Structure and dynamics of galaxies with a low surface-brightness disc – II. Stellar populations of bulges

Authors

  • L. Morelli,

    Corresponding author
    1. Dipartimento di Fisica e Astronomia, Università di Padova, vicolo dell’Osservatorio 3, I-35122 Padova, Italy
    2. INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
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  • E. M. Corsini,

    1. Dipartimento di Fisica e Astronomia, Università di Padova, vicolo dell’Osservatorio 3, I-35122 Padova, Italy
    2. INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
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  • A. Pizzella,

    1. Dipartimento di Fisica e Astronomia, Università di Padova, vicolo dell’Osservatorio 3, I-35122 Padova, Italy
    2. INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
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  • E. Dalla Bontà,

    1. Dipartimento di Fisica e Astronomia, Università di Padova, vicolo dell’Osservatorio 3, I-35122 Padova, Italy
    2. INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
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  • L. Coccato,

    1. European Southern Observatory, Karl-Schwarzschild-Straβe 2, D-85748 Garching bei München, Germany
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  • J. Méndez-Abreu,

    1. Instituto Astrofí sico de Canarias, C/Vía Láctea s/n, E-38200 La Laguna, Spain
    2. Departamento de Astrofí sica, Universidad de La Laguna, C/Astrofísico Francisco Sánchez, E-38205 La Laguna, Spain
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  • M. Cesetti

    1. Dipartimento di Fisica e Astronomia, Università di Padova, vicolo dell’Osservatorio 3, I-35122 Padova, Italy
    2. INAF–Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
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  • Based on observations made with European Southern Observatory telescopes at the La Silla Paranal Observatory under programmes 76.B-0375 and 80.B-00754.

E-mail: lorenzo.morelli@unipd.it

ABSTRACT

We present the radial profiles of the Hβ, Mg and Fe line-strength indices for a sample of eight spiral galaxies with a low-surface-brightness stellar disc and a bulge. The correlations between the central values of the line-strength indices and velocity dispersion are consistent with those known for early-type galaxies and bulges of high-surface-brightness galaxies. The age, metallicity and α/Fe enhancement of the stellar populations in the bulge-dominated region are obtained using stellar population models with variable element abundance ratios. Almost all the sample bulges are characterized by a young stellar population, ongoing star formation and a solar α/Fe enhancement. Their metallicity spans from high to subsolar values. No significant gradient in age and α/Fe enhancement is measured, whereas a negative metallicity gradient is found only in a few cases. These properties suggest that a pure dissipative collapse cannot explain the formation of all the sample bulges and that other phenomena, such as mergers or acquisition events, need to be invoked. Such a picture is also supported by the lack of a correlation between the central value and the gradient of the metallicity in bulges with very low metallicity. The stellar populations of the bulges hosted by low-surface-brightness discs share many properties with those of high-surface-brightness galaxies. Therefore, they are likely to have common formation scenarios and evolution histories. A strong interplay between bulges and discs is ruled out by the fact that, in spite of being hosted by discs with extremely different properties, the bulges of low- and high-surface-brightness discs are remarkably similar.

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