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The stellar content of I Zw 18: constraints from synthetic colormagnitude diagram modelling
Taras Yakobchuk

Main Astronomical Observatory, National Academy of Sciences of Ukraine e-mail: yakobchuk@mao.kiev.ua






Blue compact dwarf galaxy I Zw 18
NASA, ESA Y. Izotov (MAO, Kyiv, UA) and T. Thuan (University of Virginia)

Hubble Space Telescope /Advanced Camera for Surveys






HST/ACS image of I Zw 18 in F606W(V) filter
(GO: 10586, PI: Aloisi et al.)

Big ellipse: the region of the photometric analysis Smaller ellipses: the adopted inner boundaries of the Main Body and the C component Small circles: diffuse background objects, manually selected and removed from the photometry






Uncleaned CMDs [V-I, I] of I Zw 18, obtained using Aloisi et al. (left) and Izotov & Thuan (right) datasets. Apperture corrections applied, magnitudes corrected for the Galactic extinction (AV=0.101 mag).

Preliminary photometry






Diffuse background sources and false detections
CMDs [V-I, I] of the diffuse region of I Zw 18 (left) and outer sources manually selected in the inner the objects found beyond the adopted boundary (right).






CMD [V-I, I] of I Zw 18 without the background objects the photometric cuts applied (left) and the final 'clean' Near the half of the detected points disappear after

Initial contaminated and final CMDs of I Zw 18

removed and CMD (right). cleaning.






CMDs of the main body and the C component
Main stellar evolutionary sequences are marked with the dotted lines. Do we see the red giant branch stars here? And where is the TRGB?






Spatial distributions of stellar populations in I Zw 18
(a) (b) (c) (d) All stars MS, BL, SG AGB RGB?

Q.: Is the data 'deep' enough for registration of the RGB stars in the outer parts? Where is the 'halo'?



Blue compact dwarf galaxy SBS 1415+435
Hubble Space Telescope / Advanced Camera for Surveys
NASA, ESA A. Aloisi (STScI, ESA) et al.



Spatial distributions of stellar populations in SBS 1415+437
(a) (b) (c) (d) All stars MS, BL, SG AGB RGB

RGB stars occupy the largest area (though their distribution quite inhomogeneous)



The number of points on the CMD of SBS 1415+437 (right) is scaled to the statistic of I Zw 18 (left). RGB for SBS 1415+437 is easily recognizable, but not for the I Zw 18 CMD.

'Halo' populations of I Zw 18 and SBS 1415+437






Synthetic colormagnitude modelling. Ingredients.
1. Isochrone library: Marigo et al. (2008) with log(t)=6.5-10.2 and log(t)=0.01, Z=0.0004-0.001. 2. Initial mass function (IMF): =-2.35 (Salpeter (1955)) with Mmin=0.07 M and Mmax=120.0 M, =0.25 M. 3. Binary fraction: =0-0.3, flat IMF (q=0.5). 4. Simulating the observational conditions: artificial star tests (AST), recipes from Aparicio & Gallart (1995), Gallart et al. (1996). 5. Star-formation history (SFH) recovering: recipes from Dolphin (2002), Harris & Zaritsky (2001), minimization algorithm based on the downhill simplex method, uncertainties determined by MonteCarlo tests.



Isochrones superposed on the CMD of I Zw 18 for both distance moduli (m-M)=30.5 and 32.0 are near equally good at reproducing the observed distributions of stars.

Theoretical isochrones and the observed CMD






Searching the halo at D=12.6 Mpc
Synthetic CMDs of the 'artificial haloes' for a given distance, ages and total masses; (b),(d): black points ­ observed distribution; red points ­ modelled; For D=12.6 Mpc the data is well deep enough to permit the 'halo' registration if it is present.



Searching the halo at D=25.0 Mpc
Synthetic CMDs of the 'artificial haloes' for a given distance, ages and total masses; (b),(d): black points ­ observed distribution; red points ­ modelled; For D=25.0 Mpc the data is still deep enough to permit the 'halo' registration (if its stellar mass >106-7 M).



Starformation history of the main body and the C component of I Zw 18 at D=25.0 Mpc






Starformation history of the main body and the C component of I Zw 18 at D=20.0 Mpc






Starformation history of the main body and the C component of I Zw 18 at D=12.6 Mpc






Starformation history of the main body and the C component of I Zw 18 at D=16.0 Mpc






Starformation history of the main body and the C component at D=16.0 Mpc and tSFH<2 Gyr






Comparison with SBS 1415+437

The mass fraction of old stars (age>1 Gyr) for I Zw 18 (c,d) is chosen similar to SBS 1415+437.



Conclusions
1. The halo of or has available data is well deep enough to detect the extended the red giant branch stars in I Zw 18, but it is either absent very low mass (M<~105 M).

2. The distance to the galaxy is 16.0 Mpc or less, the models with the higher values show bad agreement with the observed CMDs. 3. The minimum age of the red giant branch stars in I Zw 18 is ~2 Gyr, the upper limit is uncertain due to the very poor statistics and large photometric errors. 4. The mass fraction of the old stars in I Zw 18 with ages more than 1 Gyr can be as high as ~0.5. The stars-to-gas mass ratio M*/M(HI) can be 0.1 or less.






Thank you for attention