SPITZERANALYSIS OF H II REGION COMPLEXES IN THE MAGELLANIC CLOUDS: DETERMINING A SUITABLE MONOCHROMATIC OBSCURED STAR FORMATION INDICATOR

Abstract

H II regions are the birth places of stars, and as such they provide the best measure of current star formation rates (SFRs) in galaxies. The close proximity of the Magellanic Clouds allows us to probe the nature of these star forming regions at small spatial scales. To study the H II regions, we compute the bolometric infrared flux, or total infrared (TIR), by integrating the flux from 8 to 500 µm. The TIR provides a measure of the obscured star formation because the UV photons from hot young stars are absorbed by dust and re-emitted across the mid-to-far-infrared (IR) spectrum. We aim to determine the monochromatic IR band that most accurately traces the TIR and produces an accurate obscured SFR over large spatial scales. We present the spatial analysis, via aperture/annulus photometry, of 16 Large Magellanic Cloud (LMC) and 16 Small Magellanic Cloud (SMC) H II region complexes using the Spitzer Space Telescope's IRAC (3.6, 4.5, 8 µm) and MIPS (24, 70, 160 µm) bands. Ultraviolet rocket data (1500 and 1900 Å) and SHASSA Hα data are also included. All data are convolved to the MIPS 160 µm resolution (40 arcsec full width at half-maximum), and apertures have a minimum radius of 35 ′′ . The IRAC, MIPS, UV, and Hα spatial analysis are compared with the spatial analysis of the TIR. We find that nearly all of the LMC and SMC H II region spectral energy distributions (SEDs) peak around 70 µm at all radii, from ∼ 10 to ∼ 400 pc from the central ionizing sources. As a result, we find the following: the sizes of H II regions as probed by 70 µm is approximately equal to the sizes as probed by TIR (≈ 70 pc in radius); the radial profile of the 70 µm flux, normalized by TIR, is constant at all radii (70 µm ∼ 0.45 TIR); the 1σ standard deviation of the 70 µm fluxes, normalized by TIR, is a lower fraction of the mean (0.05 − 0.12 out to ∼ 220 pc) than the normalized 8, 24, and 160 µm normalized fluxes (0.12 − 0.52); and these results are the same for the LMC and the SMC. From these results, we argue that 70 µm is the most suitable IR band to use as a monochromatic obscured star formation indicator because it most accurately reproduces the TIR of H II regions in the LMC and SMC and over large spatial scales. We also explore the general trends of the 8, 24, 70, and 160 µm bands in the LMC and SMC H II region SEDs, radial surface brightness profiles, sizes, and normalized (by TIR) radial flux profiles. We derive an obscured SFR equation that is modified from the literature to use 70 µm luminosity, SFR (M ⊙ yr −1 ) = 9.7(0.7) × 10 −44 L 70 (ergs s −1 ), which is applicable from 10 to 300 pc distance from the center of an H II region. We include an analysis of the spatial variations around H II regions between the obscured star formation indicators given by the IR and the unobscured star formation indicators given by UV and Hα. We compute obscured and unobscured SFRs using equations from the literature and examine the spatial variations of the SFRs around H II regions.

A New Electron-Density Model for Estimation of Pulsar and FRB Distances

“…The SMC images in the Barbara A. Milulski Archive for Space Telescopes 18 show an Hα shell surrounding the pulsar. There is no significant radio continuum feature at the pulsar position in the 4.8 GHz image of Lawton et al (2010). It is possible that this pulsar just happens to lie behind a relatively high-density clump of ionized gas, possibly part of the surrounding Hα shell.…”

“…The fifth, PSRJ0131−7310, has a DM of 175 cm −3 pc, despite being located right at the edge of the SMC. The diffuse Hα survey of Barger et al (2013) shows a tongue of emission covering the position of PSRJ0131−7310 and the H II region N90 (Lawton et al 2010) is nearby. The SMC images in the Barbara A. Milulski Archive for Space Telescopes 18 show an Hα shell surrounding the pulsar.…”

An analysis of star formation withHerschelin the Hi-GAL survey

“…In contrast, IR emission is a reliable tracer of the photons produced by the forming source. Lawton et al (2010) finds that, among all the IR bands, the 70 μm band is the most reliable indicator of star formation because it is able to account for IR emission from HII regions (see for example Tibbs et al 2012;Faimali et al 2012). We then consider the 70 μm monochromatic estimator developed by Li et al (2010).…”

Polycyclic Aromatic Hydrocarbons as Star Formation Rate Indicators

“…Systematic efforts began with ISO (e.g., Roussel et al 2001;Boselli et al 2004;Forster-Schreiber et al 2004;Peeters et al 2004), and continued with Spitzer, which enabled extending the analysis to fainter systems and to higher spatial detail (e.g., Wu et al 2005;Calzetti et al 2005Calzetti et al , 2007Alonso-Herrero et al 2006;Zhu et al 2008;Kennicutt et al 2009;Salim et al 2009;Lawton et al 2010). In a typical approach, the 8 μm (or similar wavelengths) luminosity or luminosity/area is plotted as a function of other known SFR indicators, to establish whether a correlation exists; an example is shown in Figure 1.…”