The resulting data set covers a time period from approximately 300 million years ago to the present. Our data show the spatial variations in star formation along the LMC bar from the southeast to the northwest and the formation of the currently observed supergiant shells surrounding the bar. We find Isserstedt's (1984) conclusion confirmed that global star formation occurred stochastically across the face of the LMC. The highly heterogeneous gas distribution prevents self-propagating star formation on a large scale.
The age structure of the LMC does not support the recently suggested scenario for bow-shock-induced star formation (de Boer et al. 1998; de Boer 1998, see p. 125). Also, the data show that 30 Dor did not form through collision of the supergiant shells LMC 2 and LMC 3. Massive star formation in 30 Dor and LMC 3 started almost simultaneously and spread later toward LMC 2.
The supergiant shells that we observe today appear to exhibit several star formation episodes, the oldest dating back 20 to 25 million years. Typical time scales for continuing large-scale star formation on length scales of 0.5 to 1 kpc seem to range from 15 to 30 million years.
Irregular galaxies undergoing interactions may be subject to additional, externally triggered star formation. The Magellanic Clouds, our closest neighbor galaxies, are interacting with each other and with the Milky Way. Close encounters (e.g, Gardiner & Noguchi 1996, Lin et al. 1995) between the galaxies appear to have left their signature in the episodic star formation history of the Large Magellanic Cloud (LMC) (see Olszewski et al. 1996 for a review).
Due to the motion of the Clouds through the Galactic halo in eastern direction their H I distribution shows compressed, high-density leading edges (Mathewson et al. 1977). De Boer et al. (1998) suggest that the combination of the space motion and rotation of the LMC leads to bow shocks that cause large-scale star formation. The model implies recent star formation in the southeast of the LMC and predicts a clockwise increase in ages.
Other scenarios for large-scale star formation in the LMC include stochastic self-propagating star formation (SSPSF; Seiden et al. 1979; Feitzinger et al. 1981; Dopita et al. 1985), collision with high-velocity clouds (Braun 1996 and references therein), and simultaneous formation of superassociations with subsequent star formation caused by shock-shock collisions (Chernin et al. 1995 and references therein).
We have studied the global recent star formation history (SFH) of the LMC using its massive non-variable supergiants and Cepheids as age tracers. We derived a color-age calibration for supergiants at LMC metallicity and a period-age relation for Cepheids. Our data cover a look-back time of 250 Myr. We use our results to age-date the supergiant shells in the LMC and to investigate predictions from the above mentioned theories for large-scale star formation.
For the most recent SFH dating back 30 Myr we used 2138 LMC supergiants from the UBV photoelectric catalogs of Rousseau et al. (1978), Isserstedt (1979, 1982a), Feitzinger & Isserstedt (1983), and Schmidt-Kaler et al. (1998). These stars comprise mostly O to A supergiants but also red supergiants. We dereddened the supergiants using the intrinsic UBV color calibration for LMC stars by Isserstedt (1982b). We then performed an age calibration of UBV colors based on Geneva models by Schaerer et al. (1993) at LMC metallicity.
For the SFH ranging back 250 Myr we used 1170 Cepheids in the LMC
from Payne-Gaposchkin (1971) and Welch et al. (1995).
To age-date these Cepheids in a manner consistent with the supergiant
calibration, we determined the ages of Cepheids in open clusters in
the Milky Way and in the Magellanic Clouds through fits of Geneva
isochrones to the cluster color-magnitude diagrams.
Our calibration,
log t [yr] = -0.7302±0.0008 ·
log P [d] + 8.5454±0.0005
is based on a new determination of ages for 9 populous clusters in the LMC
and 7 open clusters and comprises 37 fundamental-mode Cepheids.
This is the first period-age relation for Cepheids
derived through main-sequence fitting (see Efremov 1978 for a summary of
earlier work). The age range covered by the Cepheids corresponds roughly
to one rotational period of the LMC (Feitzinger 1980).
The oldest group of Cepheids (age range 150-250 Myr) indicates star formation only in the northeastern part of the bar (Fig. 1). Star formation activity is also visible above the southwestern part of today's stellar bar. 50-150 Myr ago star formation took place along the entire bar of the LMC. In more recent times, activity ceased in the northeast and moved to the southwestern portion of the bar, while above the northeastern part 30 Dor and supergiant shell LMC 3 began to form. Using star clusters with ages from integrated colors covering the past 70 Myr Dottori et al. (1996) suggest to consider these central activity regions as part of a rotating bar, whose youngest portion consists of the southwest bar and 30 Dor. However, our data do not show a similarly large bar displacement during the past 50-250 Myr.
In contrast, 30 Dor and LMC 3 formed almost simultaneously, while LMC 2 was formed more recently (15-10 Myr, Fig. 1). Oldest supergiants in this region have ages of ≅30 Myr and may have triggered subsequent compression of dense molecular clouds and the resulting starburst. The age structure does not support Chernin et al. (1996), who suggest that the collision of LMC 2 and LMC 3 created 30 Dor.
While LMC 4 and 30 Dor formed only within the last few 107 years, other structures such as LMC 7, 8, and 1 may have been sites of ongoing star formation over a much longer period (Fig. 1).
Age-dating the supergiant shells contradicts the predictions of clockwise increasing ages required by the bow-shock-induced star formation model (de Boer et al. 1998; de Boer 1998). Both transient and persistent large-scale regions are currently forming stars across the entire face of the LMC.
The Magellanic Clouds Photometric Survey will lead to accurate isochrone-based ages for thousands of star clusters, which will allow us to trace back the SFH with better resolution to much higher ages. Movies of the recent SFH of the LMC can be viewed at http://www.ucolick.org/~grebel/sfh_lmc.html.
| First version: | 29th | June, | 1998 |
| Last update: | 14th | November, | 1998 |