Pervasive Cavity-Ring Structure for Star Formation in Dwarf Irregular Galaxies

Abstract

Unsharp-mask images of HI emission from 36 dwarf irregular (dIrr) galaxies illustrate star formation in dispersed clouds and on the rims of large cavities. The cavities can extend for a radial scalelength and typically have circular or slightly sheared forms. The average surface density of cloud peaks is ~20 Msun/pc2, and, combined with their average FUV star formation rate, suggests a gas consumption time of ~3.2 Gyr. Vertical hydrostatic equilibrium calculations for 24 of these dIrrs give a typical scale height of ~400 pc, which combines with the gas and star formation surface densities to suggest an efficiency per free fall time of ~1%. These values are comparable to those in the molecular clouds of spiral galaxies, suggesting the primary difference between clouds is the presence of CO at higher metallicity in the spirals. U-B color images of the dIrrs suggest that cavity ages range between 10^7 and 10^8 years, with the longer times explaining the common lack of bright OB associations in their centers and their low expansion speeds. Most are circular because the shear time exceeds 100 Myr, although some of the HI has spiral structure. These observations suggests that star formation in dIrrs proceeds slowly in a sequential fashion in dispersed clouds and on the periphery of giant cavities that move and expand during the ~50 Myr supernova era of the previous generation. In contrast, spiral galaxies have shear times 10 times shorter and more important stellar dynamics that compresses the gas into filaments.

Figures (18)

• Figure 1: Two examples of dIrr galaxies with ultra-deep images made from U, B, and V bands on the left and HI MOM0 maps on the right. The ellipses on the optical images are at 26 mag arcsec$^{-2}$ and 29 mag arcsec$^{-2}$ as measured in V-band. The color scale on the HI images is in units of $M_\odot$ pc$^{-2}$. There are features in the HI image that are not apparent in the optical images, such as the HI cavities and the gaseous spiral structure in the outer part of DDO 47. The yellow scale bars represent 1 arcmin.
• Figure 2: (top) HI emission from galaxies showing four different HI morphologies from left to right: large dispersed cavities (DC) throughout the main disk, large outer cavities (OC) beyond the detected FUV emission, possibly in addition to DC; a large central cavity (CC), and BCDs galaxies, which have centrally concentrated HI and no obvious cavities. The color scale ranges from blue at low emission to red at high emission. The limiting projected surface densities for HI in the top panels are, from left to right: 0 to 15.1, 0 to 17.6, 0 to 29.4, and 0 to 24.5, in units of $M_\odot$ pc$^{-2}$. (bottom) Unsharp-mask images of the same four galaxies, made by subtracting a Gaussian blur from the original HI image using a Gaussian dispersion equal to one-half of the V-band radial scale length of the galaxy. The yellow scale bars represent 1 arcmin.
• Figure 3: (Rows) Galaxies from Figure \ref{['fourtypes']} with image processing to reveal morphologies of FUV emission relative to HI structure. (Columns from left to right: the FUV image with contours of FUV emission superposed; the USM image of HI emission (from Fig. \ref{['fourtypes']}, bottom) with cyan contours for the peak regions; the USM image of HI with yellow contours for the cavities, and the full HI image with all the other contours superposed. DDO 133 shows FUV emission mostly at the positions of the HI peaks with a few regions where FUV extends into the HI cavities; DDO 70 has FUV emission in large regions free of HI; CVnIdwA has FUV and HI mostly coincident as in DDO 133 and the large central HI cavity has relatively little FUV, and VIIZw403 has FUV mostly on the large central cloud with some extending north to several small clouds.
• Figure 4: Histograms of the ratios of the numbers of pixels in various regions of the 36 galaxies. These ratios are from the total pixel counts in each galaxy above or below three thresholds. On the left is the distribution of the ratio of the total HI cavity area to the total HI peak area for all of the galaxies, as defined in the USM images. In the middle is the distribution of the ratio of the USM HI and FUV peak areas. On the right is the distribution of the ratio of the USM HI cavity area to the FUV peak area. The area of brightest star formation is $\sim21$% that of the cavities defined on the USM images. Dashed lines show variations in these ratios for slightly different definitions of the thresholds (see text).
• Figure 5: Blue points: the average projected star formation rate per unit area is plotted against the average projected HI surface density (corrected for He and heavy elements) for all regions where the USM HI value exceeds the threshold given by the cyan contours on the USM HI images, from equation (\ref{['hicontour']}). Red points: Average star formation rate densities versus average HI surface densities for all the regions where FUV is detected. The small black points are like the blue points but with a 5% lower threshold USM HI intensity. Each galaxy is a distinct point. The blue and red lines are the linear bivariate (i.e., symmetric) fits to each group of points, while the dashed blue line is a linear fit to only $\Sigma_{\rm SFR}$ versus $\Sigma_{\rm HI}$. The dashed black lines show consumption times of 1 Gyr, 10 Gyr and 100 Gyr, from top to bottom.