Far Infrared Study of SN Ia host galaxies^*

L.G. Balázs¹ (PI), P. Ábrahám¹, A. Moór¹, Cs. Kiss^1,2

¹ Konkoly Observatory, Budapest, Hungary
² Max Planck Institute for Astronomy, Heidelberg, Germany

^* HTML version of the accepted Spitzer proposal

1. Introduction
The Ia type supernovae are unique tools for studying several important large scale properties of the Universe. In particular their role in proving the existence of the positive value of the Λ cosmological constant appeared to be fundamental. There are several coordinated efforts to get enough number of SN Ia of high z to give a statistically firm support for the nonzero cosmological constant (e.g. Riess aet al. (1998), Perlmutter et al. (1999), Tonry et al. (2003)). These studies suggest that it should be Ω_Λ≅0.7 and Ω_M≅0.3. These teams deduce that the Ia supernovae (SN Ia) at redshifts 0.3≤z≤1.0 give in average a ∼0.28mag bigger distance moduli than expected if Ω_M≅0.3 and Ω_Λ=0. This view seems to receive a very firm basis by the recent WMAP experiment.
Rowan-Robinson (2002) (R02) means, however, that the internal extinction in the host spiral and irregular galaxies is systematically underestimated and the conclusion of the nonzero cosmological constant, based on the statistics of Ia type supernovae, is premature yet. Accordingly, one expects a systematic difference between the z-dependence of the apparent (for extinction not properly corrected) luminosity distances of the spiral and elliptical galaxies since the later do not contain significant amount of dust. Recently Sullivan et al. (2003) did not find statistically significant difference between the objects being either in spirals+irregulars or in ellipticals+S0 types, and therefore they do not agree with R02's conclusion. Nevertheless, their sample contains only a small number of early type galaxies.
In getting firm conclusions favoring the nonzero cosmological constant the correct treatment of the intrinsic extinction of the host galaxy is a key issue in high z SN Ia studies. According to the usual assumption the properties of the cosmic dust responsible for the intrinsic extinction is the same at the low and high z hosts, i.e. follows the general reddening law obtained in our Galaxy. A critical issue in this context is the presence of the 'big grains' in the interstellar matter which does not affect considerably the optical interstellar reddening but contribute to the total extinction giving a sort of 'grey' extinction. The comparison of the thermal radiation of the interstellar dust in the far infrared (FIR), in low and high z hosts, may give important information in this respect.

2. Far-infrared observations of SN Ia host galaxies
One of the main achievements of the IRAS satellite was the discovery of galaxies anomalously bright in the FIR and probably being sites of active star formation. We correlated the comprehensive list of SN Ia of Tonry et al (2003) with the IRAS database of point sources and found 31 identifications. These sources concentrated at a redshift of z<0.1. Tonry et al. (2003) lists 130 hosts in this region. Assuming the dependence of the luminosity distance on z we extrapolated the IRAS 12, 25, 60 and 100μm brightness of our cross-identified hosts down to a realistic confusion limit. The intersection of the extrapolated luminosity distance with the 5σ level above this limit defines an upper bound for the z in the sample of Tonry et al. (2003). This limit equals to z≅1 at 12 and 25μm and z≅0.5 at 60 and 100μm. If one expects the same percentage also at high z one may estimate a number of FIR detections among the hosts in the list of Tonry et al. (2003). We get a figure of 25 galaxies in the z region not covered by the IRAS. Considering the calculated confusion limits we have to schedule about 50min observing time for all targets selected. In order to meet the 50 hours time limit of the short proposals we have to select 50% of the sources with z>0.1 in the list of Tonry et al. (2003). We may expect therefore 10-13 detections in this redshift region. We have to add some observations of a selected number of IRAS galaxies too, for reference purpose.

3. Scientific aims
As outlined in Section 2 we selected 50 targets from the list of Tonry et al. (2003) in the redshift region of z>0.1. Due to the redshift of z≅1 the IRAS photometric bands of 12, 25, 60 and 100μm are shifted into 24, 50, 120 and 200 micron. We plan therefore to use the MIPS on board of Spitzer and intend to make photometric observations with the filters of 24, 70 and 160μm. With this choice one may study the same type of dust as in the case of IRAS galaxies. Making use photometric capability of the MIPS we hope to get answers to the following scientific issues:

• The frequency of FIR hosts in the 0.1< z <1 region: Our estimated frequency of hosts of FIR excess is based on the IRAS galaxies of low redshift. The general dust content of galaxies may vary with redshift. Consequently, the fraction of FIR host might be much higher at z≅1.
• FIR dependence of the offset from the standard distance moduli - redshift relation: As we mentined in Section 1, there is an in average 0.28mag offset from the standard distance moduli - redshift relation. If the offset is caused by anomalous dust grains the FIR property of the hosts might correlate with this offset.
• Difference in the dust properties between the hosts of low and high redshifts: The planned observations in three MIPS colors make possible to compare the general properties of the dust at low and high redshifts. In particular, an enhanced radiation in the 160μm band may indicate significant big grain population in the dust content of the host galaxies.

4. General significance of the proposed observations
The sample of the SN Ia hosts gives a unique opportunity to study the star formation activity and dust property as a function of the redshift. Although, the Ia type supernovae are not directly related to the star formation the anomalous dust properties have to be taken into account at their photometry. The SN Ia sample is biased towards objects of law extinction where the bulk of the dust is only in the background. However, the direction of the observer's line of sight is fully random and in a considerable fraction of cases the influence of the intrinsic dust extinction could be quite significant. The result might serve a useful input information for studying the proper conditions for optical afterglows of gamma ray bursts (GRB). There is a sofar not fully understood dichotomy of the GRBs having or not optical after glow. A possible interpretation appears to be the absorption of the optical radiation in the interstellar dust. The suspected progenitors of these GRBs are the very massive stars strongly related to dense giant interstellar dust clouds. Normally, the observed GRBs are at higher z than the known Ia type SNs. However, the FIR results obtained for the SN Ia hosts might deliver a useful input information for planning FIR observations of GRB hosts.

5. Why Spitzer?
The high performance of the MIPS FIR photometer offers a unique possibility for measuring the properties of the interstellar dust in the SN Ia host galaxies. As we demonstrated in Section 2, Spitzer could detect hosts of FIR excess down to a redshift of z≅1. The MIPS observations at three bands allows us to make comparison between the dust properties in hosts of low and high redshifts. This will be the first possibility to make FIR studies at such objects. The results may indicate the necessity to continue similar studies at further SN Ia hosts. The results may give a firm basis for planning FIR studies of GRB host galaxies. It might demonstrate furthermore that Spitzer is capable for such type of studies.

6. Summary of proposed Spitzer observations
We proposed to observe with Spitzer 50 SN Ia of z > 0.1 from the list of Tonry et al. (2003) and 10 more ones from those cross-identified with IRAS galaxies. We hope to get MIPS photometry from all target at 24, 70 and 160μm. The calculated observing time amounts to 50 hours (MIPS photomery).