J/AJ/116/146 J/AJ/116/146 UGC 6456 HST photometry R Lynds E Tolstoy E J O'Neil Jr. D A Hunter Astron. J. 116 146 1998 1998AJ....116..146L J/A+AS/111/527 : CCD photometry of 11 dwarf irregular galaxies (Hopp+, 1995) Galaxies, photometry Photometry, CCD color-magnitude diagrams (HR diagram) galaxies: dwarf galaxies: individual (UGC 6456) galaxies: irregular stars: AGB and post-AGB 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.

HST UGC 6456VIIZw 40311 24 35.9+79 16 02

[LTO98] Identification number --- xpos x position number=1 Coordinates relative to Figure 1. North is 115{deg} clockwise from the top. The coordinates origin (0,0) is the bottom left corner. 1 pix = 0.1". pix ypos y position number=1 Coordinates relative to Figure 1. North is 115{deg} clockwise from the top. The coordinates origin (0,0) is the bottom left corner. 1 pix = 0.1". pix F336W F336W magnitude number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag e_F336W rms uncertainty on F336W number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag F555W F555W magnitude number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag e_F555W rms uncertainty on F555W number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag F814W F814W magnitude number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag e_F814W rms uncertainty on F814W number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag F336W-F555W F336W-F555W colour index number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag e_F336W-F555W rms uncertainty on F336W-F555W number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag F555W-F814W F555W-F814W colour index number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag e_F555W-F814W rms uncertainty on F555W-F814W number=2 A magnitude of 100.00 indicates that the star was not measured through that filter mag James Marcout, Patricia Bauer CDS 1998Sep22 UNKNOWN UNKNOWN From AJ electronic version J_AJ_116_146.xml