Using infrared photometry from the Spitzer Space Telescope , we perform the first inventory of aromatic feature emission ( AFE , but also commonly referred to as PAH emission ) for a statistically complete sample of star-forming galaxies in the local volume .
The photometric methodology involved is calibrated and demonstrated to recover the aromatic fraction of the IRAC 8 \mu \mathrm { m } flux with a standard deviation of 6 % for a training set of 40 SINGS galaxies ( ranging from stellar to dust dominated ) with both suitable mid-infrared Spitzer IRS spectra and equivalent photometry .
A potential factor of two improvement could be realized with suitable 5.5 \mu \mathrm { m } and 10 \mu \mathrm { m } photometry , such as what may be provided in the future by JWST .
The resulting technique is then applied to mid-infrared photometry for the 258 galaxies from the Local Volume Legacy ( LVL ) survey , a large sample dominated in number by low-luminosity dwarf galaxies for which obtaining comparable mid-infrared spectroscopy is not feasible .
We find the total LVL luminosity due to five strong aromatic features in the 8 \mu \mathrm { m } complex to be 2.47 \times 10 ^ { 10 } \mathrm { L } _ { \sun } with a mean volume density of 8.8 \times 10 ^ { 6 } \mathrm { L } _ { \sun } \mathrm { Mpc } ^ { -3 } .
Twenty-four of the LVL galaxies , corresponding to a luminosity cut at \mathrm { M } _ { \mathrm { B } } = -18.22 , account for 90 % of the aromatic luminosity .
Using oxygen abundances compiled from the literature for 129 of the 258 LVL galaxies , we find a correlation between metallicity and the aromatic to total infrared emission ratio but not the aromatic to total 8 \mu \mathrm { m } dust emission ratio .
A possible explanation is that metallicity plays a role in the abundance of aromatic molecules relative to the total dust content , but other factors such as star formation and/or the local radiation field affect the excitation of those molecules .