The UV properties of elliptical galaxies

A. Gil de Paz (UCM), B.F. Madore (OCIW), S. Boissier, A. Boselli, J. Donas, J.M. Deharveng (LAM), R. Michael Rich (UCLA)

GALEX ATLAS: images …

UV upturn: HB stars, either relatively-young metal rich stars or very-old metal-poor stars.

Young, massive stars

AGN

Origin of the UV light in galaxies

UV properties of elliptical galaxies

Gil de Paz et al. (2006)

E/S0

UV upturn, Residual Star Formation (RSF), AGN?

UV properties of elliptical galaxies UV upturn, Residual Star Formation (RSF), AGN?

Gil de Paz et al. (2006)

E/S0

UV properties of elliptical galaxies

Gil de Paz et al. (2006)

E/S0

Giant ellipticals (cD) - intermediate-mass ellipticals

- dwarf ellipticals (dE):

Transition: strong UV upturn - weak UV upturn - RSF

Boselli et al. (2005)

Dispersion in the CMD

i) (FUV-NUV): strength variations in the UV upturn

ii) (NUV-K): due to different levels of RSF (Yi et al. 2005)

Z

UV properties of elliptical galaxies Giant ellipticals (cD) - intermediate-mass ellipticals

- dwarf ellipticals (dE):

Transition: strong UV upturn - weak UV upturn - RSF

Dispersion in the CMD

i) (FUV-NUV): strength variations in the UV upturn

ii) (NUV-K): due to different levels of RSF (Yi et al. 2005)

Boselli et al. (2005)

Virgo

Discrepancies between diff. authors regarding the UV color - Mg2 correlation:

i) Aperture effects?

ii) Mg2, is not a good measure of metallicity?

iii) 2nd parameter?

iv) FUV-optical is not sensitive?

v) Sample selection?

UV properties of elliptical galaxies

Burstein et al. (1988)

Rich et al. (2005)

Boselli et al. (2005)

Virgo

UV-optical color gradients in ellipticals

In the case of Burstein et al. (1988) both the FUV-B color and Mg2 index suffer from strong aperture effects (IUE aperture, few-arcsec apertures of the Lick indices)

Would this relationship be still valid if the integrated FUV-optical or FUV-NUV colors are used?

Asymptotic

Sextractor

IUE-like

Spectral index and UV colors gradients are fundamental to solve this problem.

Donas et al. (2006)

Peculiar results for M32 by Ohl et al. (1998) using UIT data

UV-optical color gradients in ellipticals

Since M32 is believed to have an important intermediated-aged (a few Gyr old) stellar population, this would point out to that the age might be the second parameter.

M32 (GALEX)

Ohl et al. (1998)Gil de Paz et al. (2005)

However, Ohl et al. (1998) did not find a clear correlation between FUV-B and Mg2-index gradients (second parameter?)

GALEX has shown (Gil de Paz et al. 2005) that M32 is actually not peculiar in its UV properties (see also Brown 2000)

M32

Moreover, Ohl et al. Results have been recently questioned by new GALEX data.

Rhee et al. (2006)

Conclusions Luminous ellipticals show a strong UV upturn. Residual star formation dominates the dispersion in the UV-optical CMD and governs the UV properties of dE.

The origin of the UV upturn is emission from low-mass blue HB stars. The intensity of the UV upturn (and the population of the extreme blue HB) seems to be primarily correlated with metal abundance. The necessity of a second parameter is still to be determined.

Aperture effects (both in the Mg2 and the UV data) were likely playing a role in the correlation observed by Burstein et al. (1988). This correlation gets significantly worse if the integrated UV colors are used. The use of gradients is fundamental.

New FUV-NUV color gradients show a correlation with the Mg2 gradients, although with a significant dispersion. Multi-index gradients are likely to improve the situation (using e.g. Sanchez-Blázquez 2004 set).