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Spitzer-IRS Spectroscopy of the Prototypical Starburst Galaxy NGC7714

We present observations of the starburst galaxy NGC 7714 with the Infrared Spectrograph IRS on board the Spitzer Space Telescope. The spectra yield a wealth of ionic and molecular features that allow a detailed characterization of its properties. NGC 7714 has an HII region-like spectrum with strong PAH emission features. We find no evidence for an obscured active galactic nucleus, and with [NeIII]/[NeII]~0.73, NGC7714 lies near the upper end of normal-metallicity starburst galaxies. With very little slicate absorption and a temperature of the hottest dust component of 340K, NGC 7714 is the perfect template for a young, unobscured starburst


INTRODUCTION
NGC 7714 is a peculiar barred-spiral galaxy at 45 • inclination. For cz = 2798 km/s the galaxy is at distance 2 of 39.6 Mpc . Together with its post-starburst companion NGC 7715 it forms the interacting system Arp 284, which is the result of a recent (100 − 200 Myr), off-center collission between the two disk galaxies (Struck & Smith 2003). With its compact, UV-bright nucleus, NGC 7714 has been classified by Weedman et al. (1981) as the proto-typical starburst galaxy. The central region of about 330 pc has been the site of active star formation at a rate of about 1M ⊙ yr −1 for some 10 8 years. However, a recent significant increase in the star formation rate made it the dominating source of the UV flux (Lançon et al. 2001).
With its strong HeII λ4686 line (González-Delgado et al. 1995), NGC 7714 has been classified as a Wolf-Rayet galaxy. In fact, the optical and UV spectra indicate a population of about 2000 Wolf-Rayet and 20000 O-type stars, suggesting a fairly young age of the present starburst of 4 − 5 Myr (Garcia-Vargas et al. 1997). This is in agreement with earlier studies by Weedman et al. (1981) and Taniguchi et al. (1988) who estimated 10 4 O5-type stars from the Balmer and Bracket-γ line fluxes, and masses of ionized gas of 3.0 × 10 6 M ⊙ , and 1.9 × 10 6 M ⊙ , respectively.
The radio-continuum and the X-ray luminosity of 6 × 10 40 ergs s −1 require 10 4 supernova remnants in a volume of radius 280 pc (Weedman et al. Taniguchi et al. (1988) found definite evidence for a starburst-driven bipolar winds from the nucleus, nearly perpendicular to the disk plane. Although most of the activity is concentrated in the nucleus, NGC 7714 as a whole is experiencing intense star formation (González-Delgado et al. 1995). Mid-IR imaging with ISOCAM (O' Halloran et al. 2004) revealed a strong source at the nucleus surrounded by slightly extended emission out to about 30 ′′ (5.7 kpc) in diameter. From the IRAS fluxes (Surace et al. 2004) and the above distance we calculate a total infrared luminosity of L 8−1000µm = 5.6 × 10 10 L ⊙ .
Recently, Soria & Motch (2004) found two compact X-ray sources with XMM-Newton. One of them coincides with the starburst nucleus, has an X-ray luminosity of L X ≈ 10 41 erg s −1 , and shows the spectrum of a thin thermal plasma with a power-law (point-source) contribution. The variability in the power-law component hints at the presence of either a hidden low luminosity AGN or an ultraluminous X-ray source (Soria & Motch 2004).
In this letter we describe new observations of NGC 7714 with the Spitzer Space Telescope. These observations are part of an IRS guaranteed time program to obtain high signal-to-noise spectra of a large sample of different classes of nearby galaxies, that can be used for comparison with more distant systems (Devost et al. 2004, Higdon et al. 2004a, Houck et al. 2004a).
The data have been pre-processed by the Spitzer Science Center (SSC) data reduction pipeline version 9.5 (Spitzer Observer's Manual, chapter 7 3 ). The twodimensional "basic calibrated data" (BCD) constituted the basis for further processing. The BCD frames have been individually inspected by eye, and hot pixels, as well as very negative pixels, that have not already been flagged by the pipeline, were masked manually. The former could easily be identified by flipping between images at different nod positions. Tests have shown that masked pixels introduce artificially high noise in the spectra, and hence we interpolated isolated masked pixels by their nearest neighbor pixel values along the dispersion direction. Next we averaged the frames from the same nod positions (in the case of SH where four frames per nod position were available we have used the median instead). Since the low-resolution, long-slit modules contain two sub-slits, each integration provides a "free" sky spectrum -mainly zodiacal emission -in the adjacent subslit. We computed the median sky and subtracted it from the low-resolution frames.
Further processing was done within the IRS data reduction and analysis package SMART, version v.4 (Higdon et al. 2004b) -a powerful IDL package for spectral extraction and spectral analysis. The high-resolution spectra were extracted with "full aperture" extraction along the diffraction orders. The ends of each orders where the noise increases significantly were manually clipped. Within any one module the individual orders matched remarkably well and required no further finetuning, with the exception of SH order 11 which was scaled up by 5%. Finally the two nod positions were averaged. The low-resolution spectra were extracted using SMART's "interactive column extraction", which is similar to the method used in the SSC pipeline. The 3rd "bonus" order in SL and LL has not been included.
At the current stage of calibration, there remains a significant mismatch between some modules in both lowand high-resolution spectra. This mismatch is most likely due to either (i) extended source emission (as seen e.g. by O'Halloran et al. (2004)), or (ii) pointing errors that lead to flux losses that are most significant at short wavelengths (narrow slits). We also note that the lowresolution slits are narrower than the slits of the highresolution modules. At this early stage of the Spitzer mission we have no means to identify the real cause. However, the LH and LL spectra agree very well with each other, and a comparison between the IRAS 25µm flux of 3.15 Jy (Surace et al. 2004) and the LL flux in 3 http://ssc.spitzer.caltech.edu/documents/som/ the IRAS filterband of 2.61 Jy yields good agreement, taking into account that the IRAS "aperture" is much bigger and is even more susceptible to extended emission. Adopting the LH flux densities as correct, we scaled SH up by 36% to achieve an excellent match of the continuum fluxes around 19µm. We note that the flux in the [S III] line, which lies in the overlap region between SH and LH, is then the same in both modules. Similarly, the low-resolution spectra from different modules had to be scaled to match. The LL 2nd order was scaled up by 17% to match the LL 1st order; the SL 2nd order was scaled up by 40% to match the SL 1st order; and finally the resulting SL spectrum was scaled up by 17% to match the LL spectrum. As a result of this approach the low-and high resolution spectra agree very well with each other, and -even more important -we determine from the lowresolution spectrum in the IRAS 12µm band a total flux of 0.47 Jy, while Surace et al. (2004) found 0.56 Jy from IRAS -again good agreement! Figures 1 and 2 show the final IRS high-and low-resolution spectra, respectively.
The properties of the ionic and molecular features were obtained from single Gaussian fits to the high-and lowresolution spectra within SMART; the results are listed in tables 1 and 2.

The starburst properties of NGC 7714
Based on optical and near-IR spectroscopy (Lançon et al. 2001), the ISOCAM filter band ratios and PAH emission features (O'Halloran et al. 2004), it has long been argued that the nucleus of NGC 7714 is a site of intense starburst activity. However, given its low extinction, the unusually high infrared luminosity and recent reports of a possible, hidden AGN (Soria & Motch 2004) more precise diagnostics are needed to understand the processes in the nuclear region. The high signal-to-noise mid-IR IRS spectra provide the ideal diagnostic tools to characterize the nature of the underlying power source in more detail. Table 1 lists the properties of the strongest finestructure lines in the high-resolution spectrum ( fig. 1). We find 10.8 × 10 −20 Wcm −2 for the [Ne II] line while Phillips et al. (1984) found (7 ± 2) × 10 −20 Wcm −2 from the ground. For the [S IV] line we determine 1.8 × 10 −20 Wcm −2 while O'Halloran et al. (2004) quote (3.2± 0.8)×10 −20 Wcm −2 based on ISOPHOT-S low-resolution spectra. The S(0), S(1), and S(2) lines of molecular hydrogen are marginally detected and will be discussed in a subsequent paper.
Of particular interest is the [Ne III] / [Ne II] ratio as it is a good measure of the hardness of the interstellar radiation field, which is mainly determined by the most massive stars. We find [Ne III] / [Ne II] = 0.73 ± 0.05, corresponding to an effective temperature of about 3.8 × 10 4 K (Giveon et al. 2002). Based on the "ISO-SWS Starburst Sample", Thornley et al. (2000)     ratio can vary significantly as recently seen with the IRS in NGC 253 (Devost et al. 2004). We will address this important issue in a subsequent paper once we have observed a larger sample of starburst galaxies. However, with its large number of very luminous Wolf-Rayet stars, it is not surprising that the [Ne III] / [Ne II] ratio in NGC 7714 lies above the value found in most starburst galaxies.
Although [O IV] has an excitation potential of 54.9 eV, faint emission has been seen in many starburst galaxies (Lutz et al. 1998) and recently with the IRS in the ultra-luminous IR-galaxy UGC 5101 (Armus et al. 2004). We also find a marginal detection of the [O IV] line with (9.9 ± 7.2) × 10 −21 Wcm −2 . Since the location of the line coincides with a noisy pixel, we take this value as an upper limit. Hence, with [O IV] / [Ne II] ≤ 0.09 and a relative PAH(7.7µm) / continuum(7.7µm) ≈ 1.30, NGC 7714 falls below the region occupied by ULIRGs in fig. 5 of Genzel et al. (1998). Furthermore, the absence of the [Ne V] line -we place a 3σ upper limit of 3.9 × 10 −21 Wcm −2 -emphasizes the pure starburst nature of NGC 7714. On the basis of the infrared emission lines, therefore, we find no evidence for an obscured ionized region associated with an AGN, which implies that  the variable X-ray source reported by Soria & Motch (2004) is either from strong shocks associated with very recent supernova activity, or emission from a HMXB. There is growing evidence that both possibilities are intimately associated with massive young star clusters (Gao et al. 2003).
3.2. The dominant PAH features Below 13µm, the spectra are dominated by strong emission from PAHs (see e.g. Peeters et al. (2003) for a recent overview), with the strongest at 6.2µm, 7.7µm, 8.6µm, 11.2µm, 12.7µm, and 16.4µm listed in table 2. More features (e.g. at 5.7µm) appear to be present but will not be discussed here. Some of those features have also been seen in the ISOPHOT-S spectra which covered the 5 − 12µm range at a resolution of R = 90 (O'Halloran et al. 2004). The total flux we determine for the strongest feature at 7.7µm agrees with the ISOPHOT-S measurement of (8.84 ± 1.10) × 10 −19 Wcm −2 .
Shortward of 13µm the spectrum is qualitatively very similar to the well-know starburst galaxy M82. However, unlike M82, which has very strong PAH emission with respect to the warm continuum from very small grains and plausibly some extinction due to silicates at 9.7µm (Förster-Schreiber et al. 2003), the spectrum of NGC 7714 shows a rather smooth continuum and no ev-idence for silicate absorption.
With the high signal-to-noise provided by Spitzer-IRS it now becomes possible to expand the comprehensive study of fainter PAH features seen in Galactic sources to a large sample of extragalactic sources. While the strength of e.g. the 6.2µm PAH / continuum ratio is remarkably constant over a wide luminosity range in Galactic sources, others, e.g. the 11.2µm feature, which is linked to neutral PAHs, get quickly reduced in stronger ionizing radiation fields (Hony et al. 2001). Hony et al. (2001) show that the 12.7µm band correlates well with the CC stretch mode at 6.2µm for Galactic sources ranging from YSOs, H II regions, and reflection nebulae to planetary nebulae (their fig. 5). With log 10 (I 6.2 /I 11.2 ) = 0.32 and log 10 (I 12.7 /I 11.2 ) = −0.12, NGC 7714 falls right on their correlation line. We conclude that the physical conditions in NGC 7714 are similar to Galactic H II regions. In fact, a comparison with the ISO-SWS spectrum of M17 (Peeters et al. 2004) yields a perfect overall match longward of 15µm.

Dust and Extinction in NGC 7714
One of the most remarkable attributes in Figure 2 is the lack of silicate absorption in NGC 7714. This implies that the observed spectral shape of the mid-IR continuum is defined purely by the continuum emission of the hottest dust directly heated by the young, massive stars. If so then NGC 7714 might be a well-needed template for fitting relatively unobscured starbursts at low and high redshifts. Is NGC 7714 the perfect, unobscured starburst?
From the hydrogen recombination lines Paβ, Brγ, and Brα, Puxley & Brand (1994) determined A V = 1.8 ± 0.7 mag for a point source model with obscuring screen, and A V = 3.9 ± 1.7 mag if sources and dust are homogeneously mixed. For an extinction law of A 9.6µm /A V = 0.1 and if A V = 3.9 we find that τ 9.6µm = 0.36 and hence the continuum around 9.6µm would be suppressed by a factor of at most 30%. In the case of an obscuring screen, which was favored by Puxley & Brand (1994), the suppression is only 15%, which is in good agreement with the shape of our spectrum.
The long wavelength baseline available in our spectrum of NGC 7714 makes possible a very accurate determination of the continuum, which allows sophisticated fits for the dust emission which is responsible for this continuum. Figure 3 shows a three component fit to the low-resolution spectrum. Two components are warm and cool astronomical silicate/graphite (Si/Gr) dust with an MRN distribution (Mathis et al. 1977) and the optical constants given by Draine & Lee (1984) and Laor & Draine (1993). The third component is astronomical PAHs for which we use the absorption coefficient of Siebenmorgen et al. (2001). We also allowed for the possibility of an intervening, absorbing Si/Gr dust screen but found that τ 9.7µm ≤ 0.2 (consistent with the above results). The Si/Gr dust components have temperatures of approximately 153 K and 60 K, and the PAH component has a temperature of about 340 K. PAH emission clearly dominates below 9µm while the Si/Gr mix fills in the "PAH trough" at 10.5µm. The Si/Gr component then dominates at longer wavelengths. A warm and a hot component are clearly present in NGC 7714. Due to the absence of silicate absorption, we have confidence that the continuum is directly observed, without being obscured by cooler, intervening dust. This means that we have achieved a direct measurement of the hottest dust in a prototypical galaxy. This determination is important because the dust temperature is often used as a discriminant between starbursts and AGN.
This work is based [in part] on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through Contract Number 1257184 issued by JPL/Caltech.