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NGC 1705
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NGC 1705 (also ESO 158-13, IRAS 04531-5326 and PGC 16282) is a magnitude +12.8 galaxy peculiar lenticular galaxy and a Blue Compact Dwarf galaxy located 17 million light-years away in the constellation Pictor.

The galaxy was discovered by the British astronomer John Herschel using a 47.5 cm (18.7 inch) f/13 speculum reflector at Feldhausen, Claremont, Cape Town, on the 5th December 1834.

It is a member of the Dorado Group
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Right ascension 04h 54m 13.5s, Declination -53 21' 40"



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Title: Young stellar populations and star clusters in NGC 1705
Authors: F. Annibali (INAF-Oapd, Stsci), M. Tosi (INAF-Oabo), M. Monelli (IAC), M. Sirianni (ESA, Stsci), P. Montegriffo (INAF-Oabo), A. Aloisi (ESA, Stsci), L. Greggio (INAF-Oapd)

We present HST photometry of the late-type dwarf galaxy NGC 1705 observed with the WFPC2 in the U and B bands, and with the ACS/HRC in the U, V, and I bands. We cross-correlate these data with previous ones acquired with the WFPC2 in V and I and derive multiband Colour-Magnitude diagrams (CMDs) of the cross-identified individual stars and candidate star clusters. For the central regions of the galaxy, where HST-NICMOS J and H 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 V, I, J and H 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, while the younger burst (B2) started ~ 3 Myr ago, and it is still active. The stellar mass produced by B2 amounts to ~ 10^6 msun, and it is a factor of ~ 3 lower for B1. The interburst phase was likely characterised 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 arc-like 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. From the HRC data, we identified 12 star clusters (plus the SSC) in the central region of NGC 1705, 10 of which have photometry in all the UBVIJH bands. Using the GALEV models, we derived ages from ~ 10 Myr to ~ 1 Gyr, and masses between ~ 10^4 and 10^5 msun for the clusters.

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