Dave Wilman
Morphological Composition of z~0.4 Groups: The site of S0
Formation?
Wilman, Oemler, Mulchaey, McGee, Balogh & Bower
2009, ApJ, 692, 298 *
*Also described as a ‘Research Highlight’ in Nature, March 5 2009, Vol 458, Issue 7234
Decline of Star Formation at low z
e.g. Madau et al, 98
Declining SFR in individual galaxies
Lower fraction of star forming galaxies
AND
COMBINED:
Decline of Star Formation at low z
e.g. Madau et al, 98
Declining SFR in individual galaxies
Lower fraction of star forming galaxies
AND
COMBINED:
Evolution in Clusters - Blue Galaxies
Butcher & Oemler, 1984
Strongly evolving fraction of blue (star forming)
galaxies
Ellipticals and S0sBoth typically passive - but morphologies relate to dynamical state of galaxy - information about formation
Morphologies of cluster early-types
Fasano et al, 00following
Dressler et al, 97
Strongly increasing fraction of S0s
Morphology traces local densityDressler, 1980
Morphology imprinted inside groups prior to
infall?or
by cluster-centric processes?
The group regime
Postman & Geller, 1984
Morphology-Density relation
exists in low-z groups
Visual Morphological Classification
• ACS F775W coverage of 26 0.3<z<0.55 CNOC2 F-O-F groups (see Balogh talk, Carlberg et al, 01; Wilman et al, 05).• Groups velocity dispersions 100-500km/s (& a couple of ~700km/s systems)• Serendipitous Field sample (galaxies not in groups).• Classifications by Gus Oemler and John Mulchaey:
• 2 stretch levels displayed to identify high and low SB features
• Randomly ordered galaxies -> no bias with group/field environment• Classification using the MORPHS scheme:• Group into categories of classification: E (E, E/S0), S0 (S0/E, S0, S0/a, SB0/a), eSp (Sa-Sbc includes bars), lSp (Sc-Sm includes bars), Irr.
Visual Morphological ClassificationsThe most difficult classification decisions are:
E vs S0, non edge-on: Look for a break in SB profile = hard outer edge to the bulge
S0 vs Sa: Look for surface brightness irregularities in the disk (spiral arms)
Classifications highly consistent:
222/310 identical
69 ΔT=0.5, 17 ΔT=1, 2 ΔT=1.5
GIM2D Decompositions of S0s, eSpsDecompositions: McGee et al, 08
GIM2D Decompositions of S0s, eSps
S0s have MUCH higher B/T than spirals Decompositions: McGee et al, 08
Composition of Groups/Field as f(luminosity)
Early-Type Spirals (eSp)
All Spirals (eSp+lSp)
S0s:40/178 group10/109 field
Luminosity limits at: z=0.55 z=0.3
Jackknife errors
r-band luminosity
Significance of Difference between Group and Field
With Field Sample Luminosity-Matched to Groups:• Resampled field: >=f(S0)grp 0 times (>99.999%)
>=f(E)grp (85%)<=f(Sp)grp (99.95%)of which most difference for late-type spirals
• No difference to f(S0)grp, and still very significant excess, if:
• Most massive groups ignored• S0/a ignored
Radial TrendsCompare inner (<300kpc) and outer group populations at fixed luminosity.
Interesting Trends:
• More Bright Ellipticals in inner group: >99.999% confidence • More S0s in OUTER group: 97% confidence
Composition vs environment and z
Ellipticals: No clear dependence on environment OR z!
S0s: Clear dependence on zAt least as populous in Groups as Clusters
Spirals:Clear dependence on z. As populous in Clusters as Groups
to MV=-20.53, for comparison with Fasano et al, 00 cluster sample
Similar early-type fraction in the supergroup SG1120 to clusters
Kautsch et al, 2008
see also poster by Dennis Just
Bulge growth
S0s have MUCH higher B/T than spirals
Late-Type Spiral -> S0?
Poggianti et al, 2008
will need:• significant bulge growth (will be eSp for a stage)and eventually:• a truncated gas supply (stop SF)
Late-Type Spiral -> S0?
Poggianti et al, 2008
will need:• significant bulge growth (will be eSp for a stage)and eventually:• a truncated gas supply (stop SF)
Late-Type Spiral -> S0?
Poggianti et al, 2008
will need:• significant bulge growth (will be eSp for a stage)and eventually:• a truncated gas supply (stop SF)
IR-bright progenitors?Geach et al, 2008
24μm bright
galaxies mainly in infall regime
Bulge Growth by Star FormationMaximal Case:For a B/T=0 galaxy to evolve into a B/T=0.7 galaxy with total final M*=2x1011Msol:Require ~1.4x1011 Msol bulge growth in ~4 Gyr.-> SFR ~ 35 Msol/yrQuite reasonable for a dusty star former i.e. 24μm strong galaxies typically forming stars at these
rates at intermediate-high z(Bell et al, 05; Bai et al, 07; Marcillac et al, 07; Saintonge et al, 08; Geach et al, 08)
IR-bright progenitors?
Tyler et al, in preplog(LTIR/Lsun)
fiel
dgr
oup
MIPS 24μm data: No particularly bright Starbursts in
groups
All
Stripping?
Disk scaling relation: black (red) solid line best fit to field (group) galaxies. Dotted (dashed) line: effect of truncating SF in field galaxies 1 (3) Gyr earlier.
McGee et al, 08 No difference in Group/Field Disk scaling relations
IR colour reveals low level SF
0.5
SSP
0.4 [f(
8)/f
(3.6
)]
M* (x1010M◦)Wilman et al., 08
Possible lack of truly passive spirals?
Wilman et al, 08
IR colour reveals low level SF
0.5
SSP
0.4 [f(
8)/f
(3.6
)]
M* (x1010M◦)Wilman et al., 08
Possible lack of truly passive spirals?
Wilman et al, 08InfraRed Passive Sequence (IPS)
IR colour reveals low level SF
Weak / no [OII]
0.5
SSP
0.4 [f(
8)/f
(3.6
)]
M* (x1010M◦)Wilman et al., 08
Possible lack of truly passive spirals?
Wilman et al, 08InfraRed Passive Sequence (IPS)
Mechanisms in and around Clusters
Moran et al, 07
•Spirals with no UV emission tend to live in cluster regions where ram-pressure should be important. •Young S0s and UV-bright low-[OII] spirals only exist at lower density.
Mechanisms in and around Clusters
Moran et al, 07
•Spirals with no UV emission tend to live in cluster regions where ram-pressure should be important. •Young S0s and UV-bright low-[OII] spirals only exist at lower density.
Stripping
Other Mechanisms• Minor Mergers:
• A few of intermediate mass ratio minor mergers should produce galaxies with structural parameters typical of S0 (Bournard et al, 05)• Should be common:
• with halo central galaxies (but many merger events should often lead to formation of Elliptical)• in filamentary structure surrounding group / cluster
• Shapiro/Bureau et al see such events with SF / CO rich cores in SAURON S0 galaxies - often counter-rotating in stars/gas/both
• Tidal Interactions / Group-scale Harassment:• Low velocity encounters should be common in groups (but lack of IR-bright SBs)
Bournard et al., 07
Neither method directly explains lack of star formation in S0sBUTBulge growth accompanied by SMBH growth: Most low accretion (optical) AGN live in intermediate density environments
Conclusions & StatusWhat we know:• S0s common in group environment (and not limited to cores):
• Stripping processes unlikely to be important in majority of cases
• Late type spiral fraction a strong function of environment:• lSps as progenitor requires significant bulge growth
• Bright Ellipticals prefer cores of groups but otherwise exist in similar fraction for groups/field/clusters
• Mergers most common at halo centre - consistent with many/major merger hypothesis for E formation• Consistent with hypothesis that most new early types are S0s
Likely Mechanisms for S0 formation:Minor Mergers, Tidal Interactions / Group Harassment
Fingerprinting the IGM with X-ray and Optically selected groups - see poster by J. Connelly
