Abstract
Intracluster and intergalactic supernovae can be used as effective tracers of stellar
populations that are no longer associated with any particular galaxy. The origin of such
displaced populations is likely due to past tidal encounters with other galaxies (particularly in
the low velocity dispersion sub-structure cluster environment). These “hostless” supernovae
then reveal an unaccounted for baryonic population whose total scale is rather unknown.
Moreover, it is presently unclear how far low surface brightness stellar populations extend
into the intergalactic medium and the recent GALEX detections of many XUV disks shows
that the overall stellar distribution can extend for beyond the traditional optical radius. Since
most historical supernova searches focus on events occurring near to and within the optical
radius of candidate host galaxies, hostless supernova events may be potentially missed. In
view of this, we propose a search for potentially missed supernovae events through archival
WFPC2 images and source catalogs within a 3-degree radius about the Coma Cluster region.
Any new supernova detections are then likely revealing a displaced stellar population, which
can then be modeled to provide a better estimate of the total mass of intracluster/intergalactic
stars possibly contained in the total Coma environment. Images and sources will be crosschecked against other available catalogs, such as the Sloan Digital Sky Survey and the
ACS/WFC Treasury Survey of the Coma Cluster of galaxies. In addition, follow up deep
limiting surface brightness observations at these positions can be made with University of
Oregon 1 meter telescope CCD system at the Pine Mountain Observatory.
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 Scientific Justification
SN as tracers of an Intergalactic Population:
In the new concordant Cosmology, the global ratio of Dark Matter to Baryonic matter is
approximately 5 to 1. However, other than clusters of galaxies, there are few bound
potentials which have baryonic mass fractions (BMF) as large as 20%. Ways to reconcile
this include a) that most of the baryons in the Universe are in clusters or b) that there exists a
significant amount of baryons in the Intergalactic Medium (IGM). While there is now good
evidence for the warm IGM (the so-called cosmic Web), potential populations of cold
baryons whether in the form of very diffuse low surface brightness galaxies or in an even
more diffuse population of intergalactic baryons are very hard to detect and constrain. Mass
tracers such as supernovae (e.g. Sand et al., 2010; Gal-yam et al., 2003) and more recently
intracluster globular clusters (Peng et al., 2011) are useful beacons to indicate otherwise dim
populations of stars. Supernovae (SNe) are ideal tracers of mass associated with any galaxy
environment (isolated or in a group) because depending on type, the SN peak brightness can
be the equivalent of the integrated light of a galaxy. SNe fall into two main categories: core
collapse of a massive star and type Ia, an explosion of a white dwarf due to the accretion of
mass from companion star or merging of another white dwarf (e.g. Gilanov and Bodgán,
2010). Core collapse types are found within galaxies, specifically regions associated with
active star formation because the progenitors are large, massive, short-lived stars. Type Ia
SNe are found in all types of galaxies at all distances from the host galactic center and are
usually associated with stellar populations older than core collapse supernovae. Type Ia are
also brighter than most core-collapse SNe.
It is not uncommon to find supernovae outside the optical radius of the galaxy. SNe
2007fr, 2006bw, 2006bx are just a few examples of recent discoveries where the supernova
lies in these extremely low surface brightness areas of galaxies. In fact, SN 2006bx is a corecollapsed SN found well outside the optical radius of the galaxy, probably flung there via
tidal interactions with another galaxy. As more supernova surveys do blind searches, that is,
searches imaging random patches of sky, the number of SN discoveries occurring at several
optical radii from the host galaxy will increase. And due to the frequency of these objects
occurring at such large distances from the host galactic core, there must be a significant
stellar population too dim to directly observe, but dense enough for a white dwarf to interact
with a companion star and form a type Ia supernova. The Antennae (NGC 4038/4039)
galaxies are a dramatic example of how merging systems throw stellar populations far from
the central cores of the galaxies. Figure 1 shows the DSS image of the Antennae and the
location of event SN 2007sr in one of the dim arms. The Antennae are located relatively
nearby (cz ~1000km/s) therefore the disrupted arms are easy to detect, but at larger distances
a similar arm structure would be invisible. In addition, in another few hundred million years,
these tidal arms will disperse leaving only the possibility of a SN as the location beacon for
these tidally disrupted baryons. We have also identified SN 1955L at a projected distance of
51 Kpc from Arp 21 that may be another good candidate for a tidally displaced stellar
population. We are presently imaging 1955L to search for the underlying stellar population
that must accompany the SN.
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Therefore SNe occurring at large galactocentric radius can be a good indicator of dim
stellar populations inhabiting intergalactic space. For instance, while measuring the SN Ia
rate in clusters, Gal-Yam et al. (2003) found 2 intergalactic supernovae prompting intentional
searches for hostless supernovae in nearby clusters (Sand et al. 2010) and groups (McGee
and Balogh, 2010) in an effort to estimate intracluster mass. Recently, Peng et al. (2011)
found over 40,000 intracluster globular clusters (IGC) in the ACS/WFC Coma Cluster
survey. Most of this survey covered the central core surrounding the main galaxy NGC 4874
and a portion about ½ degree away from the main area of the cluster. But it is yet unclear if
IGCs trace out the entire population of intracluster stars or form their own separate
distribution.
To supplement and enhance this work, we propose to use the archival WFPC2 catalog
data and images in and around the Coma Cluster to search for the possible occurrence of
supernovae not associated with any known galaxy. The coma cluster is ideal for our
purposes because it is near enough (cz <8,000 km/s) such that galaxies are easily resolved
and potentially missed supernovae, not associated with any galaxy, can be identified. Indeed,
if all the galaxies that compromise Coma plus its surroundings have normal SN rates, then
with approximately 1014.5 stars, over a 10 year period there should be approximately 300 such
events (assume 1 SN per 100 years per 1011 stars for all types) in this overall region. We
therefore propose to supplement the Peng et al. (2011) results with a search of supernovae in
WFPC2 images and source extracted data that overlap the ACS/WFC regions as well as other
Coma regions imaged by WFPC2. The ACS/WFC was a very thorough survey, even with
only part of the data imaged due to the ACS failure. Detection of any supernova in the
archive, not associated with any known galaxy, would add another dimension to estimate the
total intracluster population of stars in Coma, which might be very large (Gregg and West
1998).
A secondary objective is related to our current work on using clusters of galaxies to
either defeat or to support Modified Newtonian Dynamics (MOND) as a credible alternative
to Dark Matter. While MOND is successful at empirically describing the rotation curves of
individual galaxies, it has several discrepancies when applied to galaxy clusters. In particular,
the observed mass measured from gas and stellar populations may be too small to keep a
cluster bound together when considering a MOND universe. However, there are two
potential difficulties that prevent using this qualitative argument as a death knell for MOND:
a) if a large intracluster mass of stars exists, (those stars are bound to the cluster but no
longer bound to individual galaxies) due to previous generations of tidal shredding, then the
total cluster mass of Coma may be poorly known; b) no one has yet worked out in detail the
proper equations that define hydrostatic equilibrium in a logarithmic potential (e.g. MOND)
and so the actual MOND mass that would be required under those conditions is unknown.
We are in the process of working out an operational framework that establishes hydrostatic
equilibrium under MOND (the concept must exist because MOND has been proven to
conserve momentum and energy – hence there is a virial in MOND). Hence, we are
proposing archival research to possibly identify intracluster SN to better estimate a total
intracluster stellar population mass in Coma. Positive detections will help with both
understanding the overall dynamical history of Coma and the amount of tidal disruption that
can occur and will produce a more accurate measure of the total baryonic mass of Coma.
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Indeed, to quote Zwicky (1961): “The Coma Cluster appears to contain much luminous and
dark intergalactic matter, which, if capably of producing supernovae,, should betray its
existence and character in this unique manner”. Fifty years later, we are proposing to test
this exact conjecture.
Fig. 1 DSS image of the Antennae galaxies with
an arrow indicating the location of SN 2007sr.
 Analysis Plan
The data we will be using are extracted from WFPC2 images defined by 3 degree radius
around the coma cluster (fig 2). The dates of the images span the lifetime of the camera, from
1994 through 2008, and are taken predominately in 450W, 606W and 814W filters
amounting to approximately 340 images with exposure times of at least 300 seconds. Since
we are concerned with point source detection and not photometry, the image data need not be
in the same filter. Catalogs will be downloaded from the Hubble Legacy Archive (HLA) and
for just the first few catalogs we will compare these to the corresponding WFPC2 images in
order to become familiar with the sources.
Except for some regions that overlap with the ACS/WFC Coma Cluster survey and
images of a few bright galaxies in the cluster taken at different dates, there is very little
overlap in the images over the lifetime of the WFPC2 camera. That is why we will compare
these regions to other published data catalogs. A magnitude limit will be imposed on objects
depending on which of the following catalogs correspond to in coverage. The catalogs we
will compare the WFPC2 data to are:
a) The ACS/WFC Treasury Survey of the Coma Cluster of Galaxies
b) Sloan Digital Sky Survey
c) BVRI imaging of the Coma cluster (Adami et al. 2006)
We will limit our search for SNe to objects brighter than 24 mag. unless we are comparing
them to the SDSS catalog, where the magnitude limit will be set at approximately 22 mag in
g band. Clearly at these faint limiting magnitudes we are also conducting a random field
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experiment in the detection of SN at distances considerably larger than Coma as any SN
occurring in Coma proper would likely be no fainter than 17th magnitude.
The Co-PI has worked extensively with image data, in particular with Sloan Digital
Sky Survey data, photometry, and data interface at SDSS.org and is also familiar with the
HLA data interface, and access to catalogs via SIMBAD. The Co-PI is also experienced in
working with very large (>500,000 rows) data sets and accessing these data sets with
numerous programs written specifically for data manipulation.
Anecdotally, the ability for a supernova to go unnoticed for at least a few weeks even
after imaged by a survey, in this case SDSS, is not unusual. Two supernovae, SN 2004df (fig.
3) and SN 2005bh (fig. 4) were imaged by SDSS prior to their discovery 2 weeks and 1
month later, respectively, by independent supernova search teams. In addition, as pointed
out by Zwicky 1961, Coma should be the best extragalactic location for detecting SNe and
yet many fewer have been detected in that region than expected. We take this as good
evidence that SNe can be easily escape detection.
Figure 2. The locations of the WFPC2 fields.
Fig. 3 SDSS g band image
of SN 2004df
Fig. 4 SDSS g band image
of SN 2005bh
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References:
Adami, C., et al. 2006, A&A, 451, 1159
Gal-Yam, A., Maoz, D., & Sharon, K. 2002, MNRAS, 332, 37
Gilfanov, M. & Bogdán, Á. 2010, Nature, 463, 924
Gregg, M. & West, M. 1998, Nature 396, 549
Hammer, D., et al. 2010, ApJS, 191, 143
McGee, S. & Balogh, M. 2010, MNRAS, 403, L79
Peng, E., et al. 2011, arXiv:1101.1000
Sand, D., et al. 2010, arXiv:1011.1310
York, D. et al. 2000, AJ, 120, 1579
Zwicky, F. 1961, PASP 73, 185
 Budget Narrative
To support this research we request the following approximate amounts:
Graduate Student Support:
o 12 Months Graduate Student stipend support @1800 per month = $21600
o Full year insurance coverage = $4500 (yeah, that’s right)
o Graduate Student Tuition Remission Academic Year = $12000
42% Overhead on items 1 and 2 above = 10962
Total Graduate Student Cost = $49K
Co-PI Support Johnson:
Two months of summer salary @3500 per month = $7000
Fringe @50% = $3500
42% overhead = 4410
Total Johnson Support Cost = $15K
Co-PI Support Bothun:
None requested:
Equipment, Travel and Publication costs:
Dedicated Image processing station for this project plus travel money to present results at
AAS meeting plus publication costs estimated to be, including overhead, $6K
Total Project Costs: $70K
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 Past HST Usage and Current Commitments
The following list of publications constitutes the results of the PI’s past usage of HST and
HST data:

The Luminosity of SN 1999by in NGC 2841 and the Nature of ``Peculiar'' Type Ia
Supernovae: 2004 Ap.J. 613, 1120

The Discovery of Cepheids and a New Distance to NGC 2841 Using the Hubble Space
Telescope: 2001 ApJ 559, 243

Structural Characteristics of Faint Galaxies Serendipitously Discovered with the Hubble
Space Telescope WFPC2: 2000 Ap J. Sup. 128, 405

Hubble Space Telescope WFPC2 Imaging of Three Low Surface Brightness Dwarf Elliptical
Galaxies in the Virgo Cluster: 1999 AJ 118, 1618

Hubble Space Telescope Wide Field Planetary Camera 2 Imaging of UGC 12695: A
Remarkably Unevolved Galaxy at Low Redshift: 1998 AJ 116, 657
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