STAR-FORMING DWARF GALAXIES: Evolutionary self-consistent models Mariluz Martín-Manjón Mercedes Mollá Ángeles Díaz Roberto Terlevich VII Workshop Estallidos, Madrid 2009
Star formation history of HII galaxies: What is the problem? HII galaxies: Metal poor systems with young massive ionizing stars HII galaxies: Metal poor systems with young massive ionizing stars Are really young galaxies? - Intermediate age and old stars, even in I Zw18 (Izotov & Thuan, 2004) -Only a few HII galaxies show EW(H 200 Å -Anti-correlation EW(H )-colour: HII galaxies show redder colours than expected at low EW NOT reproduced by SSPs NOT reproduced by SSPs (Terlevich et al. 2004) * An older underlying non ionizing population MUST exist * Then, how is the star formation history? There are 3 possible scenarios postulated: 1.Bursting SF 1.Bursting SF : short and intense SF bursts+long quiescent periods (Bradamante et al. 1998) 2.Gasping SF 2.Gasping SF : long moderated SF bursts+short quiescent periods (Tosi et al.1991) Continuous SF 3. Continuous SF : low intensity continuous SF + sporadic bursts (Legrand 2000) SSPs:Mollá et al Obs.data:Hoyos & Díaz 2006 Terlevich et al. 1991
Self-consistent Models Self-consistent Models Chemical evolution code : Chemical evolution code (Ferrini et al.92,94) : SFH, evolution of metallicity and abundances Evolutionary synthesis code + Evolutionary synthesis code (Mollá et al.2009, submitted) : S.E.D and colours. Photoionization code + Photoionization code (CLOUDY, Ferland 1998) : emission lines We use the whole information available for the galaxy sample: IONIZED GAS (present state of the galaxy) and SPECTROPHOTOMETRIC PARAMETERS (related to its SFH) in a SELF-CONSISTENT way. STAR-BURSTING MODEL STAR-BURSTING MODEL based on Martín- Manjón et al 2008 Successive bursts star formation scenario t= Gyrs, Each model is characterized by 3 PARAMETERS -Initial Efficiency ( ε ): -Initial Efficiency ( ε ): the amount of gas consumed to form stars in the first burst of SF. M tot =10 8 M sun 33% (high efficiency model) 10% (low efficiency model) -Attenuation : -Attenuation : The initial efficiency of SF is attenuated in the successive bursts : soft attenuation & strong attenuation -Time between bursts ( Δ t): -Time between bursts ( Δ t): Every burst takes place instantaneously, followed by quiet periods. Δt= 1.3 Gyr Δt=0.1 Gyr and Δt=0.05 Gyr (comparison)
Results Initial Efficiency ( ε ) principally leads the SFR and the initial oxygen abundance principally leads the SFR and the initial oxygen abundance (Hoyos et al. 2004, Hoyos & Diaz 2006). The initial efficiency also leads the behaviour of the ionized gas: of the ionized gas: Emission lines are produced by the ionizing photons of the massive stars present in the current burst. high initial efficiency (33%): high excitation and high abundance galaxies, high [OIII]/H low initial efficiency (10%): less metallic galaxies, with high [OIII]/H and low [OII]/H ratios.
Regulates the evolution of the SFR and oxygen abundance. The Attenuation of the bursts determines the contribution of the underlying non ionizing population: The Attenuation of the bursts determines the contribution of the underlying non ionizing population: A higher attenuation implies a larger contribution from the previous bursts to the total SED. The time between bursts ( Δ t) has a similar effect to the attenuation. The reduction of the time between bursts offsets the effect of increasing the attenuation. The EW(H β ) decreases more from burst to burst, however colours can be bluer. Δ t can not be less than 50 Myr Colours of the continuum and the evolution of the EW(Hb): The contribution of the underlying population to the total continuum must be higher than the contribution of the current burst which dominates the emission line spectruum. Attenuation Attenuation
BCD-QBCD connection BCD+QBCD Star-bursting model : starburst phase + quiescent period BCD+QBCD (Sánchez-Almeida et al. 08) BCD t= yr QBCD= Δt W Cairós et al (red dots) and Telles & Terlevich 1997 (cont+lin, blue squares) BCSs can only be reproduced by our models taking into account the contribution of the emission lines to the continuum colours in the BCD phase (blue dots) Corrected colours of BCDs (Mrk 370 and Telles & Terlevich pink squares) are reproduced by the model continuum colours (black dots) BCDs Host galaxy colours (Telles & Terlevich 1997,grey squares) and QBCDs (Sánchez Almeida et al. 2008) are repoduced by the continuum and interburst continuum colours. (coloured squares)
Summary & Conclusions We have made models which consist in instantaneous star formation bursts spread along 13.2 Gyr. We have made models which consist in instantaneous star formation bursts spread along 13.2 Gyr. it is necessary only to adjust 3 principal parameters In order to reproduce the observable characteristics of HII galaxies it is necessary only to adjust 3 principal parameters: *Initial efficiency *Initial efficiency: the amount of gas involved in star bursts, which leads the SFR, the oxygen abundance, an d the range of metallicity covered by the emission lines produced by the current burst. * Attenuation * Attenuation sets the contribution of the underlying continuum from the previous stellar generations. * Inter-burst time : * Inter-burst time : similar effect in colours to the attenuation: decreasing t, we produce a larger contribution of the underlying continuum, but the effect in colours could be the same as decreasing the attenuation of the bursts. Our method reproduces all observable characteristics of HII galaxies: abundances, Our method reproduces all observable characteristics of HII galaxies: abundances, colours and emission lines at the same time. QBCD colours are also reproduced. colours and emission lines at the same time. QBCD colours are also reproduced. can be reproduced by changing these three parameters The three hypothesis scenario of SF can be reproduced by changing these three parameters Bursting Bursting: We have already seen the results. Gasping : Gasping : Increasing the attenuation, and reducing t : we obtain a major contribution to the continuum of the previous bursts. This contribution will make the underlying population to be younger and then the colours to be bluer. It would be possible to find intermediate age stellar populations, younger than 1 Gyr, even after several star bursts, Continuous Continuous: Reducing the inter-burst time, and the efficiency of the burst to obtain a extremely low SFR. (see Gavilán et al. Models ) Martín-Manjón et al. MNRAS 2008 Martín Manjón et al (arXiv ) Martín Manjón et al (ASPC)