STAR FORMATION IN THE EXTENDED GASEOUS DISK OF THE ISOLATED GALAXY CIG 96

H i observations were carried out with the NRAO VLA¹¹ in its C configuration on 2005 July 23, and were combined to the D configuration data published by Espada et al. (2005). We used the same correlator setting as for the D configuration data, which provided a velocity resolution of 48.8 kHz (10.4 km s⁻¹) for the 64 individual channels after Hanning smoothing. The editing and calibration of the data were done with AIPS.¹² The data were imaged using IMAGR by combining both data sets. For the cleaning process, we applied a compromise between natural and uniform weighting (weighting parameter ROBUST = 0). The average of the line-free channels has been subtracted from all the individual channels.

The final rms noise level achieved after 4 (D configuration) plus 7 hr (C configuration) per channel is σ = 0.4 mJy beam⁻¹, with a synthesized beam of ∼169 × 156 (or 1.7 × 1.6 kpc), and P.A. = −3010. We obtained the integrated intensity (moment 0) map from the 27 channels with H i emission, to which we applied a primary beam correction (HPBW ∼30' for the VLA antennas). The moment 0 was masked using a 3σ (=1.2 mJy beam⁻¹) clipping for each single channel. The 1σ noise level of the moment 0 map is 0.02 Jy beam⁻¹ km s⁻¹, calculated as the addition in quadrature of the channel's 1σ levels.

The excellent agreement between the single-dish total flux, S = 102.96 Jy km s⁻¹ using the Green Bank 43 m Telescope (HPBW = 22'; Haynes et al. 1998), and that obtained with the VLA C+D configuration data, S = 100.3 Jy km s⁻¹, implies that we recover most of the flux. Thus, an extended gas component is not expected to be missing in our maps.

The GALEX observatory provides both a near-UV (NUV λ = 1771–2831 Å) and a far-UV (FUV λ = 1344–1786 Å) broadband filter, with angular resolutions (FWHM) of 53 and 42, respectively. The field of view of the instrument is about 125, much larger than the VLA primary beam. CIG 96 was observed in NUV for 6.385 ks and in FUV for 1.648 ks in our GALEX Guest Investigator program 065 in Cycle 5. We also combined these data with previous observations from the archive.

Foreground stars are prominent in the NUV images and we identified and flagged them easily via their UV colors (see Gil de Paz et al. 2007). We computed and subtracted the background from both the FUV and NUV maps. We corrected fluxes by adopting Galactic extinction as given by Schlegel et al. (1998), namely E(B − V) = 0.048.

Besides correcting for the foreground Galactic extinction, we must also account for radial variations in the internal extinction. The total IR (TIR) to UV ratio (TIR/UV) is a robust tracer of internal extinction, depending weakly on the dust-to-stars geometry or the extinction law (see, e.g., Witt & Gordon 2000; Buat et al. 2005, and references therein). In order to compute the TIR radial profiles, Spitzer images at 24, 70, and 160 μm would be required, but they are not available for this galaxy. Therefore, we resorted to the FUV–NUV color (i.e., the UV slope) as an indirect tracer of dust extinction. Instead of using the classical calibrations derived for starburst galaxies (Calzetti et al. 1994; Heckman et al. 1995; Meurer et al. 1999), here we made use of the recipes of Muñoz-Mateos et al. (2009), which were calibrated on a sample of normal, nearby galaxies. From the FUV–NUV color profile we infer the TIR/FUV and TIR/NUV at each radius. These ratios are then converted into extinction radial profiles in the FUV and NUV bands independently. This was done by means of the prescriptions of Cortese et al. (2008), which take into account the varying contribution of young and old stars to the dust heating. The error in the extinction is dominated by the scatter of ratios when we calibrate TIR/FUV and TIR/NUV as a function of color (FUV–NUV) and is estimated to be of the order of 0.4 mag.

We present the H i integrated density distribution in Figure 1 (lower left panel, green). The H i extent is $D_{\rm H\,\mathsc{i}}$ = 129 at a level of 0.7 M_☉ pc⁻² (1σ noise level, Section 2.1), which results in an unusually large H i to optical extent ratio of $D_{\rm H\,\mathsc{i}}$/D₂₅ ≃ 3.5. A similar ratio is that of M83, whose XUV-disk properties have been previously studied in detail (Thilker et al. 2005, 2007; Bigiel et al. 2010a).

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The VLA C+D configuration H i map reveals a large variety of structures that could not be clearly distinguished in the VLA D-configuration map due to the lower angular resolution. The inner ring, with projected diameters of 15 × 10, exhibits enhanced H i emission along the stellar spiral arms. The outer pseudo-ring, with an extent of 82 × 46, seems to be connected to the inner ring to the northeast (NE) and SW of the inner disk. Overall, the shape of the outer pseudo-ring resembles two giant spiral arms surrounding the optical disk.

Although the shape of the brighter features is quite symmetric, there is also a faint diffuse H i emission that is asymmetric. The kinematically detached component in the SW that was reported in Espada et al. (2005) at about a 6' radius is seen here as a clumpy and extended component. A more detailed analysis of the entire H i data cube will be presented in a forthcoming paper.

We present the NUV map (upper left panel, red) and FUV map (upper right panel, blue) in Figure 1 with the same field of view as the H i map. Both NUV and FUV maps show an almost identical distribution. However, while the radially averaged FUV–NUV color is ∼0.5 in the internal parts of the disk (in units of AB magnitudes per square arcsec), it reaches FUV–NUV = 0.1–0.2 in the most external regions.

First, there are two nearly symmetric arms, one starting at the E of the bar and extending toward the N, and another one starting at the W of the bar and extending toward the S. The SW arm's appearance in both NUV and FUV emission is clumpy. This is similar in appearance to that observed by Eskridge et al. (2002) in the optical and near-IR (NIR). Eskridge et al. claim that the spiral pattern is flocculent in the outer parts of the inner optical/NIR disk (r < 15). These flocculent regions are related to the connections between the outer H i pseudo-ring and the inner H i ring to the NE and SW at the P.A. of the inner disk. A central concentration is found unresolved, which is likely connected to a nuclear starburst (Martini et al. 2003).

Second, a more diffuse and extended component of UV emission is seen in the UV maps. Two symmetric spiral arms are apparent, extending from the NE and SW of the optical disk. These spiral arms join an outer pseudo-ring structure to the E and W starting at about r = 2'. The pseudo-ring structure is the same as that seen in the H i outer disk, although the star-forming regions seem to be more confined to regions with high H i surface density. Other filamentary and more chaotic structures are seen out to almost r = 6', especially to the SW and to a minor extent in the NE.

A composite image of the H i and NUV/FUV maps convolved to the resolution of the former is presented in Figure 1 (lower right panel). A scheme (over the composite image) of main features in the outskirts of CIG 96 is shown in the upper panel of Figure 2: spiral arms (blue crosses) extending from the N to the W and from the S to the E, connecting the pseudo-ring structure (red crosses). The circles indicate the southeast (SE) spiral arm and pseudo-ring (SE side as well) rotated by 180°, which shows that these features are nearly symmetrical.

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The lower panel of Figure 2 shows that the NUV (color pixel map) and H i maps (contour map) have a high degree of correlation. This tight spatial correlation of H i and UV emission remains far into the outer pseudo-ring and a diffuse component all over the H i envelope, as can be seen in the last plotted contour.

We obtained the KS law (Schmidt 1959; Kennicutt 1998) pixel-by-pixel by comparing the gas surface density (Σ_gas) map obtained from our H i data (Section 2.1) and the SFR surface density (Σ_SFR) from the extinction-corrected (radially) NUV data (Section 2.2). We converted the pixel size of both maps to 16'' (as the H i beam size).

In the outskirts of disks the gaseous component is usually dominated by H i rather than by molecular gas, so the H i content is a good estimate of the total gas content, Σ_gas ≃ $\Sigma _{\rm H\,\mathsc{i}}$, especially below the saturation limit between atomic and molecular gas at Σ_gas ≃ 12 M_☉ pc⁻² (Martin & Kennicutt 2001; Wong & Blitz 2002; Bigiel et al. 2008; Verley et al. 2009, 2010).

In order to calculate the Σ_gas, we took into account the inclination of the galaxy and multiplied by 1.36 to include the helium contribution (Walter et al. 2008; Leroy et al. 2008):

$$\begin{eqnarray*} &&\Sigma _{{\rm gas}} [M_\odot \, {\rm pc}^{-2}] = 33.6 \times \Sigma _i {\rm S}_i[{\rm Jy\,beam}^{-1}] \Delta V [\mbox{km s$^{-1}$}], \end{eqnarray*}$$

where S_i is the flux density for each channel and ΔV is the channel velocity width. The 1σ noise level for Σ_gas = 0.7 M_☉ pc⁻², using the sensitivity limit reached in our H i moment 0 map (Section 2.1).

As for the Σ_SFR map, we used the corrected NUV data (Section 2.2). In the literature, FUV is used more widely in order to derive SFR as it involves emission from young stars born in the last 100 Myr, more similar to the Hα (<10 Myr) than to the NUV emission (<300 Myr; Verley et al. 2009). However, by using NUV, we achieve a gain in sensitivity and NUV emission is not essentially different from FUV to our resolution. This approach can also be useful for other sources with more sensitive NUV data than FUV. As for the "hidden SF," i.e., the SF that we cannot correct from the UV color because it is completely obscured by dust in both bands, it is expected to be negligible in the outer parts of disks, since Prescott & Kennicutt (2008) showed that there are no such regions at radii r > 0.6r₂₅ for SINGS galaxies (Kennicutt et al. 2003).

We convolved the NUV map to match the resolution of the H i map. Following Kennicutt (1998), the Σ_SFR was calculated as

$$\begin{eqnarray*} &&\Sigma _{\rm SFR} [M_\odot \,\rm yr^{-1}\,kpc^{-2}] =10^{-0.4 \times \mu _{\rm NUV}[\rm AB\,mag \,arcsec^{-2}] + 7.41} \end{eqnarray*}$$

assuming solar metallicity, a Salpeter (1955) initial mass function (IMF), and that the SFR has remained constant over the last few 10⁸ yr, which is the typical lifetime of stars dominating the UV emission. In this equation, μ_NUV is the surface brightness (in AB magnitude units) of NUV emission. To convert to the SFR derived using the Kroupa (2001) IMF, one should multiply our SFR by a factor of 0.83. The 1σ sensitivity limit is Σ_SFR = 9.2 × 10⁻⁶ M_☉ yr⁻¹ kpc⁻². In order to avoid projection effects, we have also deprojected both the Σ_SFR and the Σ_gas maps.

Figure 3 shows the correlation between the Σ_SFR derived from both the NUV and FUV map pixels (16'' size) within the inner 400''. The Σ_SFR obtained from the FUV map has been calculated in a similar manner to the Σ_SFR from the NUV map. The error bars presented in Figure 3 represent the formal uncertainties obtained from the sensitivity limit as well as from the uncertainties associated with the extinction correction. A bisector fit to all the data points yields a slope equal to unity and an intercept close to 0 (1.02 ± 0.03 and 0.15 ± 0.01, respectively). We estimated the uncertainties using different random representations of the data taking into account the uncertainties (bootstrapping). We obtained similar uncertainties for the formal slope and intercept, 0.01 and 0.02, respectively. Finally, note that calculations of Σ_SFR using alternate SFR tracers give differences of the order of 50% (Bigiel et al. 2010b; Leroy et al. 2008; Verley et al. 2009). We do not consider this uncertainty in our error estimation.

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In Figure 4 we show the Σ_SFR versus Σ_gas plot for CIG 96. From the figure we can discern the saturation limit between atomic and molecular gas found for other galaxies at Σ_gas ≃ 12 M_☉ pc⁻².

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In order to inspect whether the local KS SF law changes as a function of radius, we calculated a linear fit for different annuli centered at α(2000) = 02^h15^m2764, δ(2000) = 06°00'091 (Leon & Verdes-Montenegro 2003). Each color in the data points of the Σ_SFR versus Σ_gas plot in the lower panel of Figure 4 represents a different annulus zone over the deprojected H i map, which is depicted in the upper panel of Figure 4.

First, in the innermost region (r < 08 or 4.8 kpc; in dark blue in the left upper panel of Figure 4), the correlation between Σ_SFR versus Σ_gas is biased since at such radii the contribution of gas in molecular phase is presumably larger than that in atomic phase. For instance, $M_{\rm H_{2}}$ = 2 × 10⁸ M_☉ using CO(1–0) observation at the Swedish–ESO 15 m Submillimeter Telescope (SEST), which is characterized by an HPBW = 42'' (radius ∼2 kpc; Elfhag et al. 1996; U. Lisenfeld et al. 2011, in preparation). If the molecular gas is distributed within a uniform distribution then the corresponding molecular gas surface density would be $\Sigma _{\rm H_2}$ ∼ 11.5 M_☉ pc⁻². The $\Sigma _{\rm H_2}$ is likely larger since enhanced emission is expected toward the nucleus, inner spiral arms, and along the bar. Should we include the molecular gas contribution, the correlation Σ_SFR–Σ_gas would likely continue above Σ_gas > 12 M_☉ pc⁻² in a standard manner, with a slope of N ∼ 1–1.4 as it is observed in other galaxies (e.g., Bigiel et al. 2008).

We find that the power index (bisector fit) of the KS law systematically changes with radius from N = 3.0 ± 0.3 in the inner disk (08–17 or 4.8–10.2 kpc) to N = 1.6 ± 0.5 in the pseudo-ring feature (33–42 or 19.8–25.2 kpc). Further out the correlations are biased since a considerably large amount of data points fall below the sensitivity limit. We detailed in Table 1 the parameters for the fit with Σ_gas as an independent variable (OLS(Y—X)) as well as the least-square bisector fit, and for annuli from 08–42 in bins of 08 (4.8–25.2 kpc in bins of 4.8 kpc). The estimated errors of the fit parameters have been calculated via bootstrapping, in the same manner as in the Σ_SFR comparison from NUV and FUV. Note that in general these parameters are not strongly affected due to spatial resolution. Bigiel et al. (2008) show that the power-law parameters vary only weakly with changing spatial resolution.

Besides the uncertainty that results from the assumption Σ_gas ≃ $\Sigma _{\rm H\,\mathsc{i}}$, note that there is another source of uncertainty regarding a possible metallicity gradient that might affect the calculated Σ_SFR (in the sense that metallicity is lower in the outskirts). Assuming a constant calibration for a given metallicity, it can be inferred that regions with a lower metallicity would have a lower Σ_SFR for a fixed UV surface brightness (Leitherer et al. 1999).

The SFE, the SF per unit gas mass, decreases with radius from values of 10⁻¹⁰ yr⁻¹ in the inner ring to ∼10⁻¹¹ yr⁻¹ in the outskirts. In Figure 5 we show the SFE as a function of radius (normalized to r₂₅ = D₂₅/2). An increase in the observed scatter is present as a function of radius, from ∼0.2 dex to ∼1 dex.

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For comparison, we have plotted in Figure 5 the derived fit for the SFE as a function of radius from Leroy et al. (2008):

$$\begin{eqnarray*} &&{\rm SFE} = 4.3 \times 10^{-10}\, {\rm yr}^{-1},\quad {\rm if}\; r< 0.4\, r_{25}; \end{eqnarray*}$$

$$\begin{eqnarray*} &&{\rm SFE} = 2.2 \times 10^{-9} {\rm exp}(-r/ (0.25 \,r_{25}))\, {\rm yr}^{-1},\quad {\rm if}\; r> 0.4 r_{25}. \end{eqnarray*}$$

Note that their calibration is valid up to 1.2r₂₅ (Leroy et al. 2008).

The agreement between this fit from Leroy et al. (2008) and our SFE radial profile is reasonably good at 0.4r₂₅ < r < 1.5r₂₅. Although our fit is slightly offset toward higher values, the slope is essentially the same in this radius range. The fit to our data and that of Leroy et al. (2008) are shown in Figure 5 as dashed lines.

From r > 1.5r₂₅, the fit by Leroy et al. (2008) would start to deviate considerably from our data, in the same manner as pointed out in other spiral galaxies by Bigiel et al. (2010b). It seems to be more constant, at least for those regions where Σ_SFR and Σ_gas are above the noise level (blue data points). For such regions the SFE ≃ 10⁻¹¹ yr⁻¹. The fit to our data is shown in Figure 5 as a green dashed line. It fits relatively well (rms ∼ 0.5 dex) over a radius range of 1.5r₂₅ < r < 3.5r₂₅. Note that the scatter is large enough to consider the fit parameters as estimates only. This region corresponds to the pseudo-ring feature. Many of the detected regions in the radius range 1.7r₂₅ < r < 3.5r₂₅ are within the kinematically detached clump located to the SW (green circles; Espada et al. 2005).

Red triangles in Figure 5 correspond to upper limits in Σ_SFR, although detected in Σ_gas, and thus upper limits in SFE. These data points would probably increase the scatter of SFE at large radii.

We derived photometric radial profiles for CIG 96 not only in the GALEX FUV and NUV bands, but also in the Sloan Digital Sky Survey (SDSS) ugriz bands and the Spitzer/IRAC 3.6 (Werner et al. 2004) and 4.5 μm bands (Fazio et al. 2004).¹³

These multi-wavelength radial profiles were fitted with the models of Boissier & Prantzos (1999, 2000) and the IMF from Kroupa (2001), which are able to predict the radial variation of several galactic properties as a function of two parameters: the spin λ (non-dimensional specific angular momentum) and the asymptotic velocity of the rotation curve V_C. Galaxies are modeled as a set of independently evolving rings, with the gas infall timescale depending on both the total galaxy mass and the local mass surface density. Gas is then converted into stars following a KS law, modulated by a dynamical term that mimics the periodic passage of spiral arms (see also Boissier et al. 2003b). Newly born stars follow a user-specified IMF (here we rely on that of Kroupa 2001). Stars of different masses die at different rates, enriching the ISM with metals that are incorporated in subsequent generations of stars. The lifetimes, yields, evolutionary paths, and spectra of stars at different radii are computed as a function of the local metallicity. An initial model was first calibrated to reproduce observables in the Milky Way (MW; Boissier & Prantzos 1999). This model was then generalized to other galaxies (Boissier & Prantzos 2000) using the scaling laws derived from the Λ-cold dark matter scenario (Mo et al. 1998).

The GALEX, SDSS, and Spitzer photometric profiles were simultaneously fitted with these disk evolution models. The internal extinction profiles in the optical and NIR bands were derived from the UV ones using an MW extinction law and a sandwich dust-to-stars geometry (further details on the fitting procedure can be found in Muñoz-Mateos et al. 2011). By minimizing the χ² between the observed and predicted photometric profiles, the best-fitting values were found to be λ = 0.048^+0.013_0.012 and V_C = 161⁺¹²_{− 9} km s⁻¹. Using other models characterized by an IMF with less massive stars (i.e., Kroupa et al. 1993) or excluding the XUV disk further out than 150'' does not successfully reproduce both photometric profiles and the observed H i rotation curve, as the ones used here do (Espada et al. 2005).

We plot in each panel of Figure 6 the deprojected surface density profile for each band considered, as well as the fits from the model. The agreement is good for all components. For UV bands, the azimuthally averaged AB magnitudes seem to be slightly underestimated in the r = 10''–15'' region, although in general they agree well.

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The KS law was expressed in Section 3.3 in the form Σ_SFR ∝ Σ^N_gas, with N varying for different annuli. On the other hand, the SFR in the model is expressed as Σ_SFR ∝ Σ^N_gas V(R)/R. To check the consistency between both definitions, in Figure 4 (bottom) we plot the temporal evolution (from 0 to 13.5 Gyr) of Σ_SFR and Σ_gas using the model, and for different radii, 5.5 (blue), 10.9 (green), and 20 kpc (red). No obvious offset is found, which indicates that this galaxy is generally following the SF law implemented in the models, which relies on average main properties of galaxies.

Note that the models used here are not necessarily good at reproducing the properties of galaxy disks out to these large radii. The model itself does not include an imposed threshold, and although it is an extrapolation, it should work reasonably well for low-density regions and outskirts as long as there is no strong threshold effects and the mode of SF does not change. In fact, these models have been tested against observations of low surface brightness galaxies by Boissier et al. (2003a).

The model provides an estimation of the rotation curve and Σ_gas as a function of radius. We compare the observed gas surface density with that predicted by the model in the upper panel of Figure 7. A small deviation is found at low radii where most of the gaseous component is in molecular phase, and at large radii from r > 15 kpc (>3') where the model underestimates the observed surface density as a result of the external pseudo-ring and the XUV emission in general.

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The latter can be probably explained as a result of a recent past event that redistributed part of the gas on a timescale shorter than that probed by the different bands, because they average over the lifetime of the galaxy. UV emission, which as H i should be sensitive to any perturbation, agrees relatively well to the model. This suggests that a recent event could have occurred, such as accretion, in order for the H i to show this deviation with respect to the model.

In the lower panel of Figure 7, it can also be seen that the agreement between the observed H i rotation curve (Espada et al. 2005) and that predicted by the model is excellent. Note that the H i observed rotation curve is not used during the fit.