分类： 天文学 >> 天文学 提交时间： 2023-02-19
Daniela Calzetti
Sean T. Linden
Timothy McQuaid
Matteo Messa
Zhiyuan Ji
Mark R. Krumholz
Angela Adamo
Bruce Elmegreen
Kathryn Grasha
Kelsey E. Johnson
Elena Sabbi
Linda Smith
Varun Bajaj
摘要： Multi-wavelength images from the Hubble Space Telescope covering the wavelength range 0.27-1.6 $\mu$m show that the central area of the nearby dwarf galaxy NGC4449 contains several tens of compact sources that are emitting in the hydrogen recombination line Pa$\beta$ (1.2818 $\mu$m) but are only marginally detected in H$\alpha$ (0.6563 $\mu$m) and undetected at wavelengths $\lambda\le$0.55 $\mu$m. An analysis of the spectral energy distributions (SEDs) of these sources indicates that they are likely relatively young star clusters heavily attenuated by dust. The selection function used to identify the sources prevents meaningful statistical analyses of their age, mass, and dust extinction distributions. However, these cluster candidates have ages $\sim$5-6 Myr and A$_V>$6 mag, according to their SED fits, and are extremely compact, with typical deconvolved radii of 1 pc. The dusty clusters are located at the periphery of dark clouds within the galaxy and appear to be partially embedded. Density and pressure considerations indicate that the HII regions surrounding these clusters may be stalled, and that pre-supernova feedback has not been able to clear the clusters of their natal cocoons. These findings are in potential tension with existing models that regulate star formation with pre-supernova feedback, since pre-supernova feedback acts on short timescales, $\lesssim$4 Myr, for a standard Stellar Initial Mass function. The existence of a population of dusty star clusters with ages $>$4 Myr, if confirmed by future observations, paints a more complex picture for the role of stellar feedback in controlling star formation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The physical properties of star cluster populations offer valuable insights into their birth, evolution, and disruption. However, individual stars in clusters beyond the nearest neighbours of the Milky Way are unresolved, forcing analyses of star cluster demographics to rely on integrated light, a process that is fraught with uncertainty. Here infer the demographics of the cluster population in the benchmark galaxy NGC 628 using data from the Legacy Extra-galactic UV Survey (LEGUS) coupled to a novel Bayesian forward-modelling technique. Our method allows analysis of a total of 1178 clusters in the LEGUS catalogue, roughly a factor of 4 more than previous studies that required severe completeness cuts to the data. Our results indicate that the cluster mass function is truncated at $\sim 10^4$ $ \mathrm{M}_{\odot}$, consistent with proposed relations between truncation mass and star formation surface density. We find that cluster disruption begins early, $\sim 10$ Myr after formation, but that it is relatively mild, with clusters requiring on average $2-3$ times their present age to disrupt; we do not find any evidence for mass dependent disruption. We also do not find convincing evidence for any radial variations in these conclusions, though we find suggestive hints that inner galaxy clusters may be more prone to disruption. Confirming or refuting these hints will require future observations to increase the sample size of outer galaxy clusters.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zefeng Li
Emily Wisnioski
J. Trevor Mendel
Mark R. Krumholz
Lisa J. Kewley
Carlos López-Cobá
Sebastian F. Sánchez
Joseph P. Anderson
Lluís Galbany
摘要： We analyse the spatial statistics of the 2D gas-phase oxygen abundance distributions in a sample of 219 local galaxies. We introduce a new adaptive binning technique to enhance the signal-to-noise ratio of weak lines, which we use to produce well-filled metallicity maps for these galaxies. We show that the two-point correlation functions computed from the metallicity distributions after removing radial gradients are in most cases well described by a simple injection-diffusion model. Fitting the data to this model yields the correlation length $l_{\rm corr}$, which describes the characteristic interstellar medium mixing length scale. We find typical correlation lengths $l_{\rm corr} \sim 1$ kpc, with a strong correlation between $l_{\rm corr}$ and stellar mass, star formation rate, and effective radius, a weak correlation with Hubble type, and significantly elevated values of $l_{\rm corr}$ in interacting or merging galaxies. We show that the trend with star formation rate can be reproduced by a simple transport+feedback model of interstellar medium turbulence at high star formation rate, and plausibly also at low star formation rate if dwarf galaxy winds have large mass-loading factors. We also report the first measurements of the injection width that describes the initial radii over which supernova remnants deposit metals. Inside this radius the metallicity correlation function is not purely the product of a competition between injection and diffusion. We show that this size scale is generally smaller than 60 pc.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Two competing models, gravitational instability-driven transport and stellar feedback, have been proposed to interpret the high velocity dispersions observed in high-redshift galaxies. We study the major mechanisms to drive the turbulence in star-forming galaxies using a sample of galaxies from the xCOLD GASS survey, selected based on their star-formation rate (SFR) and gas fraction to be in the regime that can best distinguish between the proposed models. We perform Wide Field Spectrograph (WiFeS) integral field spectroscopic (IFS) observations to measure the intrinsic gas velocity dispersions, circular velocities and orbital periods in these galaxies. Comparing the relation between the SFR, velocity dispersion, and gas fraction with predictions of these two theoretical models, we find that our results are most consistent with a model that includes both transport and feedback as drivers of turbulence in the interstellar medium. By contrast, a model where stellar feedback alone drives turbulence under-predicts the observed velocity dispersion in our galaxies, and does not reproduce the observed trend with gas fraction. These observations therefore support the idea that gravitational instability makes a substantial contribution to turbulence in high redshift and high SFR galaxies.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Galactic winds are a crucial player in galaxy formation and evolution, but observations of them have proven extraordinarily difficult to interpret, leaving large uncertainties even in basic quantities such as mass outflow rates. Part of this uncertainty arises from the relatively simplistic models to which complex wind observations are often fit, which inevitably discard much of the available information. Here we present an analysis of the wind of the nearby dwarf starburst galaxy M82 using a semi-analytic model that is able to take advantage of the full three-dimensional information present in position-position-velocity data cubes measured in the Hi 21 cm line, the CO 2-1 line, and the Ha line. Our best-fitting model produces position-dependent spectra in good agreement with the observations, and shows that the total wind mass flux in the atomic and molecular phases is approx 10 M_sun yr-1 (corresponding to a mass loading factor of about 2 - 3), with less than a factor of two uncertainty; the mass flux in the warm ionised phase is more poorly constrained, and may be comparable to or smaller than this. At least over the few kpc off the plane for which we trace the outflow, it appears to be a wind escaping the galaxy, rather than a fountain that falls back. Our fits require that clouds of cool gas entrained into the wind expand only modestly, suggesting they are magnetically confined. Finally, we demonstrate that attempts to model the wind using simplifying assumptions such as instantaneous acceleration and a constant terminal wind speed can yield significantly erroneous results.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Mass-to-flux ratios measured via the Zeeman effect suggest the existence of a transition from a magnetically sub-critical state in HI clouds to a super-critical state in molecular clouds. However, due to projection, chemical, and excitation effects, Zeeman measurements are subject to a number of biases, and may not reflect the true relations between gravitational and magnetic energies. In this paper, we carry out simulations of the formation of magnetised molecular clouds, zooming in from an entire galaxy to sub-pc scales, which we post-process to produce synthetic HI and OH Zeeman measurements. The mass-to-flux ratios we recover from the simulated observations show a transition in magnetic criticality that closely matches observations, but we find that the gravitational-magnetic energy ratios on corresponding scales are mostly super-critical, even in the HI regime. We conclude that HI clouds in the process of assembling to form molecular clouds are already super-critical even before H_2 forms, and that the apparent transition from sub- to super-criticality between HI and H_2 is primarily an illusion created by chemical and excitation biases affecting the Zeeman measurements.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Piyush Sharda
Anish M. Amarsi
Kathryn Grasha
Mark R. Krumholz
David Yong
Gen Chiaki
Arpita Roy
Thomas Nordlander
摘要： Star formation models predict that the metal-poor initial mass function (IMF) can be substantially different from that observed in the metal-rich Milky Way. This changeover occurs because metal-poor gas clouds cool inefficiently due to their lower abundance of metals and dust. However, predictions for the metal-poor IMF to date rely on assuming Solar-scaled abundances, that is, [X/O] = 0 at all [O/H]. There is now growing evidence that elements such as C and O that dominate metal line cooling in the ISM do not follow Solar scaling at low metallicities. In this work, we extend models that predict the variation in the characteristic (or, the peak) IMF mass as a function of metallicity using [C/O] ratios derived from observations of metal-poor Galactic stars and of H II regions in dwarf galaxies. These data show [C/O] < 0 at sub-Solar [O/H], which leads to a substantially different metal-poor IMF in the metallicity range where C I and C II cooling dominate ISM thermodynamics, resulting in an increase in the characteristic mass by a factor as large as 7. An important consequence of this difference is a shift in the location of the transition from a top- to a bottom-heavy IMF upwards by 0.5 $-$ 1 dex in metallicity. Our findings indicate that the IMF is very sensitive to the assumptions around Solar-scaled ISM compositions in metal-poor systems (e.g., dwarf galaxies, the Galactic halo and metal-poor stars) that are a key focus of JWST.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Zefeng Li
Mark R. Krumholz
Emily Wisnioski
J. Trevor Mendel
Lisa J. Kewley
Sebastian F. Sánchez
Lluís Galbany
摘要： In this paper we analyse the statistics of the 2D gas-phase oxygen abundance distributions of 100 nearby galaxies drawn from the Calar Alto Legacy Integral Field spectroscopy Area survey. After removing the large-scale radial metallicity gradient, we compute the two-point correlation functions of the resulting metallicity fluctuation maps. We detect correlations in the majority of our targets, which we show are significantly in excess of what is expected due to beam-smearing, and are robust against the choice of metallicity diagnostic. We show that the correlation functions are generally well-fit by the predictions of a simple model for stochastic metal injection coupled with diffusion, and from the model we show that, after accounting for the effects of both beam smearing and noise, the galaxies in our sample have characteristic correlation lengths of $\sim1$ kpc. Correlation lengths increase with both stellar mass and star formation rate, but show no significant variation with Hubble type, barredness, or merging state. We also find no evidence for a theoretically-predicted relationship between gas velocity dispersion and correlation length, though this may be due to the small dynamic range in gas velocity dispersion across our sample. Our results suggest that measurements of 2D metallicity correlation functions can be a powerful tool for studying galaxy evolution.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： On average molecular clouds convert only a small fraction epsilon_ff of their mass into stars per free-fall time, but differing star formation theories make contrasting claims for how this low mean efficiency is achieved. To test these theories, we need precise measurements of both the mean value and the scatter of epsilon_ff, but high-precision measurements have been difficult because they require determining cloud volume densities, from which we can calculate free-fall times. Until recently, most density estimates assume clouds as uniform spheres, while their real structures are often filamentary and highly non-uniform, yielding systematic errors in epsilon_ff estimates and smearing real cloud-to-cloud variations. We recently developed a theoretical model to reduce this error by using column density distributions in clouds to produce more accurate volume density estimates. In this letter, we apply this model to recent observations of 12 nearby molecular clouds. Compared to earlier analyses, our method reduces the typical dispersion of epsilon_ff within individual clouds from 0.35 dex to 0.31 dex, and decreases the median value of epsilon_ff over all clouds from ~ 0.02 to ~ 0.01. However, we find no significant change in the ~ 0.2 dex cloud-to-cloud dispersion of epsilon_ff, suggesting the measured dispersions reflect real structural differences between clouds.
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