J/ApJ/452/238 HST photometry of I Zw 18 massive stars (Hunter+ 1995)
The Massive Stars of I Zw 18 as Seen in Hubble Space Telescope Images
Hunter H.A., Thronson H.A.
<Astrophys. J. 452, 238 (1995)>
=1995ApJ...452..238H 1995ApJ...452..238H
ADC_Keywords: Photometry ; Stars, early-type
Keywords: galaxies: compact - galaxies: individual (I Zw 18) -
galaxies: stellar content - stars: early-type
Abstract:
Blue compact dwarf galaxies are tiny galaxies that are dominated by
intense star-forming regions. Thus, they have been thought to
represent a different and extreme environment for star formation
compared to the Milky Way and many other nearby galaxies. In this
paper we use images obtained with the Hubble Space Telescope to
resolve the main body of one of these galaxies, I Zw 18, into stars
for the first time. This galaxy is also one of the most metal-poor
galaxies known and is sometimes argued to be evolutionarily young.
Broadband colors are used to determine the bulk characteristics of the
resolved stellar population to an F555W magnitude of about 26, or
M_(V,0)~-4 (O9.5 V star). Narrow-band images are used to look for
emission characteristic of Wolf-Rayet stars. Color-magnitude diagrams
reveal a broad main sequence of massive stars as well as blue and red
supergiants, although there is a surprising lack of Wolf-Rayet stars.
About half the massive stars are located in two groups, corresponding
to the two knots of emission identified in ground-based images, and
the rest are distributed between these regions and in the outer parts
of the galaxy. We find that the northwestern of these two regions
resolves in Halpha images into a small H II region plus a complicated
shell structure that encircles the northern stellar association. There
are additional, larger loops and filaments of ionized gas up to 450pc
from the center of the galaxy. While the shell could be as young as
the stars it encircles, the larger ionized gas structures must be
older for reasonable models, and their presence implies that there has
been a previous generation of massive stars at least several tens of
millions of years ago. However, we find no evidence in these data for
stars that must be older than this. The larger sites of most recent
star formation are located northwest and south of the central part of
the galaxy. Thus, we find that I Zw 18 is somewhat more complicated
than had previously been thought, both in terms of its star formation
history and the state of its interstellar medium. At the same time,
however, the stellar populations look relatively normal, and the
spatial concentration of massive stars is closer to that of large OB
associations in nearby galaxies rather than to that exemplified by the
compact cluster R136 in the Large Magellanic Cloud.
Objects:
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RA (2000) DE Designation(s)
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09 34 02.1 +55 14 25 I Zw 18 = Mkn 116 = Mrk 116 = ZW I 18
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 98 398 The photometry
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 6 I6 --- Star Star identification number
8- 13 F6.1 pix Xpos *X pixel coordinate
15- 20 F6.1 pix Ypos *Y pixel coordinate
22- 27 F6.2 mag F336W []?=100.00 F336W magnitude
29- 34 F6.2 mag e_F336W []?=100.00 F336W magnitude uncertainty
36- 41 F6.2 mag F555W []?=100.00 F555W magnitude
43- 48 F6.2 mag e_F555W []?=100.00 F555W magnitude uncertainty
50- 55 F6.2 mag F814W []?=100.00 F814W magnitude
57- 62 F6.2 mag e_F814W []?=100.00 F814W magnitude uncertainty
64- 71 F8.2 mag F336W-F555W []?=100.00 F336W-F555W color
73- 80 F8.2 mag e_F336W-F555W []?=100.00 F336W-F555W color uncertainty
82- 89 F8.2 mag F555W-F814W []?=100.00 F555W-F814W color
91- 98 F8.2 mag e_F555W-F814W []?=100.00 F555W-F814W color uncertainty
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Note on Xpos, Ypos:
The X,Y coordinates are those in the F555W image, and the coordinate
system is indicated in Figure 2 of the printed paper.
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Origin: AAS CD-ROM series, Volume 5, 1995 Lee Brotzman [ADS] 11-Oct-95
(End) [CDS] 29-Jan-1996