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Search for Supernovae in Starburst Galaxies with HAWK-I

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Search for Supernovae in Starburst Galaxies with HAWK-I Astronomical Science Search for Supernovae in Starburst Galaxies with HAWK-I Matteo Miluzio1 the whole history of star formation, due to An infrared search in starburst galaxies: the wide range in the delay times of their Why? progenitors. Recently, it was claimed that 1 INAF–Osservatorio Astronomico di a significant fraction of SN Ia have a short Starburst galaxies have very high SFRs, 7 Padova, Italy delay time of about 10 years (Mannucci in the range of 10–100 MA/year, com- et al, 2006). As with CC SN, the rate of pared to the few solar masses per year such prompt SNe Ia events is proportional of normal star-forming galaxies. Many With the aim of testing the relation to the current SFR. starburst galaxies are in close pairs or between supernova (SN) rate and star have disturbed morphology, which is a formation rate, we conducted a SN sign that merging is enhancing their SFR. search in a sample of local starburst Motivations: The SN rate problem Since the ultraviolet radiation from young galaxies (SBs) where both star forma- and massive stars heats the surrounding tion rates and extinction are extremely During the last decade significant effort dust and is re-emitted in the far-infrared, high. The search was performed in the has been devoted to the measurement of the most luminous starbursts are Lumi- near-infrared, where the bias due to the cosmic SFR, using a combination nous Infrared Galaxies (LIRGS) with 1011 12 extinction is reduced using HAWK-I on of many different probes (e.g., Hopkins & < LIR <10 LA and UltraLuminous Infrared 12 the VLT. We discovered six SNe, in Beacom, 2006). Also, as a result of a Galaxies (ULIRGs) with LIR > 10 LA. excellent agreement with expectations, number of dedicated surveys, new esti- when considering that, even in our mates of the SN rate have become avail- The first attempts to measure SN rates in search, about 60% of events remain able. Although the focus was on type starburst galaxies in the optical band date hidden in the nuclear regions due to a Ia SN, a few estimates of the CC SN rates back to 1999 (Richmond et al., 1998) with combination of reduced search effi- have been published (e.g., Melinder et al., only a handful of events detected. The ciency and very high extinction. 2012). In particular, the evolution of SN first results of a systematic search in star- rates with redshift was found to track the burst galaxies in the infrared (IR) domain SFR evolution very well, considering the were reported by Maiolino et al. (2002) and Why supernova rates? large uncertainties in the extinction cor- Mannucci et al. (2003). The main conclu- rections. Botticella et al. (2008) obtained a sion was that the observed SN rate in star- The determination of the star formation good match between the observed SN burst galaxies is one order of magnitude history (SFH) of the Universe is one of and star formation rates assuming a lower higher than that expected from the blue the main goals of modern cosmology, as limit for CC SN progenitors of 10 MA. luminosity of the galaxies, but still three to it is crucial to our understanding of how ten times lower than would be ex­­pected structures in the Universe form and However, recent study of the core col- from the far-IR luminosity. They suggested evolve. The estimate of the star formation lapse SN progenitors, based on direct that the most plausible explanation for this rate mainly relies on measurements of detection on pre-explosion archival discrepancy was the extreme extinction in integrated galaxy luminosity, most often images, set a lower limit of 8 MA (Smartt the galaxy nuclear regions (AV > 25 mag) in the infrared or for specific emission et al., 2009). It was claimed that if this that hides SNe even in the IR. Recently, lines, e.g., Hα. The luminosity measure- value is adopted, the observed SN rates Mattila et al. (2007) conducted an IR ments are converted to total luminosity would be a factor of two lower than those search in starburst galaxies using ground- assuming an initial mass function (IMF) expected from the observed SFR, some- based telescopes with adaptive optics, a for the stellar population and including a times called the SN rate problem (e.g., technique that allows the nuclear regions number of corrections to account for Horiuchi et al., 2011). While we should to be observed with high spatial resolution. the presence of multiple stellar popula- keep in mind the uncertainties in SFR cal- This approach led to the discovery of a tions, non-thermal source contributions ibrations, there are also possible impor- handful of SNe, but not yet an estimate of and extinction. tant biases in the SN rate estimates. The the CC SN rate. About a dozen SNe have two most severe are the possible exist- been discovered by IR searches so far. In principle a direct test of the star forma- ence of a large population of very faint Therefore the statistics are still based on tion rate (SFR) estimate can be achieved CC SNe and/or underestimates of the very low numbers, and many of the original by directly counting the number of dying correction for extinction. In particular, questions are still unanswered. Entering massive stars, namely core collapse several authors (e.g., Mattila et al., 2012) into this debate, we started a new search supernovae (CC SNe). Indeed, because argue that up to 70–90% of CC SNe to measure the SN rate using the High of their short-lived progenitors, CC SNe remain hidden in the highly dusty nuclear Acuity Wide-field K-band Imager (HAWK-I), trace the current SFR. On the other hand, regions of starburst galaxies. If a correc- the infrared camera mounted at the ESO for an adopted SFR, measurements of tion for the hidden SN population is Very Large Telescope (VLT). the CC SN rates give information on the included in the rate calculation, the dis- mass range of their progenitors and on crepancy between SN and star formation the slope of the IMF at the high-mass rates at high redshifts seems to disap- The search strategy end. Type Ia SNe, however, are the results pear. It is unclear whether this effect is of thermonuclear explosions of white large enough to also explain the discrep- We conducted the HAWK-I search in dwarfs in binary systems, and they trace ancy observed in the local Universe. K-band, monitoring a sample of local 30 The Messenger 155 – March 2014 starburst galaxies retrieved from the IRAS - - Figure 2. K-band finding Revised Bright Galaxy Sample. We charts for two SNe dis- covered in our search selected galaxies with z < 0.07 and total (left: SN 2012hp; right: 11 infrared luminosity of LTIR > 10 LA. SN 2011ee). The insets We collected a sample of 30 starburst show the transients as they appeared in the dif- galaxies for Paranal observation, consid- $ $ ference images. ering seasonal observability. Given that the SN IR light curves evolve relatively slowly, remaining within one or two mag- nitudes of maximum for two or three ǥ months, an IR SN search did not require ǥ frequent monitoring. We planned on average three visits per galaxy per residuals that correspond to the galaxy by the location of the transient inside the semester, which implies a total of about nuclear regions, in particular when seeing host galaxy. Indeed, the magnitude limit 100 observing blocks per semester. was poor (full width at half maximum is brighter in the nuclear regions for a [FWHM] > 1 arcsecond; see Miluzio et al., typical galaxy of our sample, correspond- The programme was allocated three 2013). ing to the inner 1.5–2.0 kpc. While the observing periods and the fraction of detection limit in the galaxy outskirts is useful observing time was 100% of the largely independent of the seeing of the allocated time in the first season and Supernova discoveries and characterisa- images (typically K ~ 19 mag), this is 70% in the second and third semesters. tion not true in the nuclear region, where the In total, we obtained 210 K-band expo- magnitude limit is significantly brighter sures (with a single exposure time of During our monitoring campaign six when the seeing is poor (in the worst 15 minutes). Spectroscopic confirmation transients were detected in at least two case even 5–6 mag brighter than in the of the SN candidates was obtained with consecutive epochs separated by around galaxy outskirts; see Miluzio et al., 2013). ISAAC and X-shooter at the VLT (and in one month. We obtained spectroscopic one case at the Gran Telescopio Canarias confirmation for four of them: three as [GTC]). The average image quality was CC SN (SN2010bt; SN2012hp, see SN search simulation excellent: for about 90% of the expo- CBET2446; and SN2011ee, see CBET sures the seeing was less than 1.0 arc- 2773) and one as a type Ia SN (SN To evaluate the significance of the seconds, with an average value across 2010gp). Two candidates were too faint detected events, we developed a simu- the whole programme of 0.6 arcseconds. for spectroscopy and the transient lation tool that computes the number and Data reduction was performed by inte- classification was based on analysis of properties of the expected events based grating EsoRex (the ESO Recipe EXecu- multicolour photometry. We argued that on specific assumptions and features tion tool) with custom programs. Figure 1 they are also likely CC SN. SN e2010bt of our SN search.
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