STAR-FORMING DWARF GALAXIES: Evolutionary self-consistent models

1. STAR-FORMING DWARF GALAXIES:Evolutionary self-consistent models VII Workshop Estallidos, Madrid 2009 Mariluz Martín-Manjón Mercedes Mollá Ángeles Díaz Roberto Terlevich

2. Star formation history of HII galaxies: What is the problem? 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(Terlevich et al. 2004) * An older underlying non ionizing populationMUST exist * Then, how is the star formation history? There are 3 possible scenarios postulated: • Bursting SF: short and intense SF bursts+long quiescent periods (Bradamante et al. 1998) • Gasping SF: long moderated SF bursts+short quiescent periods (Tosi et al.1991) 3.Continuous SF: low intensity continuous SF + sporadic bursts (Legrand 2000) SSPs:Mollá et al. 2009 Obs.data:Hoyos & Díaz 2006 Terlevich et al. 1991

3. Self-consistent Models Chemical evolution code(Ferrini et al.92,94): SFH, evolution of metallicity and abundances +Evolutionary synthesis code(Mollá et al.2009, submitted): S.E.D and colours. +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 MODELbased  on Martín- Manjón  et  al  2008 Successive bursts star formation scenario t=0-13.2 Gyrs, Each model is characterized by 3 PARAMETERS -Initial Efficiency (ε):the amount of gas consumed to form stars in the first burst of SF. Mtot=108Msun 33% (high efficiency model) 10% (low efficiency model) -Attenuation :The initial efficiency of SF is attenuated in the successive bursts : soft attenuation & strong attenuation -Time between bursts (Δt):Every burst takes place instantaneously, followed by quiet periods. Δt= 1.3 Gyr Δt=0.1 Gyr and Δt=0.05 Gyr (comparison)

4. Results Initial Efficiency (ε) 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: 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.

5. Attenuation Regulates the evolution of the SFR and oxygen abundance. 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. 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. The time between bursts (Δt) has a similar effect to the attenuation. The reduction of the time between bursts offsets the effect of increasing the attenuation. The EW(Hβ) decreases more from burst to burst, however colours can be bluer. Δt can not be less than 50 Myr

6. BCD-QBCD connection Star-bursting model: starburst phase + quiescent periodBCD+QBCD(Sánchez-Almeida et al. 08) BCD t=0-107 yr QBCD= Δt Cairós et al. 2001 (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 Almeidaet al. 2008) are repoduced by the continuum and interburst continuum colours. (coloured squares) W

7. Summary & Conclusions We have made models which  consist  in  instantaneous  star  formation  bursts  spread  along  13.2  Gyr.    In  order  to  reproduce  the  observable  characteristics of HII galaxies it is necessary only to adjust 3 principal parameters:  *Initial  efficiency:  the  amount  of  gas  involved  in  star  bursts,  which   leads  the  SFR,  the  oxygen  abundance, and the range of metallicity covered by the emission lines produced by the  current burst. *Attenuation sets  the  contribution  of  the  underlying  continuum  from  the  previous  stellar  generations . *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, colours and emission linesat the same time. QBCD colours are also reproduced. The three hypothesis scenario of SF can be reproduced by changing these three parameters Bursting: We have already seen the results. 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: 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. 2008 (arXiv0901.1186) Martín Manjón et al. 2008 (ASPC)