Large Variations Seen in First Ultraviolet Spectroscopic M33 Dust Extinction Curves

Abstract

Dust extinction curves provide one of the main avenues to understanding the detailed nature of dust grains and accounting for the effects of dust on observations of many astrophysical objects. For the first time, spectroscopic ultraviolet (UV) extinction curves are measured in M33 expanding the sample of Local Group galaxies with such measurements to five. These curves are based on Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra and literature photometry from the UV to the near-infrared. The four measured curves show large variations in their UV shapes including their 2175 A bump and UV slope strengths. The average extinction of these four sightlines is lower than the averages for other Local Group Galaxies and does not follow the Milky Way R(V) dependent relationship. The variations between UV extinction shape parameters and gas-to-dust ratios for the M33 sightlines fall within the variations seen in the combined sample of UV extinction curves in the Milky Way, Large and Small Magellanic Clouds, and M31. The correlation with gas-to-dust ratio is much stronger than the correlation with global metallicity. This strengthens the picture that local conditions like radiation field density and shocks dominate over global galaxy properties like metallicity in determining the wavelength dependence of dust extinction.

Figures (7)

• Figure 1: The locations of the observed stars are shown on the Spitzer MIPS 24 image Hinz04. This image traces the dust distribution with a focus on the youngest, embedded star formation. The magenta squares give the sightlines where extinction curves were measured and the orange circles where this was not possible.
• Figure 2: The STIS spectra and photometry for the target stars are shown. The spectra have been normalized to the flux at 2800 Å, offset, and sorted by the UV spectral slope. Regions of anomalously low flux have not been plotted.
• Figure 3: The observations, model components, best fit model, and fit parameters are shown for e5. In the left top panel, the full wavelength range is shown with the unextinguished model at the top (cyan), the MW foreground extinguished model in the middle (blue), and the MW foreground and M33 internal dust extinguished model (red) overplotted on the observations (black). The right top panel gives the region around Ly$\alpha$ with the model with dust extinction only in green and the full model including the gas absorption in red. The bottom panels give the residuals between the observations and the full model.
• Figure 4: The MW foreground-corrected M33 extinction curves are plotted sorted by UV slope in these units (i.e., $C_2 / R(V) + 1$). The curves are rebinned to a resolution of 200. The FM90 fits are plotted as non-solid lines. For clarity, the curves are offset on the y-axis. Regions of low S/N, near Ly$\alpha$, and around wind lines have been masked.
• Figure 5: $A(V)$ versus $R(V)$ is shown in the upper left panel. The other panels show different FM90 parameters versus each other. The $C_1$ versus $C_2$ is not shown as our fitting technique does not include $C_1$; instead $C_1$ is related to $C_2$ using the known strong correlations between these two parameters. The data sources are MW_GCC09 Gordon09FUSE, LMC_G03 Gordon03, SMC_G24 Gordon24, M31_C25 Clayton25, and this work (M33_G25). In the right two panels, the SMC blue points with single values of $\gamma$ and $x_o$ are for the sightlines without significant 2175 Å bumps where these parameters were fixed in the fitting Gordon24.