Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aj....113..985d&link_type=abstract
Astronomical Journal v.113, p.985
Astronomy and Astrophysics
Astronomy
23
Scientific paper
Two sets of long-slit spectra are used to investigate the stellar content of the M31 inner bulge. Spectra recorded with 2.0 A FWHM resolution near 4000 Å are used to measure indices in the Rose [AJ, 89, 1238 (1984)] system out to 60 arcsec from the galaxy center along the north/south and east/west axes. Spectra with 14 Å FWHM resolution near 5500 Å are used to measure indices in the Lick system [Worthey et al., ApJS, 94, 687 (1994)] at 156 points in a 20X60 arcsec area centered on the nucleus. Systematic gradients, in the sense of weaker absorption features with increasing radius, are detected in both datasets, and the majority of gradients differ from zero at the 3σ or higher level. The gradients in M31 tend to be shallower than those in elliptical galaxies; for example, ΔMg2/Δlog(r)=-0.015±0.001 in M31 and -0.062±0.009 in nearby elliptical galaxies without strong line emission. Comparisons with globular cluster sequences indicate that the integrated M31 bulge spectrum at 4000 Å is dominated by a mixture of dwarfs and giants similar to that in globular clusters. The spectra also show enhanced 3883 Å CN absorption similar in strength to that seen in M31 globular clusters, suggesting that the CNO enrichment histories of the M31 bulge and globular cluster system are related, and systematically different from the Galactic cluster system. The spatial distribution of Hβ absorption is different from Hα, a result that is attributed to line emission. Comparisons with models generated by Worthey [APJS, 95, 107 (1994)], Buzzoni et al. [AJ, 107, 513 (1994)], and Buzzoni et al. [AJ, 103, 1814 (1992)] suggest that [M/H], mean age, and [Mg/Fe] all change with distance from the center of M31, in the sense of younger, more iron-rich populations with lower [Mg/Fe] at smaller radii. When compared with line indices measured in the nearby elliptical galaxies NGC 3379 and NGC 4472, these same models predict that age and [Mg/Fe] likely do not vary with radius. The strength of the metallicity gradient in the central regions of M31 is estimated by a direct comparison with the Worthey [ApJS, 95, 107 (1994)] models, a procedure that automatically takes into account any variation of mean age with radius, with the result that Δ[Fe/H]/Δlog(r)˜-0.5. This is significantly shallower than what is measured in the outer regions of the Galactic bulge.
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