Extragalactic stellar astronomy with

the brightest stars in the universe

Rolf Kudritzki, Fabio Bresolin, Miguel Urbaneja

Extragalactic stellar astronomy

Properties of stellar populationsEvolution of galaxiesChemical abundance and abundance pattern gradientsInterstellar extinctionDistances

Quantitative stellar spectroscopy of individual stars

in galaxies beyond the Local Group

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A supergiants – objects in transition

B8–A4

Brightest normal stars at visual light: -7 ≥ MV ≥ -10 mag

tev ~ 103 yrs L, M ~ const.

ideal to determine• chemical compos.• abundance grad.• SF history• extinction• extinction laws• distances

of galaxies

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Spectroscopic studies beyond LG – present work

• selection of targets from wide field CMDs

• HST ACS imaging

• multi-object spectroscopy Δλ ~ 4-5 A with FORS @ VLT LRIS @ Keck

Teff ~ 4%, Δ log g ~ 0.05, metallicity ~ 0.1 dex use Balmer jump for Teff

• galaxies out to 7 Mpc

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pilot study W. Gieren, G. Pietrzynski, Araucaria Project: F. Bresolin, M. Urbaneja, RPK

NGC 300NGC 300 – Sculptor Group (2 Mpc)

117 cepheids

70 blue supergiant spectra

Kudritzki, Urbaneja, Bresolin, 2007, ApJ, in prep.

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Example: early A supergiant

A2 Ia

Teff = 9500 K

log g = 1.45

HST/ACS + ground

Kudritzki, Bresolin & Urbaneja et al. 2007

SED fit E(B-V) AV

extinction law Teff

NGC 300 @ 1.8 Mpc

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Balmer series fitting: log g

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Metallicity & chemical composition

Â2i = S

N2 1

npi x

P npi xj =1 (Oj ¡ Cj )2

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χi spectral window 4497-4607Å

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Stellar metallicity gradient in NGC300

■ B0 – B3 supergiants

● B8 – A4 supergiants

--- [Z] = -0.03 – 0.44•ρ/ρ0

= -0.03 – 0.07•d/kpc

ρ0 = 9.75 arcmin ≈ 5.7kpc

[Z] = log(Z/Z_sun)

Kudritzki, Urbaneja, Bresolin, Przybilla, Gieren, Pietrzynski, 2007, in prep.

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Bresolin, Kudritzki, Mendez, Przybilla 2001, ApJ Letters 548, L159

0.2 & 0.5 solar metallicity models

A0 Ia star

V = 20.5 MV = -9

NGC 3621NGC 3621: 7 Mpc HST/ACS

Bresolin, Kudritzki, Mendez & Przybilla 2001

~19 blue supergiant candidates (VLT/FORS)

4 analyzed

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Blue supergiants as distance indicators

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Flux weighted Gravity – Luminosity Relationship (FGLR)

Kudritzki, Bresolin, Przybilla, ApJ Letters, 582, L83 (2003)

M ~ g×R2 ~ L×(g/T4) = const.

const.

with L ~ Mx ~ Lx(g/T4)x, x ~ 3 L1-x ~ (g/T4)x

or with Mbol ~ -2.5log L

Mbol = a log(g/T4) + b FGLR

a =2.5 x/(1-x) ~ 3.75

B1-A4

L,M ~ const.

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FGLR Local Group, NGC300 & NGC3621 Kudritzki, Bresolin & Przybilla, 2003,ApJL, 582, L83

Kudritzki, Urbaneja, Bresolin et al., ApJ, 2007, in prep.

Mbol = 3.75 log(g/T4eff,4) – 13.73

= 0.24

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• WFOS quantitative spectroscopy possible down to mV ~ 24.5 mag

with objects MV ≤ - 8 mag

m – M ~ 32.5 mag ~ 30 Mpc possible

chemical evolution studies SF ISM, extinction, extinction laws distances 10 objects per galaxy Δ(m-M) ~ 0.1 mag

Conclusions and 30m perspectives