Optically-passive spirals: The missing link in gradual star formation suppression upon cluster infall

Abstract

Galaxies migrate from the blue cloud to the red sequence when their star formation is quenched. Here, we report on galaxies quenched by environmental effects and not by mergers or strong AGN as often invoked: They form stars at a reduced rate which is optically even less conspicuous, and manifest a transition population of blue spirals evolving into S0 galaxies. These 'optically passive' or 'red spirals' are found in large numbers in the STAGES project (and by Galaxy Zoo) in the infall region of clusters and groups. Comment: Proceedings of "The Starburst-AGN connection" conference held in Shanghai, Oct 27-31, 2008

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arXiv:0906.0306v1 [astro-ph.CO] 1 Jun 2009
**FULL TITLE**
ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
Optically-passive spirals: The missing link in
gradual star formation suppression upon cluster infall
Christian Wolf,1Alfonso Arag´ on-Salamanca,2Michael Balogh,3Marco
Barden,4Eric F. Bell,5Meghan E. Gray,2Chien Y. Peng,6David
Bacon,7Fabio D. Barazza,8Asmus B¨ ohm,9John A.R. Caldwell,10
Anna Gallazzi,5Boris H¨ außler,2Catherine Heymans,11Knud Jahnke,5
Shardha Jogee,12Eelco van Kampen,4Kyle Lane,2Daniel H. McIntosh,13
Klaus Meisenheimer,5Casey Papovich,14Sebastian F. S´ anchez,15
Andy Taylor,11Lutz Wisotzki,9Xianzhong Zheng16
1Department of Physics, University of Oxford, Oxford, OX1 3RH, UK
2University of Nottingham, Nottingham, NG7 2RD, UK
3University Of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
4University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
5MPI f¨ ur Astronomie, K¨ onigstuhl 17, 69117, Heidelberg, Germany
6NRC Herzberg Institute of Astrophysics, 5071, Victoria, V9E 2E7, Canada
7ICG, University of Portsmouth, Hampshire Terrace, Portsmouth, PO1 2EG, UK
8EPFL, Observatoire de Sauverny, CH-1290 Versoix, Switzerland
9AIP, An der Sternwarte 16, 14482 Potsdam, Germany
10University of Texas, McDonald Observatory, Fort Davis, TX 79734, USA
11SUPA, IfA, University of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
12University of Texas, 1 University Station, Austin, TX 78712-0259, USA
13University of Missouri, 5110 Rockhill Rd, Kansas City, MO 64110, USA
14Texas A&M University, College Station, TX 77843, USA
15CAHA (Calar Alto), C/Jesus Durban Remon 2-2, 04004 Almeria, Spain
16Purple Mountain Observatory, Ch. Acad. of Sc., Nanjing 210008, PR China.
Abstract.
their star formation is quenched.
environmental effects and not by mergers or strong AGN as often invoked: They
form stars at a reduced rate which is optically even less conspicuous, and manifest
a transition population of blue spirals evolving into S0 galaxies. These ’optically
passive’ or ’red spirals’ are found in large numbers in the STAGES project (and
by Galaxy Zoo) in the infall region of clusters and groups.
Galaxies migrate from the blue cloud to the red sequence when
Here, we report on galaxies quenched by
1. The question
This meeting has already discussed the transformation of galaxies from blue
disk-like star-forming galaxies to red, spheroidal and quiescent galaxies. We
have discussed the role of AGN in triggering star formation as well as sup-
pressing it, and the role of mergers in intiating this phenomenon. Also, it was
suggested that counts of mergers or AGN may correlate with stellar mass growth
on the red sequence. In this paper, I will report on a category of galaxy that
appears to undergo star formation quenching, while definitely not undergoing
1

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Wolf et al.
major mergers and perhaps not an AGN period either. They enrich the red
sequence without the conspicuous signposts of galaxy transformation. Instead,
they highlight the role of environment in transforming galaxies.
We already know that at low redshift both star formation and nuclear ac-
tivity decline towards regions of higher galaxy density, even though the galaxy
mass dependence of the two processes is very different. It is reasonable to expect
that at sufficiently high redshift these trends have been inverted, and Elbaz et al.
(2007) already found evidence for this. The connection between the two has yet
to be clarified, which is of course the main question of this conference series.
However, star formation is expected to be regulated by a range of processes, of
which some are internal to a galaxy such as winds and ionisation from stars,
X-ray binaries, SNe and AGN; others originate from the external environment,
imposed onto a galaxy by dynamical interaction with other galaxies or a cluster
potential, or by mechanical interaction with hot intra-cluster/group gas.
An obvious question in the study of galaxy transformation concerns the
intermediate population of galaxies showing the progress of evolution in an un-
finished state and giving away clues to the physics of the events. As far as
star-formation quenching is concerned, the identification of transition objects
has not been straightforward. Their apparent absence argued for a fast process
leaving it unlikely to catch an event in progress (Balogh et al. 2004), while the
properties of most quiescent galaxies favour slow quenching, e.g. the rarity of
strong Hδ absorption. Morphological change is clearly slower, and anemic or
optically passive spirals with red colour and smooth arms have been known for
long (Van den Bergh 1976; Poggianti et al. 1999).
In the following, I present new results on red spirals from the STAGES
project (Gray et al. 2009), which combines optical SEDs and redshifts from
COMBO-17 with ACS imaging from the largest contiguous HST mosaic after
COSMOS, and Spitzer 24µm data. The data cover 30′×30′or 5×5 Mpc2at the
redshift z = 0.165 of the target cluster complex Abell 901/2, which is comprised
mainly of four cores but includes further filaments and infalling groups. The
cluster sample has very low field contamination of ∼ 10% given their extremely
low photo-z scatter of 0.005 rms. While the STAGES project includes data from
further wavelength domains such as X-ray and radio, I will only report on the
optical and 24µm data, and will thus not comment on AGN activity either.
2. Our recent work
We split the galaxies in this sample into three categories, going beyond the clas-
sical split of red and blue galaxies: Wolf, Gray & Meisenheimer (2005) already
described the properties of red galaxies after splitting them into old vs. dusty.
They found that dusty red galaxies in A901/2 have similar overall dust extinction
levels as normal blue star-forming galaxies, while old red galaxies were defined as
effectively dust-free. In a colour space where reddening due to age and dust can
be distinguished, old red galaxies form a separate structure of their own, indeed
a red sequence, while dusty red galaxies are just a continuous tail extension of
the blue cloud, and supposedly only differentiated from it by gradually lower
specific star formation with redder colour.

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Optically-passive spirals
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Figure 1.
(right) with logM∗/M⊙> 10: symbols indicate morphology and colours re-
semble SED types. The cluster contains more dusty red than blue galaxies;
they are mostly early (Sa/Sb) spirals. At fixed U-V dusty red galaxies are
bluer in M280-MU than old red ones due to young stars. A divider between
old red and dusty red is shown as an age sequence with a EB−V = 0.1 (line).
Rest-frame colours of cluster members (left) and field galaxies
Now, the HST data show as expected, that old red galaxies are almost
exclusively E and S0 galaxies, while the blue cloud contains mostly normal
spirals. The dusty red galaxies are indeed spirals with smooth arms, thus lacking
already the morphological signs of star formation in form of clumps. However,
they have both bluer UV slopes compared to old red galaxies and significant
24µm fluxes, suggesting star formation to be present. The ratio between UV
and FIR measures of star formation is correlated with the global extinction
estimate from the optical SED with a slope expected from a dust law. At fixed
mass these red spirals have on average 4× lower SFR than blue galaxies, though
the distribution covers the whole range to full SFR suppression. We thus suggest
that these objects could be spirals undergoing slow SFR quenching.
We note that Koopman & Kenney (2004) observed spirals in the Virgo clus-
ter that cover a similar range in SFR reduction; the dominant mode of transfor-
mation there seems to be an outside-in truncation of the star-forming disk inside
these galaxies. This finding is made possible by the superior spatial resolution
on galaxies as near as the Virgo cluster. Kenney et al. (2008) have corroborated
these findings with 24µm data, showing that star formation recedes towards
the centres of Virgo spirals where it is on average more obscured as well. We
speculate that these galaxies may have similar SEDs and morphology to ours if
observed at z ∼ 0.17.
The SED-based typing in COMBO-17 already showed that dusty red galax-
ies were found mostly in the cluster outskirts. From there, their fraction declines
both towards the cluster cores and towards the regular-density galaxy field. The
present work from the STAGES data set includes typing by morphology which
shows a similar picture. Here, we also resolve the trends by galaxy stellar mass,

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Wolf et al.
Figure 2.
type. Lines show mean values of star-forming galaxies in mass bins (field
dashed for comparison). Dusty red galaxies are most common in infall regions;
at fixed mass their SFR is ∼ 4× lower than that in blue galaxies.
Star formation-mass diagram of three environments split by SED
and find that red spirals are predominantly found at logM∗= [10,11]. The red
fraction among spirals increases both with galaxy mass and galaxy density in
the environment. In particular, the red spirals in the infall regions of clusters
and groups are by no means edge-on populations in contrast to field samples:
in the field, 10% of spirals are red, which is the chance fraction for a sufficiently
reddening edge-on geometry; in the cluster here, however, red spirals are more
common than blue spirals and are thus a genuinely physical phenomenon.
The STAGES findings are published in more detail in Wolf et al. (2009)
and match quantitatively results on red spirals found by the Galaxy Zoo project
(Bamford et al. 2009). While Galaxy Zoo lacks multi-wavelength data, it covers
the huge volume of the entire SDSS survey and confirms that red spirals are
a general cosmological phenomenon and not a localised freak event. The mass
dependence of their fraction suggests a time scale for the red spiral phase that
becomes longer with increased galaxy mass. Whatever process causes it, will act
more slowly on larger galaxies if infall velocity did not depend on galaxy mass.
3. Placed into context
Altogether, we see a sequence of galaxy properties with density of their environ-
ment or with proximity to cluster centres. Given cosmological structure growth
it is likely to be an evolutionary sequence even though a single-redshift cluster
snapshot can not clarify the influence of redshift evolution in the progenitors of
the galaxies seen: the property sequence along density intertwines secular galaxy
evolution with effects due to the environment. From lower to higher density we
see how the dominant galaxy type changes from blue spiral over red spiral to S0
galaxy. Along this sequence, we see the clumpiness of stellar disks as well as star
formation rates decline, and UV slopes getting redder. The U-V colours only
get redder from blue to red spirals, but then change little when ageing stellar
populations and clearing of dust have counter-acting effects. The mean redden-
ing of the overall stellar population changes little from blue to red spirals, but
disappears towards the S0s. The mean reddening of star formation may increase
a little towards the red spiral phase after which star formation disappears.

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Optically-passive spirals
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These gradual changes observed by STAGES map out the transformation
process of spirals falling into clusters more clearly now. Previously, the star for-
mation in red spirals was not acknowledged properly. As a result, transition ob-
jects had not been identified clearly and only the disappearance of star-forming
galaxies was diagnosed.
Now, Galaxy Zoo, STAGES and work on Virgo galaxies all together support
the view that a large fraction of galaxies is transformed by their environment,
even in the low-redshift Universe. They lose star formation slowly upon cluster
infall; more quickly than due to secular evolution, but more slowly than ram-
pressure stripping by hot and dense ICM would do (Moran et al. 2007). Galaxy
Zoo has shown this process to be universal, STAGES has shown the remaining
star formation level to be significant and stretching the whole range from little
to high suppression, and the Virgo galaxies provide spatial resolution to study
the processes in detail, at least for one cluster.
4.Outlook and criticism
However, whether the outside-in truncation of the star-forming disk is the main
mode of transformation, what role it plays relative to mergers and what further
modes are possible, is still unclear. We expect that a galaxy’s response to its
environment will depend not only on properties of the galaxy but also on those
of the cluster. Also, we have not yet been able to verify which role AGN may
play in this framework. We know that strong AGN are rare in our cluster, and
the strong red spiral trends with density could only suggest a strong role of AGN
if AGN were mainly caused by environmental aspects other than mergers.
Next steps could involve a test whether models of disk fading are able to
reproduce the evolutionary sequence and what level of morphological rearrange-
ment is required to make S0 galaxies. Here, it is very important to look at a
mass-resolved sample and keep progenitor bias and downsizing in mind: the S0s
of tomorrow as derived from today’s red spirals do not need to look the same as
today’s S0s that were made from past red spirals.
The STAGES data set presented here is available to the public, including
SEDs, redshifts, stellar mass and SFR estimates in catalogue form and reduced
images from COMBO-17, HST and Spitzer (Gray et al. 2009). We look forward
to more work towards understanding environmental galaxy nurturing.
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