Pre-print draft: arXiv:1012.6980

Journal: The Astrophysical Journal Volume 767 Number 1

Finding η Car Analogs in Nearby Galaxies Using Spitzer: 

I. Candidate Selection

Rubab Khan, K. Z. Stanek, C. S. Kochanek

The late-stage evolution of the most massive stars such as η Carinae is controlled by the effects of mass loss, which may be dominated by poorly understood eruptive mass ejections. Understanding this population is challenging because no true analogs of η Car have been clearly identified in the Milky Way or other galaxies. We utilize Spitzer IRAC images of 7 nearby (=<4 Mpc) galaxies to search for such analogs. We find 34 candidates with a flat or rising mid-IR spectral energy distributions towards longer mid-infrared wavelengths that emit 10^5 L_sun in the IRAC bands (3.6 to 8.0 micron) and are not known to be background sources. Based on our estimates for the expected number of background sources, we expect that follow-up observations will show that most of these candidates are not dust enshrouded massive stars, with an expectation of only 6+-6 surviving candidates. Since we would detect true analogs of η Car for roughly 200 years post-eruption, this implies that the rate of eruptions like η Car is less than the ccSN rate. It is possible, however, that every M > 40 M_sun star undergoes such eruptions given our initial results. In Paper II we will characterize the candidates through further analysis and follow-up observations, and there is no barrier to increasing the galaxy sample by an order of magnitude.

Fig. 1.— The Spectral Energy Distributions (SED) of η Car now (blue solid line, Humphreys & Davidson 1994), Object X (black solid line, Khan et al. 2011), and M33VarA (black dashed line, Humphreys et al. 2006). The black triangles mark luminosity at the IRAC band centers. Although η Car and Object X have similar luminosities up to 3.6μm, the SED of η Car is steeply rising in the IRAC bands (a≃2.6) while Object X is almost flat (a≃0.2). Object X and M33VarA (Hubble & Sandage 1953; Humphreys et al. 2006) are both dust obscured stars with comparable bolometric luminosities (Khan et al. 2011), but in the IRAC bands, Object X is much more luminous (~1.5x10^5 L_⊙) than M33VarA (~0.5x10^5 L_⊙).

Table 1 — 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. This table summarizes the properties of the targeted galaxies. The absolute magnitude M_B and Hα luminosity L(Hα) are from Kennicutt et al. (2008), and L(Hα) is converted to star formation rate (SFR) following Equation 2 of Kennicutt (1998). The foreground Galactic extinctions are from Schlafly & Finkbeiner (2011). The targeted galaxies have an integrated SFR of ~2M_⊙/year. For the assumed Salpeter IMF of Kennicutt (1998), we can convert this to the massive (M > 8M_⊙) star formation rate of ~0.014/year. The observed ccSN rate over that 20 years is ~0.15/year (0.05 < R_SN < 0.35 / year, 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≃ 0:46Mpc, 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≃0.96Mpc, 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≃1.9Mpc, Gieren et al. 2005) and M81 (D≃3.6Mpc, Gerke et al. 2011) were also studied by Khan et al. (2010). NGC2403 (D≃3.1Mpc, Saha et al. 2006) contains two sources sometimes classified as SN impostors, SN 1954J and SN 2002kg (see the review by van Dyk 2005), but any star associated with SN 1954J must be relatively low mass (~20M_⊙ rather than ≳ 50M_⊙) and shows no strong evidence for mid-IR emission, while SN 2002kg is simply a luminous variable star with little mass loss (see Kochanek et al. 2012a). Unlike the other large galaxies we studied, NGC247 (D≃3.6Mpc, Madore et al. 2009) is highly inclined. NGC7793 (D≃4.1Mpc, Tully et al. 2009) is the most distant galaxy studied.

Table 2 — We used the Daophot/Allstar PSF-fitting and photometry package (Stetson 1992) to identify point sources in all four IRAC bands and then performed photometry at the source location using both aperture and PSF photometry. We used the IRAF ApPhot/Phot tool for the aperture photometry. The aperture fluxes were transformed to Vega-calibrated magnitudes following the procedures described in the Spitzer Data Analysis Cookbook2 and aperture corrections of 1.213, 1.234, 1.379, and 1.584 for the four IRAC bands. The choice of extraction aperture aperture (R_ap) as well as the inner (R_in) and outer (R_out) radii of the local background annulus are reported in this table. We estimate the local background using a 2σ outlier rejection procedure in order to exclude sources located in the local sky annulus, and correct for the excluded pixels assuming a Gaussian background distribution. Using a background annulus immediately next to the signal aperture minimizes the effects of background variation in the crowded fields of the galaxies. We used the Daophot/Allstar package for PSF photometry. The PSF photometry fluxes were transformed to Vega-calibrated magnitudes by applying zero point offsets determined from the difference between the calibrated aperture magnitudes and the initial PSF magnitude estimates of the bright stars in each galaxy.

Fig. 2. — Integrated mid-IR luminosity L_mIR as a function of the slope a for bright sources in M81. The vertical dashed lines show the slopes of blackbodies with the indicated temperatures and peak wavelengths. The top-right (thick red) box shows the candidate selection region (L_mIR>10^5 L_⊙ and a>0). The red triangles show the sources that also satisfy the third selection criteria, that at least 30% of the integrated mid-IR luminosity is emitted between 3.6 and 5.8 μm (f>0.3). Of these, the open red triangles correspond to candidates that are known to be non-stellar in nature, and the solid red triangles represent the surviving candidates. The green open circles show sources with f<0.3 and the black cross marks represent all the other sources. The narrow clump of points at a≃-2.75 correspond to normal stars with steeply falling mid-IR SEDs, while the wider clump of points to the right correspond to sources dominated by 8μm PAH emission. The top-left box shows the region L_mIR>10^5.5 L_⊙ and a<-1 that was used to select normal stars in the M33 image (see Figure 5). The labeled blue points represent objects not in M81 that are shown for comparison: Object X (``X'', solid square), the compact cluster M 33-8 (``C'', open square), M33VarA (``A'', large open circle), η Car (``η", open star), the Carina nebula excluding η Car itself (``η-", solid circle; Smith & Brooks 2007), and the Carina nebula including η Car (``η+", spiked open circle; see Figure 6).

Fig. 3.— Integrated mid-IR luminosity L_mIR as a function of the fraction f of L_mIR that is emitted between 3.6 and 5.8 μm for bright sources in M81. The box shows the candidate selection region (L_mIR>10^5 L_⊙ and f>0.3). The red triangles show the sources that also satisfy the third selection criteria that the mid-IR SED slope is either flat or rising (a>0). Of these, the open red triangles correspond to candidates that are known to be non-stellar in nature, and the solid red triangles represent the surviving candidates. The green open circles show sources with a<0 and the black cross marks represent all the other sources. The narrow clump of points at f≃0.8 correspond to normal stars with steeply falling (negative slope) mid-IR SEDs, while the wider clump of points at f≃0.25 correspond to sources dominated by 8 μm PAH emission. The labeled blue points are same as in Figure 2.

Fig. 4.— Extragalactic contamination for M81. Here we show all sources from a 6 sq-deg region of the SDWFS survey transformed to the distance of M81. The symbols, lines, and axis-limits are the same as in Figure 2. In this SDWFS region, 449 (~75/sq-deg) sources pass our selection criteria, indicating that we should expect ~13 background sources meeting our selection criteria given our 0.17 sq-deg survey region around M81. Note that very few of the contaminating background sources have properties comparable to η Car.

Table 3 — We estimate the expected number of extragalactic contamination for each galaxy using the SDWFS survey (Ashby et al. 2009) where the nature of the sources, particularly AGNs, is also well understood from the AGES redshift survey (Kochanek et al. 2012b). We transform the apparent magnitudes of all sources in a 6 sq-deg region of SDWFS to luminosity using each target galaxy's distance modulus, determine how many of them would meet our selection criteria, and correct that count for our survey area around each galaxy. This table reports the expected surface density of extragalactic contaminants and the number expected given the survey area around each galaxy. We expect a total of ~41 extragalactic sources to pass our selection criteria across the targeted galaxies, as compared to 46 initial candidates. Figure 4, which has the same format as Figure 2, illustrates this for M81's distance. We can already identify 11 of the 46 initial candidates as extragalactic. Statistically, this means that only 6+-6 are likely associated with the galaxies. The expected numbers of contaminating sources are generally consistent with the observed numbers with the exception of NGC247, which we investigated but appears to surely be a statistical fluke. The angular distribution of the candidates relative to the galaxies is also strongly suggestive of a dominant contribution for background sources.

Fig. 5.— Mid and far-IR SEDs of the candidates in M33 (red lines) compared to the SEDs of normal stars with L_mIR>10^5.5 L_⊙ , which steeply falling SEDs (mid-IR slope a<-1, top left box of the Figure 2). The dotted portions of the SEDs correspond to the MIPS 70 and 160 μm flux upper limits. The SED of Object X is highlighted (red-black lighter dashed line) and η Car (black heavier dashed line) is shown for comparison.

Fig. 6. — The SEDs of η Car (``η'', black triangles, Humphreys & Davidson 1994), the Carina nebula excluding η Car itself (``η-'', blue squares, \citealpref:Smith_2007a, and the entire dusty complex containing η Car and other massive stars including η Car (``η+'', red circles). The first two SEDs are spline interpolated and summed to produce the third. The SED of the compact cluster M 33-8 (``C'', green dashed line, HST image in Figure 10) is shown for comparison. In Figures 2, 3, and 4 we label these η, η-, η+, and ``C'' respectively.

Fig. 7. — SEDs of four different classes of objects that met our selection criteria: a candidate dusty star in NGC2403, a star-cluster in M33, a QSO behind M81, and a galaxy behind NGC7793. Figure 10 shows IRAC and HST images of the compact cluster and the galaxy.

Table 4 — Of the 11 rejected candidates, six are AGNs or galaxies with a redshift measurement. Four have been photometrically classified as galaxies by the SDSS survey (Abazajian et al. 2009) and visual inspections of the SDSS images find diffuse extended sources consistent with this classification. One of these sources is also 0.4′′ away from a radio and X-ray source and is likely a low redshift AGN. One candidate is a well-resolved galaxy in HST images. The SEDs of the rejected candidates are shown in Figure 8 and their luminosities, SED slope, and f are listed in this table.

Fig. 8. — SEDs of sources that met our selection criteria but were rejected due to association with non-stellar sources. The dotted portions of the SEDs correspond to the MIPS 70 and 160 μm flux upper limits. The SED of  η Car (dashed blue line) is shown for comparison.

 

Table 5 — The mid-IR luminosities, slopes, and the fractions f of the 35 remaining candidates, including Object X, are presented in this table. Although we do not currently have an explanation, 5 of the candidates in M33 (M 33-3 through M33-7) are within ≲ 2.0′′ of radio-selected supernova remnants (Gordon et al. 1999). It is apparent from the SEDs of the candidates (Figure 9) that few have mid-IR luminosities comparable to η Car.

Fig. 9. — SEDs of sources that met our selection criteria and were rejected due to association with non-stellar sources. The dotted portions of the SEDs correspond to the MIPS 70 and 160 μm flux upper limits. The SEDs of η Car (dashed blue line) and Object X (dot-dashed black line) are shown for comparison. The SEDs of many sources, such as N2403-2, N247-3 and N7793-10, appear so dissimilar from the SEDs of η Car and Object X that they seem unlikely to be stellar sources. The MIPS band luminosity limits are also useful here. For example, the relatively low MIPS 70 and 160 μm luminosity limits indicate that N247-1 must have a very at or falling SED at these redder wavelengths and is probably an AGN, which is also suggested by its distance from the galaxy. On the other hand, the SEDs of M33-5, M33-8 and M81-10 are similar to η Car while N300-1, M81-5, M81-6 and M81-7 are similar to ObjectX (M 33-1). HST images show that M33-5, M33-8, and M81-10 are compact star clusters (Figure 10, top three panels). M81-5 is 0.56′′ from a variable X-ray source with maximum luminosity of 2 x 10^38 ergs/s, which is consistent with the source being an X-ray binary (Remillard & McClintock 2006). M81-7 has been classified as a galaxy by the SDSS survey but with a photometric redshift z = 0:00049 comparable to that of M81 (Abazajian et al. 2009). HST images show a region of enhanced star formation consisting of at least two components. Ancillary data also shows that N7793-3 is a High-Mass X-ray Binary (Mineo et al. 2012) with maximum X-ray luminosity of 3.9 x 10^37 ergs/s (Liu 2011). Although we do not currently have an explanation, 5 of the candidates in M33 (M 33-3 through M33-7) are within ≲ 2.0′′ of radio-selected supernova remnants (Gordon et al. 1999).

Fig. 10.— IRAC and HST images of the compact stellar clusters M33-5, M33-8, and M81-10, and the background galaxy N7793-2. The clusters are resolved in the HST images with FWHM of ~0.87′′≃4.1pc (M33-5), ~0.77′′≃3.6pc (M33-8) and 0.34′′≃6.1pc (M81-10). They are very luminous (few x 10^7 L_⊙) and their SED shapes are very similar to η Car (Figure 9).

Table 6 — 9 of the candidates from Table 5 also have 2MASS photometry and their near-IR magnitudes are presented in this table  along with the UBVRI magnitudes of the closest optical counterparts for the three candidates in M33 for which there is a publicly available catalog (Massey et al. 2007).