The Impact of Star Formation Histories on the Inner Dark Matter Density Slopes of Galaxies

TL;DR

We address how the time structure of star formation shapes the inner dark matter density profile in galaxies, testing the roles of burstiness and SFH duration with NIHAO and FIRE-2 simulations. The authors quantify burstiness with a SFR criterion over short vs. long timescales and measure the SFH duration via $M_{\star, post}/M_{\star, pre}$, then show these factors correlate with inner slope deviations from the stellar-to-halo mass baseline. They derive a Tollet-like fit augmented by the SFH-duration term, reducing the prediction error and aligning both simulation suites in their core-formation predictions. The results imply that baryonic feedback timing and temporal structure are key secondary drivers of cusp-core transformations and offer observationally testable forecasts for resolved SFHs in dwarf galaxies.

Abstract

Aims. We aim to investigate the connection between star formation histories (SFHs) and the inner dark matter density profiles of simulated galaxies. In particular, we test whether the burstiness and temporal distribution of star formation influence the formation of cored versus cuspy dark matter profiles. Methods. We homogeneously analysed simulated galaxies from the NIHAO and FIRE-2 projects. For each galaxy, we derived dark matter density profiles and measured the logarithmic slope in the inner region of the dark matter halo (1-2% of R$_{\rm vir}$). To characterise star formation burstiness, we introduced a criterion based on comparing the star formation rate (SFR) averaged over two distinct timescales. We further quantified the duration of SFHs by computing $M_{\star, \rm post}$ / $M_{\star, \rm pre}$, the ratio of stellar mass formed after versus before the epoch of reionisation at redshift z $\sim$ 6.5. Results. Homogeneous analysis reveals that inner slope versus stellar-to-halo mass ratio trends for NIHAO and FIRE-2 galaxies are in much better agreement than reported in previous works. The burstiness and duration of the SFH explain the scatter in the inner slope versus stellar-to-halo mass ratio relation, revealing that galaxies with above average burstiness and more extended SFHs are more efficient at developing cored dark matter profiles. In contrast, galaxies with smoother SFHs and earlier stellar mass assembly tend to maintain cuspier dark matter profiles. We present an analytic expression that improves predictions for the inner slope using the parameter $M_{\star \rm,post}$ / $M_{\star \rm,pre}$, which reduces the mean squared error in both simulation suites relative to previous formulations based solely on the stellar-to-halo mass ratio.

Figures (10)

• Figure 1: Inner slope of the dark matter density profile, measured between 1% and 2% of the virial radius, as a function of the stellar-to-halo mass ratio. Results are shown for galaxies from the NIHAO (blue) and FIRE-2 (orange) simulations. Solid lines represent fits following Eq. \ref{['eq:tollet']} with parameters from Table \ref{['tab:params-FIREandNIHAO']} and the 1-$\sigma$ scatter around the fits is indicated with shadowed regions. The trends are compared to literature fits from dicintio_mstar_mhalo, Tollet and Lazar.
• Figure 2: Bursty mass fractions as a function of stellar mass for galaxies in NIHAO (top panel) and FIRE-2 (bottom panel) simulation suites. Black dashed lines show the result of fitting the bursty mass fraction to the stellar mass with a second degree polynomial. Circles are coloured by their absolute deviation from the fit, which we define as burst deviation.
• Figure 3: Inner slope of the dark matter density profile, measured between 1% and 2% of the virial radius, as a function of the stellar-to-halo mass ratio. The top panel shows results for NIHAO galaxies and the bottom panel shows results for FIRE-2 galaxies compared to the literature fit from Lazar. Circles for each galaxy are colour coded by their burst deviation (the difference between actual and expected bursty mass fraction at a given stellar mass). Separate fits are shown for galaxies with above- (red dashed lines) and below- (blue dashed lines) average burstiness. Black dashed lines show the fits to the full sample of galaxies from each suite and gray bands represent the 1-$\sigma$ scatter around the fit. All fits follow Eq. \ref{['eq:tollet']} with parameters shown in Table \ref{['tab:params-FIREandNIHAO-burst']}.
• Figure 4: Ratio of post- to pre-reionisation stellar mass ($M_{\star, \rm post}/M_{\star, \rm pre}$) as a function of total stellar mass for galaxies in NIHAO (top panel) and FIRE-2 (bottom panel) simulation suites. Black dashed lines show second degree polynomial fits. Circles are coloured by their relative deviation from the fit, which we define as SFH deviation.
• Figure 5: Inner slope of the dark matter density profile, measured between 1% and 2% of the virial radius, as a function of the stellar-to-halo mass ratio. The top panel shows results for NIHAO galaxies and the bottom panel shows results for FIRE-2 galaxies. Circles for each galaxy are colour coded by their SFH deviation (the relative difference between actual and expected post- to pre-reionisation stellar mass ratio at a given stellar mass). Separate fits are shown for galaxies with more extended (red dashed lines) and less extended (blue dashed lines) SFHs. Black dashed lines show the fits to the full sample of galaxies from each suite and gray bands represent the 1-$\sigma$ scatter around the fit. All fits follow Eq. \ref{['eq:tollet']} with parameters shown in Table \ref{['tab:params-FIREandNIHAO-mpostpre']}.