Issue |
A&A
Volume 492, Number 1, December II 2008
|
|
---|---|---|
Page(s) | 3 - 22 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20077621 | |
Published online | 11 September 2008 |
Extremely massive young clusters in NGC 1365
1
Observatório Nacional, Rua General José Cristino 77, 20921-400 São Cristovão, Rio de Janeiro, Brazil e-mail: egallian@on.br
2
Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
3
European Southern Observatory, Casilla 19001, Santiago 19, Chile
4
Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, Bât. 709, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France
5
INAF – Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
6
INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
Received:
9
April
2007
Accepted:
20
July
2008
Context. In a previous work, three bright mid-infrared/radio sources were discovered in the nuclear region of starburst/AGN galaxy NGC 1365.
Aims. The present study aims to confirm that these sources are indeed young and massive “embedded” clusters, and derive their physical parameters, such as extinction, age and mass.
Methods. Using ISAAC and VISIR at the VLT we obtained maps and low resolution spectra in the near- and mid-infrared. The resulting datasets are first interpreted by comparing the observations with images and spectra of the close-by young cluster R136 in the Large Magellanic Cloud and then by using model predictions for both the nebular emission lines and the spectral energy distribution of the sources.
Results. We produce maps of the region containing the
three sources in the R, J, Ks, L' bands and at
12.8 and perform their accurate relative
positioning. We also provide spectra in the ranges
1.8-2.4
, 3.3-4.0
, 8.1-9.3
and
10.4-13.2
. The spectral energy distribution of the
three sources rises with wavelength. Emission lines from
ionised hydrogen and molecular hydrogen are detected, as
well as PAH emission. A conspicuous [NeII] 12.8
line
is also observed, while neither the [ArIII] 8.9
nor
the [SIV] 10.4
lines are detected. This provides a
stringent constraint on the age of the sources: we argue
that they are relatively evolved young clusters
(6-8 Myr). Owing to their ionising photon emission rates
and ages, they must be extremely massive clusters (of the
order of 107
). Their mid-infrared spectral
energy distribution suggests the presence of two
components: (1) an optically thin component, with a
continuum comparable to that of R136; and (2) an optically
thick component which might be related to subsequent or
on-going episodes of star formation. We anticipate that
these sources are good candidates for evolution according
to a bi-modal hydrodynamical regime, in which matter is
trapped at the centre of a compact and massive cluster and
generates further star formation.
Key words: ISM: dust, extinction / ISM: HII regions / galaxies: star clusters / galaxies: individual: NGC 1365 / infrared: galaxies
© ESO, 2008
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