 30').
-Mass distribution parameters strongly depend on the inner
slope of RCs (Blais-Ouellette et al., 2001; Amram&Garrido, 2002).
Hα ROTATION CURVES
→provided by the GHASP survey.
GHASP ≅ Gassendi HAlpha survey of Spirals
• 220 nearby isolated Spirals and Irregulars.
• Fabry-Perot observations at the O.H.P (France).
• 2D Hα velocity fields (1.7''/pixel; 5.8''x5.8'')
• Rotation curves with high spatial (3'') and spectral
resolution (around 5 km/s) combined with HI rotation
curves for the spatial extension.
• Accurate determination of the inner slope of the RCs.
see poster N°2.33 about GHASP
THE STATE OF THE ART…
• Observational point of view :
-Dwarfs and Irregulars mainly studied (Blais-Ouellette et
al., 1999, 2001; Swaters, 1999; Côté et al., 2000; de Blok &
Bosma, 2003; Gentile et al., 2004).
-Dark halos have constant central density : galaxies have
core halos.
• Theoretical point of view :
-Dark halos are cuspy (Moore et al., 1994, 1998, 1999;
Navarro et al., 1996, 1997) : the density of dark matter
increases in the central parts.
• Our sample : 23 galaxies ranging from Irr to Sb galaxies.
Dark matter density profiles
the density profiles family of Zhao (1996) :
ρ (r) =
ρo
r α
(1 + ( ) )
ro
β −γ
α
r γ
( )
ro
Two profiles adopted :
NFW (cuspy halo) :
(α, β, γ) = (1,3,1)
Isothermal sphere (core halo) : (α, β, γ) = (2,3,0)
ro, ρo : free parameters
Difference between the NFW and the Isothermal sphere profiles
Our 23 hybrid RCs are best fitted with a density profile of core
halo.
← IsoT sphere profile
Very good fit
NWF : not able to fit the
inner parts of the RC →
Letting the inner slope of the dark matter, γ, free to vary,
no value of γ>0.8 was found!
No correlation has been found between γ and VMAX (the
maximal rotational velocity) : disagreement with the models of
core halos formed by violent ejection of baryonic matter
(Navarro et al., 1996; Gelato & Sommer-Larsen, 1999; van den Bosch et al.,
2000).
CONCLUSION
•Our results based on high resolution kinematical data
(combining Hα and HI data) suggest that dark halos
have constant central density.
•No value of γ above 0.8 : in total contradiction with the
results deduced from cosmological simulations which
predict values of γ>1.
•This work extends to Sc/Sd galaxies the result
previously found for dwarf and irregular galaxies.