Bidimensional Spectroscopy of Nearby Starbursts

Abstract

The observed star formation events in nearby galaxies take place under very di†erent conditions. While in some cases the star formation rate (SFR) appears to have been nearly constant for several billion years, other ones have experienced large bursts. These starbursts must have been quite brief : their SFR is so large that the gas reservoir would be depleted on a timescale much shorter than the Hubble time (L. Searle, W. L. W. Sargent, & W. G. Bagnuolo, 1973, ApJ, 179, 427). Starbursts are easily detected, especially when they are young, due to the presence of a large number of massive stars in a relatively small volume. These stars produce a blue continuum and ionize the surrounding gas, resulting in emission lines in the visible spectrum. An example of how easy it is to detect a starburst can be seen in the fact that two of the most conspicuous structures in the Local Group, 30 Doradus and NGC 604 (in the LMC and in M33, respectively), belong to this class. Another example is that Ðve starbursts located in a galaxy quite a bit farther away, M101, are bright enough to deserve their own NGC numbers (5447, 5455, 5461, 5462, and 5471). The objective of this work was the study of the physical and kinematical properties of starbursts, paying special attention to their spatial variations and to how those varia- tions a†ect global results when they are not taken into con- sideration. To accomplish this, we have used bidimensional spectroscopic data obtained with multiple long slits. The wavelength range covers part of the red and blue spectral ranges, including some of the most important emission lines which originate in the warm ionized gas : Ha ,H b ,( OIII) j5007, (N II) j6584, (S II) j6717, and (S II) j6731. We also have information regarding the intensity of the red and blue continua and of the W-R j4686 bump. We have been able to obtain detailed synthetic maps of some physical proper- ties such as the gas density and excitation and the extinction by dust and concluded that some of the results obtained by one-point (or a few points) spectroscopy are clearly incor- rect. Furthermore, the high spectral resolution of our spectra (B0.4 pixel~1) has allowed us to study the kine- A � matics of the ionized regions. Among other results, this work sheds some light on the creation of hot bubbles in the interstellar medium (ISM) and on the origin of the super- sonic movement of the warm ionized gas. The spectroscopic data have been complemented with images obtained with the WFPC2 aboard the Hubble Space T elescope. The sample used in this Ph.D. thesis consists of two very di†erent objects : NGC 4214 and NGC 604. NGC 4214 is a galaxy apparently similar in shape and size to the LMC, rich in gas, and with several starbursts. On the other hand, NGC 604 is a giant H II region located in the third largest spiral galaxy of the Local Group, M33. We have chosen these two objects in part to cover di†erent types of star- bursts, and we have discovered that they share many char- acteristics. The number of long slits used was 12 for NGC 4214 and 10 for NGC 604. The most important conclusions obtained in this work