Bibcode
Annibali, F.; Tosi, M.; Monelli, M.; Sirianni, M.; Montegriffo, P.; Aloisi, A.; Greggio, L.
Referencia bibliográfica
The Astronomical Journal, Volume 138, Issue 1, pp. 169-183 (2009).
Fecha de publicación:
7
2009
Número de citas
36
Número de citas referidas
30
Descripción
We present Hubble Space Telescope (HST) photometry of the late-type
dwarf galaxy NGC 1705 observed with the Wide-Field Planetary Camera 2
(WFPC2) in the F380W and F439W bands and with the Advanced Camera for
Surveys/High-Resolution Channel (HRC) in the F330W, F555W, and F814W
broad-band filters. We cross-correlate these data with previous ones
acquired with the WFPC2 in the F555W, F814W bands, and derive multiband
color-magnitude diagrams (CMDs) of the cross-identified individual stars
and candidate star clusters. For the central regions of the galaxy,
where HST-NICMOS F110W and F160W photometry is also available, we
present U, B, V, I, J, H CMDs of the 256 objects with magnitudes
measured in all bands. While our previous study based on F555W, F814W,
F110W, and F160W data allowed us to trace the star formation history of
NGC 1705 back to a Hubble time, the new data provide a better insight on
its recent evolution. With the method of the synthetic CMDs, we confirm
the presence of two strong bursts of star formation (SF). The older of
the two bursts (B1) occurred between ~10 and 15 Myr ago, coeval to the
age of the central super star cluster (SSC). The younger burst (B2)
started ~3 Myr ago, and it is still active. The stellar mass produced by
B2 amounts to ~106 M sun, and it is a factor of ~3
lower for B1. The interburst phase was likely characterized by a much
lower level of SF rather than by its complete cessation. The two bursts
show distinct spatial distributions: while B1 is centrally concentrated,
B2 is more diffused, and presents ring and arclike structures that
remind of an expanding shell. This suggests a feedback mechanism, in
which the expanding superbubble observed in NGC 1705, likely generated
by the 10-15 Myr burst, triggered the current strong SF activity. The
excellent spatial resolution of the HRC allowed us to reliably identify
12 star clusters (plus the SSC) in the central ~26'' × 29'' region
of NGC 1705, 10 of which have photometry in all the UBVIJH bands. The
comparison of the cluster photometry with the GALEV populations
synthesis models provides ages from ≈10 Myr to ≈1 Gyr, and masses
between ≈104 and 105 M sun. The
conspicuous cluster population in the central regions, with one SSC, one
populous cluster, and several regular ones, confirm the strong
star-forming activity of NGC 1705.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by AURA for
NASA under contract NAS5-26555.
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