Transcript Study properties of dwarf galaxies in and around the nearby Lynx

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)
Introduction
The galaxy evolution versus the global environment
(clusters, groups, voids) is a long-studied problem.
Voids are delineated by luminous (L > L*) massive
objects and populated by dwarfs. The galaxy
densities and other parameters vary significantly from
one void to another.
One expects that not only galaxy interactions occur in
voids much less frequently, but also DM halo and
galaxy formation from density perturbations can be
somewhat retarded due to the lower void mean
gravitational potential.
Motivation. Lynx-Cancer void
Despite the study galaxies in voids is an actual direction, not
much is done so far. Almost all large voids 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
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Nearby voids
Lynx-Cancer void
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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 fraction Mg/(Mg+M*),
3. ages of the oldest visible stellar population T_star.
Lynx-Cancer void cosmography.I
Lynx-Cancer void cosmography.II
10-degree Declination slices in Lynx-Cancer void
Sample galaxies. Images
Sample galaxies. Properties
Observational data
Major part of this region is covered by SDSS 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 rest are
from our HI observations at NRT (Nancay) with J.-M.
Martin (in preparation).
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A caveat: LSB galaxies are rare objects among SDSS spectral
targets. Therefore the LSB dwarfs with known velocities are
underrepresented. HI blind survey may help. But ALFALFA
will cover only less then a half of the respective sky section
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Lynx-Cancer void: some unusual galaxies
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•
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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 pair close 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)
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
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 ]
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DDO 68. Summary
DDO 68 summary
Edge-on LSBD SDSS J0926+3343
SCORPIO slit position
SDSS finding chart
2D spectrum of J0926+3343
BTA spectral results
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Spectra of HII regions «a» and «b» with SCORPIO.
Estimates of O/H by classic T_e method give mean value
of 12+log(O/H)=7.12+-0.02.
SDSS J0926+3343
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SDSS J0926+3343 is one
of two galaxies with the
record low O/H (of many
thousands known to
date). This is situated
inside the Lynx-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
Metal deficiency in void dwarfs
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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
Colors of dwarfs in the void.
SAO 0822+3545 (unknown O/H)
Colors of dwarfs in the void.
SDSS J0737+4724 (12+log(O/H)~7.3)
Colors of dwarfs in the void.
SDSS J0843+4025 (12+log(O/H)=7.59)
Colors of dwarfs in the void.
SDSS J0859+3923 (12+log(O/H)=7.57)
Colors of dwarfs in the void.
SDSS J0911+3136 (12+log(O/H)=7.51)
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 metaldeficient. 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.
Prospects and future research
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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.
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)
Thank you for attention!