UV Properties of Type Ia Supernova and their Host Galaxies

5 MB·English

by  Brad E. Tucker

UV Properties of Type Ia Supernova and their Host
Galaxies
Brad. E. Tucker1, The ESSENCE Project
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Abstract Type Ia Supernova(SN Ia) are a powerful, etal.1996;Riessetal.1996;Goldhaberetal.2001;Pri-
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1 albeit not completely understood, tool for cosmology. etoetal.2005;Guyetal.2007;Jhaetal.2007;Conley
Gaps in our understanding of their progenitors and de- et al. 2008). However we still lack a clear understand-

] tailed physics can lead to systematic errors in the cos- ing of the progenitor system (Hillebrandt & Niemeyer
O
mological distances they measure. We use UV data in 2000) and their use to understand the nature of Dark
C
two contextto help further ourunderstanding of SN Ia Energyisbecominglimitedbysystematics(Astieretal.
.
h progenitorsandphysics. Weanalyzeasetofnearly700 2006; Wood-Vasey et al. 2007) rather than statistics.
p light curves, and find no signature of the shock heating It is widely held that the progenitor of a SN Ia is a
o- of a red giant companion, predicted by Kasen (2010), carbon/oxygen white dwarf (WD) which accretes mass
r casting doubt as to frequency of this SN Ia channel. in a binary process until a thermonuclear runaway be-
t
s We also use UV imaging of high redshift host galaxies gins - typically as the white dwarf’s mass approaches
a of SN Ia to better understand the environments which
[ the1.4M(cid:12)Chandrasekharlimit. Incontrasttocorecol-
SNIaoccur. Weshowthatsomehigh-zellipticalgalax- lapse SN, pre-explosion imaging has not turned up ex-
1 ieshavecurrentstarformation, hinderingeffortstouse
plosion progenitors, confirming the relatively faint na-
v
them as low-extinction environments. We show cosmo-
6 ture of SN Ia progenitor systems (Maoz & Mannucci
logical scatter of SN distances at large effective radii
9 2008). The mostly widely held model, the single de-
3 in their hosts is significantly reduced, and argue this is generate (SD) scenario, has a non-degenerate compan-
2 due to the smaller amounts of dust affecting the SN Ia.
ion, such as a red giant (RG) or main sequence (MS)
1. Finally,wefindatwocomponentdependanceofSNdis- star depositing material onto the WD. This compan-
0 tancemeasurementsasafunctionoftheirhostgalaxy’s
ionstarshouldremainrelativelybrightandpotentially
1 FUV-V color. This indicates that both the age and
1 with signatures of rotation and spatial motion after
metallicity/mass of the host galaxy maybe important
: the explosion. (Wheeler et al. 1975; Fryxell & Arnett
v ingredients in measuring SN Ia distances.
1981; Livne et al. 1992). Ruiz-Lapuente et al. (2004);
i
X
Gonz´alez Hern´andez et al. (2009) have identified a po-
ar 1 Introduction tentialcompanioncandidateintheremnantofSN1572
whichhasahighspatialvelocityandsomeothersigna-
The use of type Ia Supernova (SN Ia) in cosmology is turesexpectedforaSNIa companionstar. Kerzendorf
a long and arduous road. The empirical relation of the etal.(2009)haveinvestigatedfurtherthepotentialcan-
light curve shape of a SN Ia to its peak absolute mag- didate and other stars in SN1572, and while unable to
nitude has become a cornerstone of measuring the cos- rule out the candidate entirely, they argue it is an un-
mic acceleration of the Universe (Phillips 1993; Hamuy likely SN Ia companion star.
A second possible scenario is the double degener-
ate (DD) scenario, in which two WDs coalesce, exceed-
Brad. E.Tucker,TheESSENCEProject
1The Research School of Astronomy and Astrophysics, Aus- ing the Chandrasekhar limit (Iben & Tutukov 1984;
tralian National University, Mount Stromlo Observatory, Webbink 1984). Howell et al. (2006) and Hicken et al.
via Cotter Road, Weston Creek, ACT 2611, Australia; (2007)believetohaveobservationalevidenceofaSNIa
[email protected]
exploding due to the DD scenario, based on a few ex-
ceptionalobjectsthatappeartohavesynthesizedmore
2
than a Chandrasekhar Mass of 56Ni, and are estimated hereafter K10 and references therein) has calculated a
to have ejected approximately two solar masses of ma- model applying similar physics to the companion stars
terial. These claims have some model dependence, but of SN Ia.
seem compelling. Despite these instances, such SN are
unusualanddonotrepresenttheaveragepopulationof 2.1 The Model
SN Ia’s.
Investigating the environments of SN Ia allows an The model in K10 uses plausible non-degenerate com-
alternative way to examine SN Ia properties, while panions: 1−3M and 5−6M main sequence (MS)
(cid:12) (cid:12)
providing a potential avenue to help alleviate system- sub-giantsand1−2M redgiants(RG).Uponexplosion
(cid:12)
atic problems in their application as distance indica- oftheWD,theejectafromtheSNIawillexpandfreely
tors. Early studies suggested that SN Ia are more untilhittingthecompanion. Astheejectahitsthecom-
frequently associated with younger stellar populations panion,abowshockisformed,heatingandcompressing
in star-forming late type galaxies (Oemler & Tinsley the remaining ejecta into a thin shell. The companion
1979), and prefer the disks of the galaxies rather than subsequentlydivertstheincomingflowofmaterialfrom
bulges (Wang et al. 1997). However, McMillan & Cia- the SN, creating a hole in the ejecta. This hole allows
rdullo (1996) observed that SN Ia’s occur far from the the thin shell of heated ejecta to rapidly escape. Af-
spiral arms of late-type galaxies, and in galaxies with ter a further time, the remaining ejecta fills in the hole
low star formation rates (SFR). These observations ar- created by the companion.
gue for a delay between the formation of the systems Whenthecompanioncarvesoutaholeintheejecta,
and the subsequent explosion as a SN. Hamuy et al. the shocked ejecta radiates, and this radiation can es-
(1996) found a loose correlation between the SN Ia de- cape through the cavity in the ejecta. Both the radius
cline rate and the host galaxy type. They found the of the shocked stellar material and the ejecta cavities
faint, fast declining SN occur in early-type galaxies, are large in the case of RG companions, allowing a sig-
whereas the bright, slower declining SN Ia occur ex- nificant fraction of the radiation to escape, producing
clusively in late-type star-forming galaxies. This trend an observable signature on a SN Ia light curve. The
hasbeenconfirmedwithotherstudies(Neilletal.2009) effects for MS companions are significantly less.
andathigherredshifts(Sullivanetal.2006). Gallagher The emission from the prompt burst is mostly in
etal.(2005)foundthatfast,faintdecliningSNIa’swere softX-rays,howeverenergynotradiatedintheprompt
only in galaxies without any significant star-formation burst will be emitted in the following hours and days
activity. However, a follow up study (Gallagher et al. in the UV and optical. The strength of the observable
2008) found that those elliptical galaxies actually had emission will depend on the viewing angle of the ob-
some star-formation, contrary to previous thoughts. server, with the strongest emission seen with a viewing
Studies using nearby SN Ia’s (Kelly et al. 2010), SN anglelookingdirectlyatthehole. K10calculatesabout
Ia’s in the 0.1 < z < 0.3 range (Lampeitl et al. 2010) 10% of all SN Ia have a favorable viewing angle with
and at higher redshifts (0.3 < z < 1.0) (Sullivan et al. strong detectable emission in the ultraviolet and B op-
2010) have all found that the luminosity of SN Ia, cor- ticalbands. Figure1showsthestrengthoftheshocked
rectedforlightcurveshape,dependsonthehostgalaxy escaped emission at the ideal viewing angle, θ =0deg,
stellar mass (which correlates metallicity). While the in the ultraviolet for the RG scenario.
trends are weak, fitting for the trend can improve SN
Ia distances by ≈5% (Sullivan et al. 2010). 2.2 Observations
To test this prediction, we compiled 695 SN Ia light
2 Progenitor Signatures in UV Light Curves curves from several surveys (Tucker et al. 2011b) over
the redshift range of 0.03 < z < 1.0. The data con-
Recent developments in the observations of core- sistsoftheCenterforAstrophysics(CfA)1(Riessetal.
collapse SN have opened up a new means to under- 1999), CfA2 (Jha et al. 2006), and CfA3 (Hicken et al.
standingtheprogenitorsystemsofSN.Aftertheexplo- 2009)datasets,theSloanDigitalSkySurvey-II(SDSS-
sion of a core-collapse supernova, the shock wave prop- II)(Friemanetal.2008)firstyeardatarelease(Kessler
agates through the envelope of the star, emitting first et al. 2009), the Equation of State SuperNova trace
in x-rays then the ultraviolet and optical. This early Cosmic Expansion (ESSENCE) (Miknaitis et al. 2007;
shockwavehasnowbeenobservedintheX-raywiththe Wood-Vasey et al. 2007; Narayan et al. 2011) and the
accidental discovery of SN 2008D by SWIFT (Soder- Supernova Legacy Survey (SNLS) (Astier et al. 2006).
berg et al. 2008; Modjaz et al. 2009). Kasen (2010,
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FromK10,weexpect,10%ofalllightcurvesshould
show the signature of the companion star. If all SN Ia
have RG companions, our data set should have ≈ 70
light curves with this feature. At t (cid:46) 7 days, where
the predicted effects are most profound, we should be
able to see the signature of the companion, with the
signature increasing as t ⇒ 0. Examining our light
curves in UBV, we detect no signature in any filter
for any event. While our sampling greatly decreases
in early time measurements, we have a large enough
sample to yield a few detections if a majority of SN Ia
haveprogenitorsinvolveRGcompanions. Ourdataset
observations are well fit by the Leibundgut template,
showingtheanalysisisconsistent. Tuckeretal.(2011b)
will publish formal limits on the fraction of SN Ia with
Red Giant companions.
Fig. 1 Calculated model from K10 for a viewing angle of
Θ=0degintheU filter. Wecanseetheshockedemission’s
signatureinSNIalightcurves. TheRGcompanionhasthe
biggest effect and should be easily discernible in observed
SN Ia light curves.
Tobeabletocompareallofthedatasets,wemustcor-
rect them to a common frame. This requires us to cor-
rect for extinction due to dust (both in the host galaxy
and the Milky Way), time dilation, different filter sys-
tems and redshift filter corrections (k-corrections). We
transformallofthelightcurvesintorest-frameUBVRI Fig. 2 Acompilationof695lightcurvesfromtheCfA1,2,
and 3 surveys, SDSS-II, ESSENCE, and SNLS supernova
at z = 0 using the SALT2 package (Guy et al. 2007).
surveys. The light curves have been run through SALT2 in
Withthetransformations,wecancompareittothepre-
order to bring them to a common system, UBVRI. The
dictions of K10 and to the ”normal” SN Ia template,
black line is the Leibundgut template of a standard SN Ia.
the Leibundgut template (Leibundgut 1989)
Itisevidentthereisnodetectablesignaturefromtheemis-
sion due to the shocked ejecta escaping a hole created by
2.3 Discussion the companion. There should be ≈ 70 light curves that
havethisfeature,withtheRGscenariobeingthestrongest
Figure 2 shows our complied light curves from all of contributor. Despitethedecreaseinsamplingatearlytimes
the surveys in rest-frame UBVRI, with the black line (where the feature occurs), no hint of the presence is seen,
ruling out the RG scenario.
showing,asreferencetheLeibundguttemplateofanor-
mal SN Ia. For our purposes, we treat the U filter as
beingapproximatedbythenear-ultravioletfiltercalcu-
lation as calculated in K10, with the B and V filters
being direct comparisons to K10’s model predictions. 3 UV Properties of Ia Host Galaxies
Light curve variations are larger after maximum than
before, because variations in the amount and location The Equation of State SuperNova trace Cosmic Ex-
ofenergyejectioncausedbythedecayof56Niaremost pansion(ESSENCE)projectwasa6yearNationalOp-
pronounced after maximum light. (Kasen & Woosley tical Astronomy Observatory (NOAO) survey program
2007; Hayden et al. 2010). While global fits to U and (Miknaitis et al. 2007, hereafter GM07) which discov-
B bands show some dispersion due to intrinsic varia- ered 228 high redshift SNe Ia, 0.2 < z < 0.8. The pri-
tion and the effects of dust, these departures are much mary goal of ESSENCE is to measure the dark energy
smaller than the signal predicted by K10. equation of state parameter, w = P/(ρc2), to better
4
understandthenatureofdarkenergy(Krisciunasetal. broad-band photometry to any rest-frame magnitude
2005;Davisetal.2007;Wood-Vaseyetal.2007). Using system. We make a simplifying assumption that the
the MOSAIC II CCD on the 4-m Blanco Telescope at spectral energy distribution of any galaxy we observe
the Cerro Tololo Inter-American Observatory (CTIO), can be described as a linear combination of locally ob-
ESSENCE took repeated images in broad-band John- served galaxy types, ranging from ellipticals to star-
son-CousinRandI filtersinfourfields,eachcovering bursts. We use 11 galaxy SED templates, covering
2 square degrees. Supplemental imaging in Johnson- different morphological types (Kinney et al. 1996) and
Cousin B and V and SDSS z(cid:48) for the purposes of host also a range of starburst galaxies with various extinc-
galaxy analysis was also undertaken. Additionally, we tion levels (Calzetti et al. 1994) obtained from Blan-
have Galaxy Evolution Explorer (GalEx) data in the ton et al. (2003). These templates cover a wavelength
far-UV(FUV)andthenear-UV(NUV)(Martinetal. rangeof100-1000nm,meaningwecantransformobser-
2005). TheGalExMediumImagingSurvey(MIS)over- vations between Johnson-Cousin (Bessell 1990), SDSS,
laps with the majority of our ESSENCE fields, achiev- 2 Micron All Sky Survey (2MASS) Cohen et al. 2003;
ing a sensitivity mAB ≈23 (Morrissey et al. 2005). Skrutskie et al. 2006, and Galaxy Evolution Explorer
In the case of BVRIz(cid:48) images, we first stack the (GalEx)(Martinetal.2005;Morrisseyetal.2005)pho-
individual images in order to maximize our sensitivity. tometric systems..
Given the large quantity of R and I filters, we place We cover the range of all galaxies using a series
criteria on both the full width half maximum(FWHM) of linear combinations and define a χ2 statistic based
(5 pixels or 1.35(cid:48)(cid:48)) and the sky background levels. For on the quality of fit to a template. We allow a zero-
all other filters, no image restrictions are placed. We point(ZP)offsetduetothecombinedeffectsofintrinsic
then convolve individual images to a common PSF, luminosity, distance, and template brightness, which
and transform to a common coordinate system. These we marginalize over. This allows us to define a set of
transformed and convolved images are combined us- acceptable galaxy templates with varying fractions of
ing two pixel rejection methods, one that uses an av- each galaxy type, and the allowed range of zero-points,
erage sigma rejection and another that uses a mini- fitbyourobserver’sframephotometrywithin∆χ2 =1
mum/maximum rejection. Both methods give zero-
. Wealsoallowupperlimits,aswehavenon-detections
points,consistentwithin0.02magnitudesofeachother.
inGalExofsomehostgalaxies. Lastly,wealsocalculate
For each SN, we identify all galaxies within a 10(cid:48)(cid:48)(≈60
theprobabilityofatemplatebasedonagoodness-of-fit
Kpc) radius of the SN positions. We identify the cor-
test, providing us with a confidence of galaxy type.
responding host galaxy by comparing the peak SN flux
We also calculate physical parameters for our host
withthegalaxyluminosity. Insomecases,ahostgalaxy
galaxies. Firstly we calculate the separation from the
couldnotbeunambiguouslyidentified,orwasobscured
galactocentric position and the SN position using the
by a foreground object. Due to the large FWHM for
respectivepositions. Wethencalculatethegalaxyscale
GalEx,werequirethattheFUV andNUV havedetec-
length and size using the respective radii in SExtrac-
tionsinbothfiltersandpositionswithin2(cid:48)(cid:48)oftheoptical
tor, FLUX RADIUS for our 50% our effective radius,
positions. Thisconfirmsthepresenceofthehostinthe
and the KRON RADIUS for our total galaxy size. We
UV and limits mis-identification.
normalizethegalactocentricseparationbytheeffective
Photometric measurements are done using Source
radius, enabling us to compare the position of objects
Extractor (SExtractor) (Bertin & Arnouts 1996), and
withtheirhostgalaxieswhichaccountsforthevariation
weusetheMAG AUTOphotometry,whichisbasedon
of host galaxy size.
Kron radius (Kron 1980). Graham & Driver (2005)
Ourstarformationrates(SFRs)arecalculatedusing
showed that the Kron photometry in SExtractor is
the empirical relation from Salim et al. (2007). Salim
accurate to 90% ± 5% of the total galaxy flux. We
etal.(2007)usedalargesetofgalaxieswithGalExUV
perform relative photometry using catalogs from the
and SDSS optical imaging to develop an empircal con-
ESSENCE project, limiting the calibration stars to
versionbetweenL andSFRusingBruzual&Char-
16.5 < m < 21.0, and fit the zero-point from this FUV
R lot (2003) models with a Chabrier (2003) initial mass
catalog for each galaxy. We correct for Milky Way ex-
function(IMF).Usingthisconversion,wealsocalculate
tinction using the Schlegel et al. (1998) dust maps.
SFR upper limits for host galaxies with null detections
Comparing galaxies at various distances is not triv-
in GalEx. However, due to contamination from blue
ial because the spectral energy distributions (SED) of
horizontal branch stars which emit in the FUV filter,
the galaxies are highly varying and unknown while
we need to apply a nominal correction for this added
broad-band filters cover different parts of the spectrum
flux (Brown et al. 2000). While the added flux is negli-
at different redshifts. We wrote a k-correction proce-
gible compare to the emission from a starburst galaxy,
dure(Tuckeretal.2011a)thatconvertsobservedframe
5
it is necessary for an early type galaxy with little or no Lastly, we examine the properties of the cosmology
UV emission. fitsasafunctionofthehostgalaxyproperties.,toseeif
Lastly, we determined stellar masses of our host wecanseethestellarmass/metallicityeffectasseenby
galaxiesusingourB−V colorandtherelationsofBell others. Figure6showsHubbleresidualsasafunctionof
& de Jong (2001) and their preferred model which uses FUV −V color. Weseeatwocomponenttrend,depen-
Bruzual & Charlot (2003) with a Z = 0 metallicity, dent on FUV −V, where both the bluest host galaxies
Scaled Salpeter IMF and star formation bursts in the (ones with high specific rates of star formation), and
formation epoch. We use determined V band stellar the reddest galaxies (old, red-dead galaxies) have light
mass, however Bell & de Jong (2001) normalize their curve shape corrected distances which are systemati-
relations so that all filters yield the same stellar mass cally brighter than the average SN Ia. This trend of
so all filters should give the same value. As these rela- brighter distances as a function of blue host galaxy
tions are best used for disk galaxies, we fit each galaxy FUV −V colors been previously undetected again due
using a S´ersic profile (S´ersic 1963), where in we deter- to the lack of UV imaging. Nomoto et al. (2003) pre-
minetheS´ersicindexeitherasanexponentialprofileor dicted that both metallicy and age are important in-
thedeVaucouleursprofilewhichisappliedforelliptical gredients in making a SN Ia. The blue FUV −V host
galaxies. Thisfittingenablesustoapplyanominalcor- galaxies suggest young stars, while the red FUV −V
rection, making the application of the Bell & de Jong host galaxy color correlates with both the mass, mean
(2001) relations relevant. stellar age, and metallicity of the host. With good UV
In figure 3, we compare our values of stellar mass data, these trends can be removed from SN cosmology
and SFR. It is interesting to note that some early type fits.
systems have relatively high SFRs. This has not been
previously seen in photometric studies as they did not
use UV imaging as we have. However, this effect has
been seen in some elliptical galaxy spectra Gallagher
et al. (2008). At z > 0.5, the universal SFR density
has already increased by a factor of 4, and it is more
common to see star formation associated with early-
type galaxies at such redshifts (Rich et al. 2005). UV
photometry is one of the best indicators of SFR (Ken-
nicutt 1998; Adelberger & Steidel 2000) and so we are
abletoobtainaaccuratemeasurementofthetrueSFR
of SN Ia host galaxies.
Figure 4 shows how are SN are distributed in their
respective galaxies with respect to the type. At sepa-
rations>2R ,aSNisoccurringawayfromthebulk
Eff
of its host galaxy, and consequently the bulk of stars
and extinction. Approximately 20% of the SN Ia oc-
cur at these distances, suggesting a population of SN
Ia occurring in lower extinction environments.
Figure5plotsthisseparationwithrespecttoHubble
residual. Here, a Hubble residual of 0 means that the
SN distance was a perfect fit to the average cosmologi- Fig. 3 DeterminedstellarmassesversusSFR.Thegalaxy
cal values. A positive residual means the distance was type is based on our SED fit. The size represents the cor-
underestimated, and it is fainter than expected while a responding confidence in the fit. As expected, our sample
negative value is a SN that is brighter than expected. consists of a large number of starburst galaxies. What is
Not surprising, the scatter for objects occurring near somewhatsurprisingisthelargenumberofellipticalgalax-
ies with higher than expected SFRs. However, this is seen
the center of their host galaxies (R/R < 2) have a
Eff in other studies, whereas elliptical galaxies at z > 0.5 are
largerscatterontheHubblediagram(Riessetal.1998).
frequently undergoing active mergers and do have residual
Thisprobablyisaconsequenceofmoresignificantdust
star formation.
extinction experienced by SN Ia in these areas, as ob-
jects (R/R > 2) have less scatter where extinction
Eff
is less likely. This potentially provides a way to use
SN Ia more effectively for cosmological measurements,
circumventing some of the extinction problems.
6
Fig. 4 Rest-frame FUV − V color compared with the
Fig. 5 Normalized supernova separation compared to the
galactocentric separation of the SN normalized by the ef-
Hubbleresidual. Theresidualistheoffsetfromthebestfit
fective radius. By normalizing it, we are able to com-
cosmology, where a positive value is a SN Ia that is fainter
pare the relative positions of SN in all galaxies types. At
thanexpected,andanegativevalueisbrighter. Weseethat
R/R > 2, the SN are occurring environments with less
Eff SNwithR/R >2havebetterfitstotheaveragecosmol-
Eff
stars and extinction, where we see ≈20% of the SN occur-
ogy. At these large radii, we expect the average extinction
ring at these distances.
of these objects to very low.
4 Conclusions
The UV portion of the spectrum contains highly use-
fulpiecesofinformation. Bylookingatearlytimelight
curves in the ultraviolet, we do not see a signature of
a red giant companion, as predicted by Kasen (2010),
castingdoubtonthefrequencyofthischannelasapro-
genitor of SN Ia. Ultraviolet imaging of SN Ia host
galaxies has yielded a wealth of information, includ-
ing a two component residual in Hubble diagrams as a
function of fost galaxy FUV −V color. The ultravio-
let has also shown some early-type systems have some
star-formationanditisprobablynotpossibletoassume
that objects occurring in these systems are free of ex-
tinction. What is possibly better will be to use the SN
Ia’s position within its host galaxy to help choose low
extinction candidates. With additional and improved
UV data, such as that of the World Space Observatory
- Ultraviolet (WSO-UV) (Shustov et al. 2009, 2011),
measurements of Dark Energy and our understanding
of SN Ia will be greatly improved. Fig. 6 The host galaxy FUV −V color versus the SN Ia
Hubbleresidual. Theresidualistheoffsetfromthebestfit
cosmology, where a positive value is a SN Ia that is fainter
than expected, and a negative value is brighter. We see a
two component fit of SALT2 derived distances versus host
Galaxy FUV −V color.
7
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