J/AJ/116/146 UGC 6456 HST photometry (Lynds+ 1998)
Star formation in and evolution of the blue compact dwarf galaxy UGC 6456
determined from Hubble space telescope images
Lynds R., Tolstoy E., O'Neil E.J.Jr, Hunter D.A.
<Astron. J. 116, 146 (1998)>
=1998AJ....116..146L 1998AJ....116..146L (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, photometry ; Photometry, CCD
Mission_Name: HST
Keywords: color-magnitude diagrams - galaxies: dwarf -
galaxies: individual: UGC 6456 - galaxies: irregular -
stars: AGB and post-AGB
Abstract:
Photometry on the UVI system has been performed on the resolved
stellar content of the blue compact dwarf galaxy UGC 6456 using Wide
Field Planetary Camera 2 (WFPC2) images obtained with the Hubble Space
Telescope. The resulting color-magnitude diagram (CMD) goes to about
V=27.5 and reveals not only a young population of blue main-sequence
stars and blue and red supergiants, but also an older evolved
population of red giants and a fairly well represented asymptotic
giant branch. The distance to the galaxy is estimated from the tip of
the red giant branch to be 4.5Mpc, placing it about 1.5Mpc farther
away than the major members of the M81 Group, with which it is usually
associated. The youngest stars are generally associated with H ii
regions shown on our Halpha image and are largely confined to the
745pc field of our PC images. A comparison of their distribution in
the CMD with theoretical isochrones suggests ages from 4 to 10Myr. The
population of older stars is found throughout all WFPC2 camera fields
and seems to show an elliptical distribution with an aspect ratio of
about 2.4 and an exponential falloff in surface density with distance
from a center of symmetry that is not far from the centroid of the
youngest stars. Theoretical modeling of the CMD at a metallicity of
Z=0.001 suggests star formation in the age interval 1-2Gyr, a strong
burst in the interval 600-800Myr, and a lower rate of star formation
up to the present. The evidence is compatible with a scenario
beginning with the formation of a population of low-metallicity stars,
enriching a major residual of prestellar material that subsequently
fueled an active episode of star formation. That burst of star
formation must have been particularly spectacular and may be related
to the activity we now see in the distant blue dwarf galaxies revealed
in deep imaging.
Objects:
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RA (2000) DE Designation(s)
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11 24 35.9 +79 16 02 UGC 6456 = VIIZw 403
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 89 22273 The Galaxy Photometry
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See also:
J/A+AS/111/527 : CCD photometry of 11 dwarf irregular galaxies (Hopp+, 1995)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- [LTO98] Identification number
7- 12 F6.1 pix xpos x position (1)
14- 19 F6.1 pix ypos y position (1)
21- 26 F6.2 mag F336W ?=100.00 F336W magnitude (2)
28- 33 F6.2 mag e_F336W ?=100.00 rms uncertainty on F336W (2)
35- 40 F6.2 mag F555W ?=100.00 F555W magnitude (2)
42- 47 F6.2 mag e_F555W ?=100.00 rms uncertainty on F555W (2)
49- 54 F6.2 mag F814W ?=100.00 F814W magnitude (2)
56- 61 F6.2 mag e_F814W ?=100.00 rms uncertainty on F814W (2)
63- 68 F6.2 mag F336W-F555W ?=100.00 F336W-F555W colour index (2)
70- 75 F6.2 mag e_F336W-F555W ?=100.00 rms uncertainty on F336W-F555W (2)
77- 82 F6.2 mag F555W-F814W ?=100.00 F555W-F814W colour index (2)
84- 89 F6.2 mag e_F555W-F814W ?=100.00 rms uncertainty on F555W-F814W (2)
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Note (1): Coordinates relative to Figure 1. North is 115° clockwise from
the top. The coordinates origin (0,0) is the bottom left corner.
1 pix = 0.1".
Note (2): A magnitude of 100.00 indicates that the star was not measured through
that filter
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History: From AJ electronic version
(End) James Marcout, Patricia Bauer [CDS] 22-Sep-1998