1 Introduction

Some recent surveys aimed at detecting SNe (in the optical) have found that starburst galaxies do not show evidence for an enhanced SN rate, when the rate is normalized to the B luminosity of the galaxies, i.e. starburst and quiescent have the same SN rate in terms of SNu (Richmond et al. 1998). This result is puzzling, since the presence of a burst of star formation should boost the SN rate much more than the optical luminosity (in addition to the fact that the optical luminosity is diluted by the quiescent, evolved stellar population). One possible explanation for the shortage of SNe in star forming galaxies is that most SNe are obscured by the large amount of dust which generally affects starburst systems. In the following we will focus on the latter scenario.

A survey for SNe in the infrared, where dust extinction is greatly reduced, would allow one to detect the obscured SNe which are possibly missed in optical monitoring campaigns. This strategy was first proposed by van Buren & Norman (1989) and discussed more recently also by Mattila & Meikle (2000). The first K-band (2.2 $\mu$m) monitoring campaign was attempted by van Buren et al. (1994). They discovered SN 1992bu by comparing four near-IR images of NGC 3690 (this SN was not confirmed spectroscopically). More recently, a search in the K'-band was attempted by Grossan et al. (1999); they failed in detecting any new SN and they ascribe their negative result to the poor resolution of their camera.

We started a K'-band ( $\rm 2.1~\mu$m) monitoring campaign of a sample of $\sim$35 Luminous Infrared Galaxies (LIRGs) aimed at detecting obscured SNe elusive in optical surveys. LIRGs are galaxies characterized by far-IR (FIR) luminosities in excess of $10^{11}~L_{\odot}$. According to optical and IR studies the main energy source of this class of objects seems to be starburst activity, at least up to luminosities of ${\sim} 10^{12.5}~L_{\odot}$ (e.g. Sanders & Mirabel 1996; Genzel et al. 1998). The contribution to the IR luminosity from hidden active galactic nuclei (AGNs) is matter of debate. However, if most of their luminosity is powered by star formation, then the inferred star formation activity would imply an average rate of about one SN per year per object, within our sample (Mattila & Meikle 2000).

Figure 1: Left: Ks-band image of NGC3256 obtained with SOFI in November 1999. Right: Ks-band image obtained in January 2001 where SN 2001db has been detected. Both images are $35''\times 35''$ in size.

Figure 2: Upper panel: Ks-band light curve of the SN 2001db. The solid line is the light curve of the SN 1980k (type IIL) whose epoch has been shifted by 65 days (i.e. B band peak at -65 days), and extinguished by AK =0.56 mag (as derived in Sect. 2.2). This is nearly the same as the average light curve found by Mattila & Meikle (2000, MM01), shifted by 55 days. The dotted line is the light curve of SN 1999em, whose epoch is shifted by 90 days and whose magnitude is made brighter by 1 mag (besides the extinction of AK =0.56). The shift of the template light curve was determined to best fit the observed light curve. Lower panel: upper limits on the V magnitude for SN 2001db compared with the V-band light curves corresponding to those in the upper panel (extinguished by AV =5.6 mag).

The survey, started in late 1999, was carried on with the NTT-ESO, TNG and Kuiper/Steward telescopes. In this paper we report the discovery of two infrared SNe: SN 2001db (Maiolino et al. 2001) and SN 1999gw (Cresci et al. 2002), and provide a preliminary estimate of the SN production in Luminous Infrared Galaxies. A more detailed description of our survey and a more accurate estimate of the SN rate will be given in a forthcoming paper (Mannucci et al. in prep.).

 

 

Table 1: Log of observations and photometry of the two SNe discussed in this paper.

SN 2001db
Date	Tel./Instr.	type	band/ $\Delta \lambda$	mag
09 Jan. 2001	NTT/SOFI	imag.	Ks	$16.03\pm0.20$
08 Feb. 2001	NTT/SOFI	imag.	Ks	$16.93\pm0.20$
18 Mar. 2001	NTT/SOFI	imag.	Ks	$17.33\pm0.25$
01 Apr. 2001	NTT/SOFI	imag.	Ks	$17.74\pm0.30$
17 Apr. 2001	VLT/FORS1	imag.	open	V>22
21 Apr. 2001	VLT/ISAAC	spec.	1.08-1.35 $\mu$m
16 May 2001	VLT/FORS1	spec.	4500-9100 Å
16 May 2001	VLT/FORS1	imag.	open	V>22
SN 1999gw
Date	Tel./Instr.	type	band/ $\Delta \lambda$
16 Dec. 1999	TNG/ARNICA	imag.	Ks	$17.45\pm0.20$
26 Jan. 2000	TNG/ARNICA	imag.	Ks	$18.41\pm0.30$
12 Feb. 2000	TNG/ARNICA	imag.	Ks	>18.6
08 Feb. 2001	TNG/NICS	imag.	Ks	>18.6