The environment of HII galaxies

Recent morphological studies (Telles \& Terlevich 1994) of HII galaxies, i.e. dwarf galaxies dominated by a very luminous starburst, have indicated that luminous HII galaxies tend to show distorted morphology suggestive of tidal interactions triggering the present starburst while low luminosity HII galaxies tend to be instead symmetric and regular. To check the tidal origin of the starburst in HII galaxies, we have searched for companions in the neighbourhood of a sample of 51 HII galaxies. We found that only 12 HII galaxies have a neighbour within a projected distance of 1 Mpc and 250 \kmsec~ in velocity difference, and of these 12, only 4 have a luminous (M$_B <$ --19) neighbour. Surprisingly, isolated HII galaxies tend to be of high luminosity and disturbed morphology while HII galaxies with neighbours tend to be low luminosity regular HII galaxies. Furthermore, the metal abundance and the equivalent width of the emission lines in HII galaxies do not depend on the presence of a companion. These results are opposed to simple expectations if interaction with a bright companion is the main mechanism triggering the starbursts. We have also found a loose group of HII galaxies with no luminous companion. For this, there is the additional difficulty of understanding how these starbursts are synchronized on time scales of less than $10^7$ yrs in systems separated by $\sim$ 1-2 Mpc.Comment: 7 pages, uuencoded, compressed, postscript file. (three figures included). Also available via anonymous ftp from ftp://cast0.ast.cam.ac.uk/pub/etelles/environ.ps.

The Local Environment of HII Galaxies

We address the question of whether violent star formation in HII galaxies is induced by low mass companions by describing statistically their local environment as estimated by the correlation function. We argue that even if low mass companions were mainly intergalactic HI clouds, their optical counterparts should be detectable at faint limits of the Automatic Plate Measuring Machine scans. We then cross-correlate a large sample of HII galaxies with the APM faint field galaxy catalogue. The preliminary results are all consistent with HII galaxies being a randomly selected sample of normal faint field galaxy with no extra clustering. This suggests that at least in these dwarf starburst galaxies star formation is not triggered by tidal interactions and may have a different origin.Comment: 4 pages, 2 Postscript figures, uses moriond.sty. To appear in ``Dwarf Galaxies and Cosmology'', Eds. T.X. Thuan, C. Balkowski, V. Cayette, J. Tran Than Van, Editions Frontieres (Gyf-sur-Yvette, France

High resolution spectroscopy of H II Galaxies: Structure and Supersonic line widths

We present high resolution echelle spectroscopy of a sample of H II galaxies. In all galaxies we identify different H(alpha) emitting knots along the slit crossing the nucleus. All of these have been isolated and separately analyzed through luminosity and size vs diagnosis plots. We find that in all cases, for a particular galaxy, the bulk of emission comes from their main knot and therefore, at least for the compact class galaxies we are dealing with, luminosity and sigma values measured using single aperture observations would provide similar results to what is obtained with spatially resolved spectroscopy. In the size vs plots as expected there is a shift in the correlations depending on whether we are including all emission in a single point or we split it in its different emitting knots. The problem of a proper determination of the size of the emitting region so that it can be used to determine the mass of the system remains open. From the data set gathered, using the highest surface brightness points as recently proposed by Fuentes-Masip et al. (2000), the best luminosity vs correlation turns out to be consistent with a Virial model.Comment: 14 pages, 4 figures, ApJ accepted. Also available from http://www.daf.on.br/~etelles/papers/wht.ps.g

The Stellar Populations of HII galaxies: A tale of three bursts

We present a UV to mid-IR spectral energy distribution study of a large sample of SDSS DR13 HII galaxies. These are selected as starburst (EW(H$\alpha) > 50$\AA) and for their high excitation locus in the upper-left region of the BPT diagram. Their photometry was derived from the cross-matched GALEX, SDSS, UKDISS and WISE catalogues. We have used CIGALE modelling and SED fitting routine with the parametrization of a three burst star formation history, and a comprehensive analysis of all other model parameters. We have been able to estimate the contribution of the underlying old stellar population to the observed equivalent width of H$\beta$ and allow for more accurate burst age determination. We found that the star formation histories of HII Galaxies can be reproduced remarkably well by three major eras of star formation. In addition, the SED fitting results indicate that: i) in all cases the current burst produces less than a few percent of the total stellar mass: the bulk of stellar mass in HII galaxies have been produced by the past episodes of star formation; ii) at a given age the H$\beta$ luminosity depends only on the mass of young stars favouring a universal IMF for massive stars; iii) the current star formation episodes are {\it maximal} starbursts, producing stars at the highest possible rate.Comment: 12 pages, 14 Figures, Accepted for publication in Astronomy & Astrophysic

The Internal Kinematics of the HII Galaxy II Zw 40

We present a study of the kinematic properties of the ionized gas in the dominant giant HII region of the well known HII galaxy: II Zw 40. High spatial and spectral resolution spectroscopy has been obtained using IFU mode on the GMOS instrument at Gemini-North telescope. We have used a set of kinematics diagnostic diagrams, such as the intensity vs. velocity dispersion intensity vs. radial velocity, for global and individual analysis in sub-regions of the nebula. We aim to separate the main line broadening mechanisms responsible for producing a smooth supersonic integrated line profile for the giant HII region. The brightest central region (R ~ 50 pc) is responsible for sigma derived from a single fit to the integrated line profile. The dominant action of gravity, and possibly unresolved winds of young (<10 Myr) massive stars, in this small region should be responsible for the characteristic Halpha velocity profile of the starburst region as a whole. Our observations show that the complex structure of the interstellar medium of this galactic scale star-forming region is very similar to that of nearby extragalactic giant HII regions in the Local Group galaxies.Comment: 40 pages, 13 figures. Accepted for publication in The Astrophysical Journa

The Morphology of HII Galaxies

We present CCD images of a sample of 39 HII galaxies taken at the Danish 1.54m telescope on La Silla. The images are used to analyse the morphology of these emission line dwarfs, and the structural properties of the knots of star formation and of the underlying galaxy. The sizes of the starbursts are measured. We propose a morphological classification based on the presence or absence of signs of tails, extensions, or distorted outer isophotes. This criterion segregates the objects into two broad morphological types with different physical properties: the more disturbed and extended (type I) HII galaxies having larger luminosities and velocity dispersions than the more compact and regular (type II) objects. The relative position of HII galaxies and of a sample of dwarf elliptical galaxies in the [R -- sigma] diagram support the hypothesis of a possible evolutionary link between the two types of galaxy.Comment: 11 latex pages, including 5 postscript figures within the text (106 Kb). MNRAS accepted. Image figures not included in the text can be sent to you upon request or found in http://www.iagusp.usp.br/~etelles/papers/images