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Study evolutionary status of dwarfs in the nearby Lynx-Cancer void. Preliminary results

Pustilnik S.A. , Tepliakova A.L. (SAO RAS) Kniazev A.Y. (SAAO, Cape Town). Study evolutionary status of dwarfs in the nearby Lynx-Cancer void. Preliminary results. Introduction. The galaxy evolution versus the global environment ( clusters , groups , voids ) is a long-studied problem.

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Study evolutionary status of dwarfs in the nearby Lynx-Cancer void. Preliminary results

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  1. Pustilnik S.A. , Tepliakova A.L. (SAO RAS) Kniazev A.Y. (SAAO, Cape Town) Study evolutionary status of dwarfs in the nearby Lynx-Cancer void. Preliminary results

  2. Introduction The galaxy evolutionversus the global environment (clusters, groups, voids) is a long-studied problem. Voidsare delineated byluminous (L > L*) massive objectsand populatedby dwarfs. The galaxy densities and other parameters vary significantly from one void to another. One expects that not only galaxy interactionsoccur in voids much less frequently, but also DM halo and galaxy formation from density perturbations can be somewhat retarded due to the lower voidmean gravitational potential.

  3. Motivation. Lynx-Cancer void Despite the study galaxies in voids is an actual direction, not much is done so far. Almost all largevoids with D > 20-30 Mpc are quite distant! Due to selection effects one can see and study in them only a small, brighter part of void galaxies (M_B < ~ --17), and those with strong emission-line spectra. More typical and fainter dwarfs (M_B=--12 to --16) remain largely unexplored. The only nearby large ‘the Local Void’ of Tully et al. (2008) is close, but pretty empty. A.Fairall (1998) described several small nearby voids (with sizes of D~10 Mpc) in Cetus, Cepheus, Volans, Monoceros

  4. Nearby voids

  5. Lynx-Cancer void Pustilnik et al. (2003) described Lynx-Cancer void (with `conditional’ center at RA~8.5 h, Dec~+30, D ~14 Mpc, with the total extent D ~> 10 Mpc). The known to date void dwarf galaxy population consists of ~50 objects with M_B down to -12 (Pustilnik & Tepliakova, in preparation). The important advantage of this region is that it is well covered by the SDSS images and spectra. The project goals Comparative study of evolutionary status of dwarfs in this void and in nearby denser structure elements. Parameters: 1. gas metallicity (HII regions O/H), 2. gas mass fractionMg/(Mg+M*), 3. ages of the oldest visible stellar populationT_star.

  6. Lynx-Cancer void cosmography.I

  7. Lynx-Cancer void cosmography.II

  8. 10-degree Declination slices in Lynx-Cancer void

  9. Sample galaxies. Images

  10. Sample galaxies. Properties

  11. Observational data Major part of this region is covered bySDSS DR7. This provides a) radial velocities for many dwarfs; b) five-filter good quality images with some photometry; c) small fraction of galaxies with spectra suitable for O/H determination. Own spectral survey with 6-m telescope (BTA) for O/H (in preparation). Integrated HI parameters – partly from literature,the restare from our HI observations atNRT (Nancay) with J.-M. Martin (in preparation). ===================================== A caveat: LSB galaxies are rare objects among SDSS spectral targets.Therefore the LSB dwarfs with knownvelocities are underrepresented. HI blind survey may help. But ALFALFA will cover only less then a half of the respective sky section

  12. Lynx-Cancer void: some unusual galaxies First very low O/H galaxy BCG HS 0822+3542 O/H=7.4, M_B~-12.9 (Kniazev et al. 2000) LSB dwarf SAO 0822+3545very blue, M_B~-13.3 (at d~14 kpc), interacting with HS 0822+3542. SDSS ugri colors vs PEGASE2 evolutionary tracks: no stars with T > 2-3 Gyr.A dwarf pairclose to the void center (Pustilnik et al. 2003). DDO 68 – a nearby dwarf (M_B=-15.3) with record low O/H (7.14, Pustilnik+05, Izotov+06). SDSS ugri colors vs PEGASE2  T < 1 Gyr (Pustilnik etal. 2008). HI maps with GMRT: recent merging of two very gas-rich objects (Ekta etal. 2008). SDSS J0926+3343 – edge-on LSBD with M_B~-12.9, O/H=7.12 and ugri colors  T ~1--3 Gyr. At D of only ~1.6 Mpc from DDO 68 (Pustilnik etal. 2009, MNRAS)

  13. Filaments, “groups”, pairs • ~80% of void dwarfs enter to pairs, or/and belong to filaments with lengths of 1-2 Mpc. Groups are rare. About 20% of dwarfs look like isolated

  14. Summary of spectral data In total, for ~35 late-type dwarfs O/H is determined either by direct T_e method (with [OIII]4363 line), or by semi-empirical method of Izotov & Thuan (2007) For 17 of them: Zo/35 < Z < Zo/10 For 4 galaxies of these 17: 12+log(O/H) < 7.35: SDSS J0926+3343 O/H=7.12 DDO 68 (J0956+2849) O/H=7.14 SDSS J0812+4836 O/H=7.28 SDSS J0737+4724 O/H~7.30 ---------------------------------------------------------------------- [ … plus adjacent I Zw18 (J0934+5514) O/H=7.17 ]

  15. DDO 68. Summary

  16. DDO 68 summary

  17. Edge-on LSBDSDSS J0926+3343 SCORPIO slit position SDSS finding chart 2D spectrum of J0926+3343

  18. BTA spectral results • Spectra of HII regions «a» and «b» with SCORPIO. Estimates of O/H by classic T_e method give meanvalue of 12+log(O/H)=7.12+-0.02.

  19. SDSS J0926+3343 SDSS J0926+3343 is one of two galaxies with the record low O/H (of many thousands known to date). This is situatedinside theLynx-Cancer void, at the distance of~1.6 Mpc from DDO 68, the next known lowest O/H galaxy. Blue SDSS ugri colors for outer regions of SDSS J0926+3343, as compared with PEGASE2 evolutionary tracks indicate ages of the oldest stellar popul. of ~1—3 Gyr

  20. Metal deficiency in void dwarfs • The basic relation “O/H vs M_B” as derived on the Local Volume groups and field dwarfs (van Zee & Haynes 2006, the upper dashed line) and O/H, observed in void dwarfs • The line shifted by 1 mag (or ~0.2 dex on O/H) better accounts for possible systematic lower O/H in the void • Several extremely low O/H dwarfs: very rare special cases

  21. Colors of dwarfs in the void. SAO 0822+3545 (unknown O/H)

  22. Colors of dwarfs in the void. SDSS J0737+4724 (12+log(O/H)~7.3)

  23. Colors of dwarfs in the void. SDSS J0843+4025 (12+log(O/H)=7.59)

  24. Colors of dwarfs in the void. SDSS J0859+3923 (12+log(O/H)=7.57)

  25. Colors of dwarfs in the void. SDSS J0911+3136 (12+log(O/H)=7.51)

  26. Conclusions About 50 dwarfs are currently found within the Lynx-Cancer void. Their study allows much deeper probe of void galaxy properties down to absolute magnitudes of M_B~-12.0 in the volume of >~500 Mpc^3 and comparison with galaxies in more typical environments in the Local Volume. Our preliminary results indicate the sizable effect of void environment: * Late-type dwarfs in LC void are on mass more metal-deficient. Several outstanding void dwarfs are the most metal-poor known. * Parameter M(HI)/L_B is enhanced for void galaxies. * Several dwarf galaxies have outer colors consistent with the ages of the oldest visible stars of ~1--3 Gyr.

  27. Prospects and future research • The nearby voids are good places to address cosmologically related issues of N-body simulations of galaxies and their structures. • It appears that the least evolved and “young” galaxies favor the void environment => the way for hunting for local `cousins’ of high-redshift young galaxies! • Number and spatial distribution of void galaxies bears clues for CDM cosmology dwarf galaxy formation. • LSB dwarfs are expected to be the main population in voids, but currently are not well represented. Need in means of their efficient selection and redshift/distance measurement.

  28. Our collaborators • We acknowledge the contribution to the project • from J.-M.Martin (Obs. Paris, Nancay RT), • Y.Lyamina (Rostov U.) and E.Shaldenkova (SAI, Moscow)

  29. Thank you for attention!

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