SMC Paper

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[edit] Far Ultraviolet Diffuse Emission from the Small Magellanic Cloud

[edit] Abstract

We present the first observations of far ultraviolet (FUV: 1000 -- 1150 A) diffuse radiation from the Small Magellanic Cloud (SMC) based on observations made with the Far Ultraviolet Spectroscopic Explorer (FUSE). We have adopted the data analysis method of Murthy & Sahnow (2004) for extraction of diffuse surface brightnesses from the FUSE spectra. The diffuse radiation is primarily due to light from hot stars scattered by the interstellar dust grains. The FUV diffuse surface brightness in the SMC ranges from around 103 photons cm-2 s-1 sr-1 A-1 to as high as 2.5 x 105 photons cm-2 s-1 sr-1 A-1. The FUV diffuse fraction emitted from the SMC is much higher than the Large Magellanic Cloud (LMC: Pradhan et al. 2010) mostly due to the less number of hot stars in the SMC compared to the LMC. We have estimated the fraction of the total radiation in the field emitted as diffuse radiation which is about 40% -- 50%. We find that, in contrast to the LMC, in the SMC the fraction of light scattered in the FUV is almost same as the fraction of stellar radiation going into heating the interstellar dust.

[edit] Introduction

The Small Magellanic Cloud (SMC) is a nearby dwarf galaxy (~ 60 Kpc; Hilditch 2005) oriented nearly face-on to us. The foreground Galactic extinction is low (E(B-V) = 0.02 mag; Hutchings 1982) furnishing an unimpeded view of its small scale structure. The SMC, itself, contains significant amount of dust and gas but with a low metallicity (Z ≈ 0.005; Dufour 1984; Asplund et al. 2004), low dust-to-gas ratio (8 times smaller than the MW; Bouchet et al. 1985), and a strong interstellar ultraviolet (UV) radiation field (4 -- 10 times higher than that in the solar neighborhood; Vangioni-Flam et al. 1980). The ISM of the SMC is similar to that of high redshift galaxies because of its low metallicity and therefore may be a stepping stone to our understanding of the ISM in them (Witt & Gordon 2000). Dust in the SMC is quite different from that in either the MW or the LMC as shown, for instance, by the absence of 2175 A bump (Gordon et al. 2003). Models of the dust in the SMC typically assume that it is dominated by silicates with the absence of the 2175 A bump attributed to a lack of carbonaceous dust (Weingartner & Draine 2001).

There have been a number of observations of the SMC in the near ultraviolet (Nandy et al. (1978), Vangioni-Flam et al. 1979), Maucherat-Joubert et al. (1980), Cornett et al. (1994)) who have mapped the surface brightness and integrated magnitudes of the bright regions of the SMC. Here, we present the first observations of diffuse FUV emission from the SMC. These were serendipitous observations made with the Far Ultraviolet Spectroscopic Explorer (FUSE) and include several different environments in the SMC, from those near hot stars to those further out in the edges of the galaxy. The diffuse emission tracks the interaction of the radiation field with the dust and is an important input into models of distant galaxies (da Cunha, Charlot & Elbaz 2008). The SMC offers an opportunity to test these models at high spatial resolution and to distinguish the different components of the galaxy.

[edit] Observations and Data Analysis

We have used serendipitous observations made by the FUSE spacecraft to measure the diffuse emission from the SMC in the FUV (912 - 1200 A). The FUSE instrument and its mission have been discussed by Moos et al. (2000) and Sahnow et al. (2000). It consisted of four optical channels with each channel consisting of a mirror, a focal plane assembly (FPA) and a diffraction grating. Two of the primary mirrors were coated with LiF and aluminium and the other two with SiC with the spectrum imaged onto four detectors. Observations could be made through three different apertures: the high-resolution aperture (HIRS: 1.25" × 20"); the medium-resolution aperture (MDRS: 4" × 20"); and the low-resolution aperture (LWRS: 30" × 30"), with all three obtaining data simultaneously. Thus even though a source may have been in the MDRS or HIRS aperture, the diffuse background could still be measured through the LWRS aperture 90" away. Only the very brightest backgrounds could be observed with the smaller MDRS aperture or with the less sensitive SiC channels. The faintest backgrounds that could be observed were on the order of 2000 ph cm-2 s-1 sr-1 A-1.

There were a total of 220 observations in and around the SMC but most were of stars through the LWRS aperture leaving 30 pointings from which we could extract the diffuse background. These observations were from two classes of targets: point sources located in either of the MDRS or HIRS apertures; or blank areas of the sky where the spectrographs were allowed to thermalize before an instrumental realignment. The observational details of these pointings are given in Table 1 with the locations plotted in Figure 1. Most of the regions observed are either active areas of star formation are HII regions, such as NGC346 and NGC330.

The data selection and analysis procedure have been explained in detail elsewhere (Murthy & Sahnow 2004). We began with the raw photon list and processed the data through the latest version of CalFUSE (v 3.2; Dixon et al. 2007). Murthy & Sahnow (2004) found that the standard background subtraction in CalFUSE overestimated the instrumental background for faint extended sources and instead estimated it from the counts in the detector just off the spectrum. The background was subtracted from the data which was then collapsed into two wavelength bands per segment, avoiding airglow lines (Feldman et al. 19xx). This resulted in a total of 6 bands (Table 2) from 3 segments. The noise was too high to observe the diffuse background in segment 2B and so we didn't use it. In addition, a few of the data points in the 2A segment were anomalous and were also rejected.

Our target locations marked by '+' signs on a 160 micron image from Gordon & Witt (2009) of the SMC in Figure 1. The surface brightness measured in the FUSE bands show a strong correlation between them with the correlation coefficients better than 0.9. Our observed surface brightnesses (Table 1) range from a minimum of 1200 photons cm-2 s-1 A-1 sr-1 to as high as 2.5 x 105 photons cm-2 s-1 A-1 sr-1 in NGC 346, the youngest and largest HII region in the SMC. We have estimated the level of Galactic background at these wavelengths from the Voyager maps of Murthy et al. (1999) to be about 1000 photons cm-2 s-1 A-1 sr-1, below the FUSE detection limit.

We added observations from the Ultraviolet Imaging Telescope (UIT; Stecher et al. 1992) to extend our data into the near UV. UIT observed four 40' diameter fields which encompassed most of the bar of the SMC in at 1615 A (Cornett et al. 1997) with an angular resolution of 3" (Figure 1). Nine of our FUSE targets fell within the area covered by the UIT observations and we added the diffuse UIT flux by rebinning the 1.13" UIT pixels over the 30"x30" FUSE-LWRS aperture. These fluxes are listed in Table 1 and are highly correlated (r = 0.88) with the FUSE surface brightness (Figure 2).

[edit] Result and Discussion

Most of the FUV diffuse targets are present on the Bar region of the SMC except a bunch of bright FUV diffuse targets (nine targets) that are located in the north-east of the SMC which is a supernovae remnant. Although this region is off the SMC bar, we find this region very bright in UV. This is in close proximity to the N66 which is the largest star forming region in the SMC with numerous O type stars (Massey et al. 1989; Walborn et al. 2000) and also contains some of our bright diffuse targets. It is likely that some part of the diffuse light comes from the stars far away from the observed region where the dust grain is located. This is evident from the calculations of Cole et al. (1999) and Pradhan et al. (2010) for the LMC, where they found that much of the stellar radiation is non-local i.e., the diffuse light is actually the scattered light of distant stars by local dust. The north-east region is bright in IR 160 micron as seen in figure 1 (Gordon et al. 2009) showing that adequate amount of dusts are present which scatter the radiation from the OB stars of N66.

UIT observed the four fields of the SMC covering the complete bar regin for which Cornett et al. (1997) have derived stellar photometry. We measured the total FUV flux in the those UIT images directly by summing the fluxes in pixels and calculated the total stellar flux at 1615 A by summing the flux from the stars present using the star catalog (Cornett et al. 1997). Then calculated the total diffuse emission which is sum of contributions from faint stars as well as the dust scattered emissions. Cornett et al (1997) predicted that 22% of the diffuse emission is contributed by the stars fainter than UV magnitude 14.5 at 1615 A. We measured the dust scattered diffuse emission by subtracting the faint star contribution. We then extended this to the FUSE bands by scaling the diffuse radiation by the ratio between the FUSE and UIT observed fluxes (Figure 2) and used the stellar radiation field model of Sujatha et al.(2004) that is based on Kurucz models (Kurucz 1992) to extrapolate the stellar flux into the FUSE spectral regime. We used these to calculate the fraction of diffuse light escaping the SMC, defined as the diffuse emission divided by the total emission (diffuse + stellar), in each region and over the entire SMC bar (Fig. 9) with an estimated uncertainty of 20% -- 30%. In all cases, the behaviour of the diffuse fraction is almost the same, rising by 10% from 1000 A to 1150 A and a further 50% from 1150 to 1615 A, suggesting that the albedo of the dust increases by about the same factor, in agreement with the theoretical predictions of Weingartner (2001) for a mix of spherical carbonaceous and silicate grains over the same wavelength range.

Combining all the observations together, we find that 34% of the total radiation that escapes the SMC bar at 1000 A is diffuse rising to an astonishing 63% in the UIT bands at 1615 A. Witt and Gordon (2000) have modelled the scattered light in the SMC using a multiple scattering code predicting that the ultraviolet scattered radiation is in the range of 25% to 50% of the total integrated radiation coming from the SMC bar. We have plotted their modelled FUV scattered fraction along with our observed fraction (Fig. 8). Although their modelled value for a shell type dust geometry with various degrees of clumpiness is not matching the approximate observed value of diffuse fraction from the SMC completely, it matches with the diffuse fraction of NGC 330, a bright region in UV in the SMC bar.

The shape of the FUV diffuse fraction in both the LMC and the SMC are very much similar in the FUSE as well as the UIT wavelength bands. This fraction in the LMC is significantly less where only 5% of the total radiation at 1000 A and 40% at 1615 A is diffuse (Pradhan et al.2010). The UV emission in the LMC is dominated by the SN 1987A region and by two morphologically analogous and immense star forming regions, i.e., the 30 Doradus (the Tarantula Nebula), and the N11 region (a complex ring of emission nebula), at diametrically opposite ends of the prominent central bar. One possibility could be the relatively lower value of albedo (about 1.5 times; Weingartner & draine (2001)) of LMC dust compared to the SMC dust, which would lessen the scattering of FUV radiation that contribute to the diffuse FUV light. The FUV diffuse emission in both the galaxies are due to the scattering of starlight from the OB associations and the number of young hot stars in each of the UIT regions in the SMC are less than the LMC causing a lower value of stellar flux and a higher value of diffuse fraction. Multiple scattering from dust in the SMC also can't be ruled out as one of the reason for the high value of diffuse fraction and to incorporate this a suitable model of the diffuse radiation would be needed.

The LMC and the SMC show regional variation in diffuse emission and diffuse fraction due to varying composition and distribution of dust as well as the variation of the number density of young hot stars. The diffuse fraction in NGC 267 and NGC 292 of the SMC is relatively higher than that of NGC 346 and NGC 330 (Figure 2) because of less number of young stars in the former regions than the latter. Diffuse flux is quite less compared to the stellar flux in NGC 346 and NGC 330 and so is the diffuse fraction as the total flux is dominated by stellar flux. Even though stars are less in NGC 292 and NGC 267, scattering of the light coming from the stars outside the region are scattered by the local dust making the diffuse flux to be high. Similar situations have been observed in the UIT fields of the LMC. The values of diffuse fraction are less in the crowded regions, 30 Doradus, SN 1987A and N11 (4% -- 10%) and more in less crowded region, N70 (24% -- 45%) because of less number of exciting stars in the latter.

The H II regions are accomplished copious Hα emitters with hot massive stars at the centre. The catalog of the H II regions in the SMC was given by Davies et al. (1976) and the integrated Hα flux for them was calculated by Kennicut & Hodge (1986) defining the circular aperture sizes. We have computed the integrated FUV diffuse flux at FUSE bands for 36 H II regions that were used by Cornett et al. (1997). We found a good correlation of 0.81 between the integrated diffuse FUV emission and Hα emission from HII regions of the SMC (Fig.5) which indicates that much of the Hα emission in the SMC are produced from the scattering of the dust grains. The correlation of Hα emission with FUV flux at high latitudes dust clouds was observed by Seon et al. 2010 and the Hα radiation in the dust clouds is as a result of scattering of Hα emission with its source off the lines of sight (Mattila et al. 2007 and Lehtinen et al. 2010) which is substantiated by Wood & Anderson (1999) from their model calculation suggesting that 20% of the total Hα emission in high galactic latitude is produced due to the scattering of dust which is illuminated by Hα having its source elsewhere.

In the figure 5, the deviating points below the best fit line with lower H-alpha emission are from NGC 330 where as the points with higher value of diffuse FUV flux above the best fit line are from NGC 346. Both NGC 346 and NGC 330 are FUV bright with NGC 330 less bright in Hα emission (Cornett et al. 1997); the brightest star in NGC 330 has age of more that 15 Myr and the same in NGC 346 is less than 10.2 Myr as is obtained from Cornet et al. (1994). The FUV diffuse flux is indirectly connected with star formation as it correlates with Hα radiation which traces the star formation.

[edit] Conclusion

[edit] Text that has been cut

A chemical enrichment study of the SMC by Carrera et al. (2008) shows that the more metal-rich younger stars are concentrated in the central regions of the galaxy and the most active star-forming region is NGC 346, located towards the northern end of the SMC Bar. Recently, the Spitzer Space Telescope with a better sensitivity and angular resolution than the previous far infra-red missions has surveyed the ISM of the SMC in great detail in the wavelength range 2 μm to 160 μm (Bolatto et al. 2007; Leroy et al. 2007; Simon et al. 2007; Gordon et al. 2009; Sandstrom et al. 2010; Bonanos et al. 2010, Loon et al. 2010).

from the Sky Survey Telescope (S2/68) on board the TD-1 satellite. This was followed by the ultraviolet observations of the Magellanic Clouds of and Maucherat-Joubert et al. (1980) from ELZ instrument on board the D2B Aura satellite. Cornett et al. (1994) studied the distribution of dust from the Ultraviolet Imaging Telescope (UIT) on board the Space Shuttle Columbia finding an increase in E(B-V) from the north-east to the south-west in the SMC. The diffuse ultraviolet light escaping from galaxies is a diagnostic of the amount of dust and the interaction of the radiation field with the dust. It affects the appearance of external galaxies and also plays an important role in deciding the amount of radiation escaping from the galaxies that ionizes the intergalactic medium (IGM). The SMC provides an ideal location to probe the scattering of starlight by interstellar dust grains. It is far enough that local effects are not one of the best studied and is close enough that different components can be studied in great detail. Recently, Cole et al.(1999a) and Pradhan et al. (2010) have studied the far ultraviolet diffuse emission from its bigger and nearby sibling, the LMC. Pradhan et al.(2010) found that the diffuse FUV emission is 5 -- 20% of the total UV radiation of the LMC whereas Cole et al.(1999b) developed a three-dimensional scattering code for the analysis of the observed radiation finding a complex relationship among the diffuse radiation, the stars and the dust distribution.

This report extends the work of Pradhan et al. (2010) for the LMC in the wavelength range of 1000 - 1150 A.

FUSE has obtained high resolution spectra (\lambda/\Delta \lambda \approx 20,000) of Galactic and extragalactic sources, the ISM of the MW and nearby galaxies and the intergalactic medium before its unfortunate demise.

that were further divided into eight different segments; the SiC 1A, SiC 2A, SiC 1B and SiC 2B segments covering the wavelength range 905 A to 1105 A and the LiF 1A, LiF 2A, LiF 1B and LiF 2B segments covering the wavelength range 1000 A to 1187 A. Each segment consists of

Interstellar dust grains both absorb and scatter light of hot stars. Far ultraviolet (FUV) diffuse emission is the scattering part of the starlight from the dust grains whereas the absorbed part of the starlight by dust is seen as emission in infra-red. Dust in the SMC is quite different from that in either the MW or the LMC as shown, for instance, by the absence of 2175 A bump (Gordon et al. 2003). Models of the dust in the SMC typically assume that it is dominated by silicates with the absence of the 2175 A bump attributed to a lack of carbonaceous dust (Weingartner & Draine 2001). Stanimirovic et al.(2000) used high resolution Infrared Astronomical Satellite (IRAS) data in conjuction with Parkes telescope H I data to describe the diffuse cool dust and gas, and concluded with the fact that dust in the SMC is dominated by large grains. The authors found a quite low value of dust-to-gas ratio and dust mass compare to the MW. Bot et al.(2004) modelled the SEDs of dust to decompose the emission into the contributions by PAHs, VSGs, and large grains, finding a substantial 60 $\mu$m excess that could be caused by an enhanced interstellar radiation field in the SMC, or due to a change in the grain size distribution with respect to the Galaxy. Spitzer results report the properties of the dust, gas, IR sources, and the abundance of Polycyclic Aromatic Hydrocarbons (PAHs) which is a major component of dust in the ISM of the SMC. Using the Spitzer’s photometric and Spectroscopic Survey data of the SMC and modeling the dust spectral energy distributions (SEDs) and emissin spectrum with Draine & Li (2007) dust model, Sandstrom et al. (2010) found a higher fraction of PAHs within molecular clouds (q(PAH)~ 1% - 2%) than in diffuse ISM (q(PAH) ~ 0.6%) that the PAHs and the average PAHs fraction is half the value of the MW. From the Spitzer survey of the SMC, Leroy (2007) studied dust emission in the far-infrared finding an excess of emission which is tracing H2 gas but no CO in the molecular clouds leading to conclusion that the intense radiation field (UV photons) destroys the CO while H2 is survived by shelf-shielding (Maloney 1988)in the SMC.

FUV diffuse observations in the LMC are already presented (Pradhan et al 2020) and here we are presenting the results of the SMC observations and comparison with the LMC results.

Of the 200 pointings of FUSE in the SMC, first all the observations with a star in the LWRS aperture were rejected. Still there were other other observation where possibility of a star in the LWRS aperture was there. For them, the FWHM of the source was used to decide whether there exists a point source in the LWRS aperture; the criteria being FWHM of 20 pixels for a point source and 30 pixels for a diffuse source filling the aperture.

For the purpose of measuring the diffuse FUV emission, the observations made by the LiF LWRS aperture have been considered because of its large field of view and high throughput compared to the other apertures. Data obtained from 1A and 1B have been used for the measurement of the diffuse background flux as the sensitivity in 2B was too low to provide useful information and the wavelength range of 2A overlaps with 1B.

We have included all the observations for completeness despite the fact that the detection limit of the FUSE diffuse emission is about 2000 photons cm-2 s-1 A-1 sr-1 and the diffuse flux less than this value may be significantly contaminated. One such contaminant may be the galactic contribution towards the SMC line of sight. The Galactic contribution of diffuse radiation estimated from Voyager observations by Murthy et al.(1999) is less than 1000 photons cm-2 s-1 A-1 sr-1 which is much below the detection limit of the FUSE and we have neglected it in our estimation of the FUV diffuse emission from the SMC.

[edit] Reference

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[edit] Figures

Figure 1 : SMC 160 micron image from Gordon & Witt 2009 with the location of FUSE observations marked by '+' signs and the UIT fields are represented by circles.
Figure 1 : SMC 160 micron image from Gordon & Witt 2009 with the location of FUSE observations marked by '+' signs and the UIT fields are represented by circles.
Figure 2 : Correlation between FUSE (1B1) and UIT surface brightness is shown. The correlation coefficient is 0.88. The best fit is the line with slope 0.72 and an offset of -10103 photons cm-2 s-1 sr-1 A-1. Similar plots are obtained for the other three FUSE bands. Errors in the observations (Table 1) are small (relative to Y-axis scale) to be visible and are not shown
Figure 2 : Correlation between FUSE (1B1) and UIT surface brightness is shown. The correlation coefficient is 0.88. The best fit is the line with slope 0.72 and an offset of -10103 photons cm-2 s-1 sr-1 A-1. Similar plots are obtained for the other three FUSE bands. Errors in the observations (Table 1) are small (relative to Y-axis scale) to be visible and are not shown
Figure 3 : Variation of diffuse fraction against FUSE and UIT wavelength bands for the SMC Bar. Dust albedo (dashed line) and cross section (dot-dashed line divided by 4.85 × 10-22) from model calculations by Weingartner & Draine (2001) are also ploted. The error bars were empirically calculated by taking the extremes of the observed fluxes and range from 20% to 30% of the data.
Figure 3 : Variation of diffuse fraction against FUSE and UIT wavelength bands for the SMC Bar. Dust albedo (dashed line) and cross section (dot-dashed line divided by 4.85 × 10-22) from model calculations by Weingartner & Draine (2001) are also ploted. The error bars were empirically calculated by taking the extremes of the observed fluxes and range from 20% to 30% of the data.
Figure 4 : Comparison of Diffuse FUV fraction of the LMC and the SMC.Dashed line represents albedo of the SMC and the dot-dashed line represents the albedo of the LMC.
Figure 4 : Comparison of Diffuse FUV fraction of the LMC and the SMC.Dashed line represents albedo of the SMC and the dot-dashed line represents the albedo of the LMC.
Figure 5 : Integrated FUV diffuse flux from the HII regions is plotted against H-alpha flux of the same region obtained from Kennicutt & Hodge (1986).
Figure 5 : Integrated FUV diffuse flux from the HII regions is plotted against H-alpha flux of the same region obtained from Kennicutt & Hodge (1986).
Figure 6 : Same as fig 5 barring one point. Both NGC 346 and NGC 330 are FUV bright with NGC 330 less bright in H-alpha emission Cornett et al. (1997). The deviating points with lower H-alpha emission in the plot are from NGC 330 where as the points with higher value of FUV flux are from NGC 346. The brightest star in NGC 330 has age of more that 15 Myr and the same in NGC 346 is less than 10.2 Myr as is obtained from Cornet et al. (1994).
Figure 6 : Same as fig 5 barring one point. Both NGC 346 and NGC 330 are FUV bright with NGC 330 less bright in H-alpha emission Cornett et al. (1997). The deviating points with lower H-alpha emission in the plot are from NGC 330 where as the points with higher value of FUV flux are from NGC 346. The brightest star in NGC 330 has age of more that 15 Myr and the same in NGC 346 is less than 10.2 Myr as is obtained from Cornet et al. (1994).
Figure 7 : Diffuse fraction of HII regions.
Figure 7 : Diffuse fraction of HII regions.


Figure 8 : Diffuse fraction from individual regions as well as from the SMC (approximate value) & the model calculation of Witt & Gordon (2000).
Figure 8 : Diffuse fraction from individual regions as well as from the SMC (approximate value) & the model calculation of Witt & Gordon (2000).
Figure 9 : Diffuse fraction after subtraction of the faint star contributions.
Figure 9 : Diffuse fraction after subtraction of the faint star contributions.
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