Targeted Galaxies

Table 1: Properties of Targeted Galaxies

Galaxy	Distance	$M_{B}$	$E(B-V)$	$\log_{10} L(H{\alpha})$	SFR (H${\alpha}$)	Known ccSN
	(Mpc)		(erg/s)	(M$_\odot$/yr)	($<20$years)
NGC6822	0.46	$-14.9$	0.24	39.1	0.01	...
M33	0.96	$-18.8$	0.04	40.6	0.33	...
NGC300	1.9	$-17.7$	0.01	40.1	0.11	...
NGC2403	3.1	$-18.7$	0.04	40.8	0.44	SN2004dj (IIP)
M81	3.6	$-20.1$	0.08	40.8	0.46	SN1993J (IIb)
NGC247	3.6	$-18.2$	0.02	40.3	0.17	...
NGC7793	4.1	$-18.5$	0.02	40.6	0.33	SN2008bk (IIP)

There are a number of sources of archival Spitzer data for nearby galaxies. In the Local Group, SAGE (Surveying the Agents of a Galaxy's Evolution) and SAGE-SMC (Meixner et al.2006; Gordon et al.2007) surveyed the LMC and SMC. Barmby et al. (2006) surveyed M31 with first results for massive stars discussed by Mould et al. (2008). M33 was observed at several epochs which allowed for mid-IR variability studies of M33 stars (Thompson et al.2009; McQuinn et al.2007). The Spitzer Infrared Nearby Galaxies Survey (SINGS, Kennicutt et al.2003) made a comprehensive mid-IR imaging and spectroscopic survey of 75 galaxies, many of them within $10\;$Mpc. The Local Volume Legacy Survey (LVL, Dale et al.2009) surveyed a total of 256 nearby galaxies, including all known galaxies inside a sub-volume bounded by $3.5\;$Mpc and an unbiased sample of S-Irr galaxies within a larger, and more representative, $11\;$Mpc sphere. The ongoing Spitzer Survey of Stellar Structure in Galaxies ($S^4G$, Sheth et al.2008) is collecting data for $\sim2300$ galaxies within $40\;$Mpc using the warm Spitzer ($3.6$ and $4.5 \micron$) bands.

For our paper, we selected 7 galaxies spanning a range of mass, morphology, distance, and star formation history. Since this is a pilot study, the sample is deliberately eclectic rather than focused on a sample maximizing the star formation rate per galaxy. Ultimately we would like to examine all nearby galaxies rather than just a few. Table1 summarizes the properties of the targeted galaxies. The absolute magnitude $M_B$ and H$\alpha$ luminosity L(H$\alpha$) are from Kennicutt et al. (2008), and L(H$\alpha$) is converted to star formation rate (SFR) following Equation2 of Kennicutt (1998). The foreground Galactic extinctions are from Schlafly & Finkbeiner (2011). The targeted galaxies have an integrated SFR of $\sim2\,\,$M$_\odot\,$year$^{-1}$. For the assumed Salpeter IMF of Kennicutt (1998), we can convert this to the massive ($M>8\,\,$M$_\odot$) star formation rate of $\sim0.014$year$^{-1}$. The observed ccSN rate over that 20 years is $\sim0.15$year$^{-1}$ ( $0.05 < R_{SN} < 0.35$year$^{-1}$, at 90% confidence). Since the ccSN rate should agree with the massive-star formation rate, this is a significant discrepancy for which we have no obvious explanation, and such mismatches are also found in other contexts (e.g., Horiuchi et al.2011).

The nearest of our targeted galaxies, NGC6822 (D$\simeq0.46$Mpc, Gieren et al.2006), is a barred irregular galaxy (de Vaucouleurs et al.1991). We included this small galaxy in our sample as an interesting nearby test case for examining large numbers of smaller, lower metallicity systems. M33 (D$\simeq0.96$Mpc, Bonanos et al.2006) was previously studied by both Thompson et al. (2009) and Khan et al. (2010) to search for dusty stars that are much redder but less luminous than the stars we are searching for in this paper. NGC300 (D$\simeq1.9$Mpc, Gieren et al.2005) and M81 (D$\simeq3.6$Mpc, Gerke et al.2011) were also studied by Khan et al. (2010). NGC2403 (D$\simeq3.1$Mpc, Saha et al.2006) contains two sources sometimes classified as SN impostors, SN1954J and SN2002kg (see the review by van Dyk2005), but any star associated with SN1954J must be relatively low mass ($\sim 20 \,$M$_\odot$ rather than $\gtrsim 50 \,$M$_\odot$) and shows no strong evidence for mid-IR emission, while SN2002kg is simply a luminous variable star with little mass loss (see Kochanek et al.2012a). Unlike the other large galaxies we studied, NGC247 (D$\simeq3.6$Mpc, Madore et al.2009) is highly inclined. NGC7793 (D$\simeq4.1$Mpc, Tully et al.2009) is the most distant galaxy studied.

For M33, we used the six co-added epochs of IRAC data from McQuinn et al. (2007) that were used by Thompson et al. (2009) and Khan et al. (2010), and the MIPS data retrieved from the Spitzer Heritage Archive. For NGC300 and NGC247, we used the data from the LVL survey (Dale et al.2009). For NGC6822, NGC2403, M81, and NGC7793, we used the data from the SINGS survey (Kennicutt et al.2003). We utilize the full mosaics available for each galaxy. Table2 shows the different pixel scales of the images retrieved from the Spitzer archive for M33, and those provided by the SINGS and LVL surveys for the other six galaxies.