A Study of star formation in nearby starburst galaxies a thesis submitted for the degree of Doctor of Philosophy in the faculty of science, Bangalore University, Bangalore Swara Ravindranath [Ph.D Thesis]

Thesis Supervisor T. P. Prabhu

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1999

A starburst galaxy is defined as a galaxy whose luminosity is dominated by an intense
episode of star formation, and is characterized by a very high star formation rate which
cannot be sustained over its lifetime. To explain their very high Far-Infrared luminosities
(1010 to 1012 L0 ), star formation rates of rv 10-100 Me yr-l would be required and at
such a high rate the molecular gas in the galaxy would be exhausted in less than 1/10 of
the Hubble time. Hence the starburst phase must be a transient phenomena in galaxies.
Since gas is efficiently converted to stars in starbursts, and massive stars return most
of the processed material back into the ISM through stellar winds and supernovae in a
short time, they are mainly responsible for the metal-enrichment of the Universe. Thus,
starbursts being the most extreme cases of massive star formation, they are very important
for understanding the formation and chemodynamical evolution of galaxies.
Starburst phenomenon has been found to be very dominant at high redshifts (which
implies large lookback times) and most of the ground-based large telescopes and spacetelescopes
have observational programmes devoted to the study of star-forming galaxies
at intermediate and high redshifts. These observations essentially sample the integrated
properties of the star-forming regions. Inorder to underst.and and interpret the integrated
spectra, colors and luminosities in terms of the IMF, SFR and ages of the'ionizing clusters,
it is very important to study t,he nearby starbursts in detail.
The work presented in this thesis a.ims at studying the physical properties of t.he nebular
gas and the stellar populations in a sample of nearby starburst galaxies. The observational
technique includes broad-band BV R and narrow-band Ha imaging, and opticalNIR
spectroscopy. All the observations required for the present study were obtained using
the 2.34-m Vainu Bappu Telescope at the Vainu Bappu Observatory in Kavalur, India.
The distribution of ionized gas in the galaxies were analysed based on the Ha emission
morphology and the distribution of dust was traced using the B / R color maps. The star
formation rates for the star-forming regions have been derived from the Ha luminosities.
The ages of the young stellar populations have been determined based on the evolutionary synthesis models.
A general introduction to the field is presented in Chapter 1 and the observationsand reduction techniques are described in Chapter 2. A case study of the massive starformation in the IR bright galaxy NGC 972 is presented in Chapter 3. The H-alpha imageshows active star formation in the circumnuclear regions and the disk of this galaxywhile the color map reveals the complex dust distribution. The results from aperturephotometry are presented for the individual HII regions and using evolutionary synthesismodels we estimate that the current episode of star formation is less than 6.5 Myr old. Based on the He\( fluxes we derive and the spectroscopic emission line ratios, we confirmthe starburst nature of the nucleus of NGC 972. Nuclear starbursts and circumnuclear star formation are usually triggered by galaxy-galaxy interactions or are associated withstrong bars. However, NGO 972 is an isolated galaxy and the optical images do not showany evidence for a bar. This prompted an investigation of the NIR morphology of thisgalaxy, since the NIR images sample the underlying stellar populations (low-mass starsand their evolved stages) better. The K-band image did show a weak bar-like structure within the circum nuclear ring of star-forming regions. From the information available in the literature, we found that the molecular gas content in this galaxyisvery high. Basedon the evidence that the nucleus of NGO 972 is undergoing a starburst with strength comparable t.o that in the prototype starburst M82, the presence of active circumnuclear star format.ion and large molecular content, we propose that the activity in NGC 972 is likely to have been triggered by a, merger with a gas-rich companion. A study of the statistical properties of HI! regions in the galaxy NGC 1365 was carried out and the results are discussed in Chapter 4. We obtain a power law slope of a =-2.29±O.11 for the HII region luminosity function which is consistent with that found for galaxies.of similar Hubble type. We find that the size distribution can be well-fitted by anexponential law. From the steep slopes of the HII region luminosity function and the small characterestic diameters derived from the size distribution for NGC 1365 and other earlytype galaxies in the literature, it appears that some dynamical phenomena are responsible for inhibiting the formation of "supergiant" HII regions in these galaxies. The ultraviolet photons from massive stars can ionize a finite volume of gas and this region of completely
ionized gas is refered to as the "Stromgren sphere". Such an HI! region is ionizationbounded
and the Hc~ luminosity scales as the cube of the Stromgren radius. A simple linear
least square fit to the luminosity versus radius of the HII regions in logarithmic units for
NGC 1365, yeilds a slope close to 3 at low luminosities, while the slope tends to get
steeper at higher luminosities. The sizes of the luminous HII regions which contain more
number of massive stars are limited by the amount of material available for ionizing by
UV photons. Thus it appears that the low luminosity HII regions are ionization bounded
while the high luminosity HI! regions are density bounded.
Optical-NIR spectroscopy of a sample of starburst nuclei were carried out with an
aim to study the physical properties of the photoionized gas and to estimate the ages
of the young stellar populations present in these nuclei (Chapter 5). We determined
the electron densities, ionization parameters, and the nebular oxygen abundances using
diagnostic emission line ratios. We then compared the properties of starburst nuclei with
that of HII nuclei and disk HII regions. We find that t'he electron densities are high and
the excitation levels are also slightly higher in starbursts. The low-ionization lines are
enhanced in starbursts compared to disk HII regions and we find that the photoionization
models which include the effects of dust can account for the enhancement of these lines in
the spectra. For a few nuclei where we could determine the radiation softness parameter,
we find that the temperatures of ionizing stars decrease with increasing metallicities and
is less than 40,000 K. This would place the upper mass limit of the IMF in these regions
at Mu :5 25 Me!). We have derived the star formation rates from the Ha luminosities.
The presence of evolved massive stars in the red supergiant (RSG) phase was identified
through the large equivalent widths of the Ca II triplet lines in the NIR. From the observed
Hex luminosity, Ha equivalent widths and Ca II triplet line equivalent widths, it appears
that the nuclear starbursts have a composite population consisting of young massive
stars which are less than 3 Myr old, co-existing with an evolved RSG population. We
use diagnostic diagrams involving Ca II absorption line and Hex emission line equivalent
widths to infer ages of 6-8 Myr for the evolved population.