Star Formation and Physical Properties of Starburts Galaxies:
Results for NGC 3690

Daniel Devost

Département de physique et Observatoire du mont Mégantic
Université Laval, Québec, QC G1K 7P4, Canada
Electronic-mail: ddevost@phy.ulaval.ca

Abstract:

Age differences are derived between the various super star clusters of NGC 3690 using the two color diagram (B-H) vs (H-K) and previously published data on the physical properties of the gas. This method offers a quick and easy way to establish age differences down to a time resolution that is limited by the data on extinction. One of the infrared knots, suspected to be a galaxy nucleus, is found to be a starbursting region. The validity of the method is checked by comparing the equivalent width of the H$\alpha$ line with the color maps of NGC 3690.

Introduction

  Although known to be very important in the evolution of galaxies, the star formation process is usually poorly accounted for in galactic evolution models. ``Recipes'' (e.g. Kennicutt 1998) are provided to account for the global characteristics of the star formation rate versus the interstellar medium (ISM); those are derived from the global or azimuthally averaged properties of observed galaxies while star formation is a local phenomenon. Little is known about the local aspects of the star formation process. The way the star formation propagates, the dynamics of the star formation, and its relation to the physical properties of the ISM are usually parameterized as a percolation phenomenon (Seiden, Schulman & Gerola 1979; Seiden & Gerola 1982) or regulated by feedback (Navarro & Steimetz 1997; Silk 1997). Both are ad-hoc parameterization of a very complicated process.

One of the most difficult task in the quantification of the star formation process is to assign precise ages to star forming regions. Several parameters of a stellar population other than its age, can cause a variation of the integrated magnitude and colors of a SSC. Extinction and metallicity are the two main factors that can affect the observed characteristics of a SSC and produce a degeneracy with age.

Starburst galaxies are ideal objects to study the local properties of various star forming regions and their effects on the properties of a galaxy. They form many compact ($\sim$ 2-5 pc), luminous and blue (-11 > M_UV > -18) super star clusters (SSC; Meurer et al. 1995) that radiate tremendous amounts of light in all wavebands. The study of the SSC integrated light with the (B-H) vs. (H-K) two color diagram (BHK diagram) combined with data on the physical properties of the gas with the nebular lines, provides the opportunity to lift a reasonable amount the degeneracies associated with extinction and metallicity. The time resolution that follows is then strictly limited by the uncertainty on the extinction correction.

Chronometry of NGC 3690

NGC 3690 is part of the merging system ARP 299. Several young SSCs (Meurer et al. 1995) are thought to be in proximity of an older stellar population (Wynn-Williams et al. 1991). It is not clear whether ARP 299 is the merging result of three galaxies, or if the collision of two galaxies produced a young starbursting region, which is being confused with the nucleus of a third galaxy. To differentiate between the two scenarios, an age determination of the various sources is essential. Let's see how the analysis with the BHK diagram combined with nebular data on the physical properties of the gas resolves part of this problem.

A basic study on the nebular properties of ARP 299 has been done by Mazzarella & Boroson (1993). Their data shows surprisingly uniform oxygen abundances at about a third solar. The logarithmic extinction differences at H$\beta$ between the various regions they studied are of the order of $\Delta c \simeq$ 0.2 - 0.3. The data on oxygen abundance lifts age-metallicity degeneracy and the restriction on extinction allows a time resolution of 5 Myr.

 

Figure 1:  (a) Left panel: CFHT K band image of NGC 3690. The bright regions are identified based on the convention of Wynn-Williams et al. (1991). North is up and east is left. The total field of view is 1 $\times$ 1. (b) Right panel: BHK color diagram of NGC 3690. The solid curve is the theoretical model at 0.25 solar metallicity. The observed points shown are those brighter than M_B = -10 and M_K = -14. The observations have been corrected for E(B-V) = 0.16. For these colors, the extinction vector is parallel to the old branch.

The BHK diagram of NGC 3690 was constructed with observations done at the Observatoire du mont Mégantic (B band; March 1996) and at the Canada-France-Hawaii Telescope (H and K bands; January 1997). Figure 1(a) shows the K image of ARP 299 labeled with the conventional symbols of Wynn-Williams et al. (1991) and Figure 1(b) compares the observed colors of the galaxy (only the points with M_B and M_K $\geq$-10 and -14 respectively) with the models of Leitherer & Heckman (1995). The models shows the behavior of these colors for an instantaneous burst of star formation forming 10⁶ M$_\odot$ of stars whose mass ranges from 1 to 120 M$_\odot$ according to a Salpeter initial mass function. All the stars have a metallicity of 0.25 solar.

The best fit of the theoretical tracks to the data gives a global extinction correction E(B-V) = 0.16, which is in agreement with the value derived by Meurer et al. (1995) and with the value inferred from the data of Mazzarella & Boroson (1993). If I assign an age to all the points in Figure 1 (b), I obtain the age map of Figure 2 (a). With the hypothesis that extinction differences do not significantly affect the age derived, I find three generations of stellar populations: i) ages smaller than 5 Myr (light grey), corresponding to region C; ii) ages between 5-15 Myr (medium grey), for region B2, and iii) ages older than 15 Myr (black), for B1.

 

Figure 2:  (a) Left panel: SSC age distribution in NGC 3690. The age map (greyscale) is shown superposed to the contours of the B image. The oldest regions are the darkest. In this picture, region C is the youngest (< 5 Myr) while region B1 is the oldest (> 15 Myr). Region B2 seems to be of intermediate age. Notice that object B1 does not show in the B image while it is the brightest in K. North is up and east is left. (b) Right panel: Contour map of EW(H$\alpha$) superposed to the wide R band image in NGC 3690. The highest region of EW(H$\alpha$) is coincident with object C, confirming the results of the BHK diagram analysis. Each point on this image is a fiber of 0.4 for a total field of view of 12$\times$ 10. North is up and east is left. Due to bad fiber response, the contours near the edge of the image are not smooth. These regions cannot be considered to contain valid data.

Discussion

From this analysis, I can rule out region C as being a galaxy nucleus. The BHK diagram clearly categorizes it as a very young region. The age determination for those points is quite robust, since these are lying on the red side of the young branch (Figure 1 (b)). A further test of the validity of this approach can be made with the 2-D spectroscopy of MOS-ARGUS. The equivalent width of H$\alpha$(EW(H$\alpha$)) is known to be a very strong function of age (Leitherer & Heckman 1995; Devost, Roy, & Drissen 1997). Figure 2 (b) shows a contour map of EW(H$\alpha$) for NGC 3690, derived from MOS-ARGUS spectroscopic observations superposed to the wide R band image. Notice that the only valid contours are the ones away from the edges of the image; contours close to the edge are not reliable due to bad fibers and discontinuity in spatial sampling. The smooth contours indicate a direct correlation between object C and the highest EW(H$\alpha$) region located to the north of the image. Region B2, also seen in the wide R band image, is an intermediate age object according to EW(H$\alpha$) and the BHK diagram analysis.

Further analysis must be made to classify object B1. It may be the nuclei of NGC 3690; however, very high extinction hiding a young dust embedded stellar cluster cannot yet be ruled out. According to the data of Figure 1 (b), extinction values for points with (B-H) > 1.3 and (H-K) > 0.6 could correspond to visual extinction as high as A_V $\sim$1.5 - 2 mag. The fact that these points lie in this part of the diagram, combined with the very high K band surface brightness of the region favors the young and buried star forming region hypothesis for B1.