Received 15th March 1998
1. Introduction
The Phoenix dwarf galaxy is a low luminosity system situated in the Local
Group at a distance of ∼400 kpc.
It lacks obvious signs of current star formation, such as a conspicous bright,
blue stellar population or H II regions.
H I surveys have detected small amounts of gas in Phoenix
(Carignan et al. 1991; Young & Lo 1998), although the value
MH I/LB∼0.1 is
significantly lower than for typical dIrrs.
The color-magnitude diagram (CMD) of Phoenix reveals a dominant old,
metal-poor population and a small amount of blue young stars (Ortolani &
Gratton 1988; Van de Rydt et al. 1991; Martínez-Delgado et al. 1998a).
In addition, there is some evidence for an intermediate-age population,
indicating a complex star formation history for this galaxy.
2. The young population of Phoenix
Our Phoenix CMD shows that the recent star formation is located in the central
170 pc, in contrast with the complete absence of young stars in the outer
region of the galaxy (see Martínez-Delgado et al. 1998a).
A significant amount of young main sequence and/or blue-loop (BL) stars are
located in the blue-plume (BP), which can be explained by a burst of
star formation produced less that a 150 Myr ago.
In addition, a considerable number of Helium-burning BL stars with ages
∼0.5-1 Gyr are observed.
The overall distribution of these two young populations match quite well
the shape of a disk-like component found in our study of the structural
parameters of Phoenix based on the analysis of a digitized photographic
plate (Martínez-Delgado et al. 1998b).
The distribution of these two different populations can provide important
insights about the spatial variation of the star formation history within
the galaxy.
To analyze this issue, we have created stellar density maps for BP and BL
stars, following a method similar to that used by Dohm-Palmer et al. (1998).
The BP and BL stars were selected from our CMD with 21<I<24;
-0.6<(V-I)<0 and 22.6<V<22.1;
0.7<(V-I)<1.0, respectively.
The map of positions of these stars was then convolved with
a two-dimensional Cauchy kernel, using a smoothing parameter
of hx=hy=15 pc.
The resulting stellar density maps for BP and BL stars are plotted in
Fig. 1.
The results indicate that the most recent star formation is predominantly
located in the West part of the disk of the galaxy, while the older BL stars
are only found in the East region.
Moreover, a significant number of BP stars are clustered in two associations,
indicating that the star formation took place in localized regions of
the galaxy.
One of these associations was previously noticed by Canterna & Flowers
(1977) in their pioneering study of Phoenix.
This stellar distribution resembles those observed in dIrr galaxies (e.g.
Sextans A) rather than dSph and indicates that Phoenix may have looked
like a typical dIrr a fews Myr in the past.
Furthermore, the observed gradients in BP and BL stars suggest the star
formation has moved with time from East to West, yielding an age progression
acrosss the disk of Phoenix.
This spatial variation with time is consistent with a propagation of the star
formation over the inner disk of Phoenix.
Interestingly, a cloud of H I has been found by Young &
Lo (1998) situated at 6' SW from the center of the galaxy, forming a curved
structure that wraps around its border.
No H I was detected coincident with the optical body
of the galaxy.
However it is not clear if this cloud is associated with Phoenix or with
the Magellanic Stream.
If it belongs to Phoenix, this galaxy could be a good evidence that a burst
of star formation can blow-out the gas from a dwarf galaxy, preventing further
star formation (Dekel & Silk 1986).
[Click here to see Fig. 1!]
References
- Carignan C., Demers S., Côte S., 1991, ApJ 381, L13
- Canterna R., Flowers P.J., 1977, ApJ 212, L57
- Dekel A., Silk J., 1986, ApJ 303, 39
- Dohm-Palmer R.C., Skillman E.D., Saha A., Tolstoy E., Mateo M.,
Gallagher J., Hoessel J., Chiosi C., Dufour R.J., 1998, AJ 114, 2527
- Martínez-Delgado D., Gallart C., Aparicio A., 1998a, in 'Galactic
Halos', Zaritski D. (ed.) ASP, San Francisco, in press
- Martínez-Delgado D., Gallart C., Aparicio A., 1998b, in preparation
- Ortolani S., Gratton R.G., 1988, PASP 109, 1405
- Van de Rydt F., Demers S., Kunkel W.E., 1991, AJ 102, 130
- Young L.M., Lo K.Y., 1998, ApJ, in press
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| First version: | 08th | August, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)