Pressure and Star Formation in LITTLE THINGS Dwarf Irregular Galaxies

TL;DR

This study shows that dwarf irregulars in the LITTLE THINGS survey follow Sigma_SFR–Sigma_g and Sigma_SFR–P relations similar to spirals, despite much lower gas surface densities and a dominant dark-matter halo in the inner disks. Using azimuthal averages and 1.5'' and 244 pc pixelations, the authors demonstrate that these relations are largely resolution-independent from ~20 to ~400 pc, implying scale-free star formation physics. The analysis finds an average molecular fraction f_mol ≈ 0.23 and a roughly constant star formation rate per molecule across dIrrs, with the molecular consumption time ~3.8 Gyr and the HI consumption time ~16.6 Gyr, placing dIrrs in between purely atomic and molecular-dominated regimes. They also argue that CO in these metal-poor environments traces dense, self-gravitating cores, aligning with dense-gas tracers like HCN in spirals, and emphasize the significant role of dark matter in shaping midplane pressure and, hence, star formation activity in dwarfs.

Abstract

The surface densities of star formation, Sigma_SFR, in 24 dwarf irregular (dIrr) galaxies from the LITTLE THINGS survey are combined with gas surface densities and midplane pressures to examine the correlations found previously for spiral galaxies. The pressure is the weight of the disk inside the gas layer, including gas, stars, and dark matter, which usually dominates disk gravity in dIrrs. We compare the results to the outer part of M33, which has similar local properties but a slightly higher metallicity, enabling the detection of CO. All the data are convolved to the HI beam, but to study the effects of resolution, the galaxies are examined first with average radial profiles, and then with maps having 1.5" pixels and 244 pc pixels. The correlations are found to be independent of resolution from 24 pc to 424 pc. The average ratio of molecular to atomic surface density is estimated to be 0.23+-0.1, from the H_2 surface density in M33 compared to the HI surface density at the same Sigma_SFR in the dIrrs. With this ratio, the average star formation rate per molecule is about the same for all the dIrrs, and a factor of 2 less than the rate in M33. The pressure in dIrrs is so low that CO is essentially a dense gas tracer, with the same surface density threshold at the low metallicities of dIrrs as HCN has in spiral galaxies. As a result, CO regions in dIrrs should be strongly self-gravitating.

Figures (8)

• Figure 1: The HI KS relation (left) and $\Sigma_{\rm SFR}-P$ relation (right) for 24 dIrr galaxies (blue dots and blue fitted lines). The fits are given by Eqs. \ref{['eq:ks']} and \ref{['eq:spfit']}. Also on the left are the $\Sigma_{\rm SFR}-H_2$ relation for the outer part of M33 (cyan), for all annuli in M33 (red circles and fitting line), and for all gas in M33 (red crosses). The red dots are for WLM with a star formation rate from FUV (like the others), and with gas surface densities for just the CO mass in the star-forming region (left-hand dot), the dark gas in this region (central dot) and the total gas in this region (right-hand dot). WLM is shown by the red dot in the right-hand panel as well, along with the outer M33 fit in cyan and two models from kim24.
• Figure 2: Histograms of the dark gas factors, defined to be the ratios of the derived dark gas surface densities to the observed HI surface densities, for each annulus in the dIrrs. On the left, they are calculated from the shift between the outer M33 $\Sigma_{\rm SFR}-H_2$ relationship and the dIrr observations at the same $\Sigma_{\rm SFR}$. On the right they are calculated by supplementing $\Sigma_{\rm g}$ in equation (\ref{['finalP']}) until $P$ for the dIrrs equals $P$ for the outer part of M33 at the same $\Sigma_{\rm SFR}$. Negative dark gas factors on the right are for dIrr observations at $P$ larger than the M33 pressure. The dark gas fractions from the KS relation are typical for dIrrs, derived by other methods, but some of the dark gas fractions from the $\Sigma_{\rm SFR}-P$ relation are higher.
• Figure 3: The average values for the pressure in the outer part of M33 (cyan line) and the dIrrs (blue curve), and for the terms in the pressure equation for the dIrrs (red, green, black lines), evaluated for $\Sigma_{\rm SFR}$ in units of $M_\odot$ pc$^{-2}$ Myr$^{-1}$ between $10^{-3}$ and $10^{-4}$. The pressure variations are relatively small, which is consistent with the good correlation between $\Sigma_{\rm SFR}$ and $P$. High values of $\rho_{\rm DM}$ are compensated by low values of $\Sigma_{\rm g}$. The points on the far right represent values for the outer disk of M33 using the same color scheme. These points for M33 pressure are averages over outer disk pixels which have $\Sigma_{SFR}$ in the same range as considered for dIrrs, while the cyan line for M33 pressure is the value from the fit to the $\Sigma_{\rm SFR}-P$ relation at $\log \Sigma_{SFR} = -3.5$.
• Figure 4: Pixel values in the HI KS relation for 24 dIrr galaxies: (left) $1.5^{\prime\prime}$ pixels, (right) the same with 244 pc pixel values superposed in red. The black line is the molecular KS relation for the outer part of M33; the cyan lines are bivariate fits to the data with solid lines for the $1.5^{\prime\prime}$ pixels and dashed lines for the 244 pc pixels; the orange segmented line is the azimuthally-averaged radial distribution from Figure \ref{['kssp']}. Galaxies are ordered by increasing HI resolution size in pc, as indicated.
• Figure 5: Pixel values in the $\Sigma_{\rm SFR}-P$ relation usinf $1.5^{\prime\prime}$ pixels (left) and 244 pc pixels superposed in red (right). The black line is the molecular KS relation for the outer part of M33; the cyan lines are bivariate fits to the data with solid lines for the $1.5^{\prime\prime}$ pixels and dashed lines for the 244 pc pixels; the orange segmented line is the azimuthally-averaged radial distribution from Figure \ref{['kssp']}. Galaxies are ordered by increasing HI resolution size in pc, as indicated.