Several open questions on galaxy formation and evolution have their roots in the lack of a universal star formation law that could univocally link the gas properties, such as its density, to the star ...formation rate (SFR) density. In a recent paper we used a sample of nearby disc galaxies to infer the volumetric star formation (VSF) law, a tight correlation between the gas and the SFR volume densities derived under the assumption of hydrostatic equilibrium for the gas disc. However, due to the dearth of information about the vertical distribution of the SFR in these galaxies, we could not find a unique slope for the VSF law, but two alternative values. In this paper, we use the scale height of the SFR density distribution in our Galaxy adopting classical Cepheids (age ≲200 Myr) as tracers of star formation. We show that this latter is fully compatible with the flaring scale height expected from gas in hydrostatic equilibrium. These scale heights allowed us to convert the observed surface densities of gas and SFR into the corresponding volume densities. Our results indicate that the VSF law ρSFR ∝ ραgas ρ SFR ∝ ρ gas α $ \rho_\mathrm{SFR} \propto \rho_\mathrm{gas}^\alpha $ with α ≈ 2 is valid in the Milky Way as well as in nearby disc galaxies.
ABSTRACT
Using Large Binocular Telescope deep imaging data from the Smallest Scale of Hierarchy Survey (SSH) and archival Hubble Space Telescope data, we reveal the presence of two elongated stellar ...features contiguous to a bar-like stellar structure in the inner regions of the dwarf irregular galaxy NGC 3741. These structures are dominated by stars younger than a few hundred Myr and collectively are about twice as extended as the old stellar component. These properties are very unusual for dwarf galaxies in the nearby Universe and difficult to explain by hydro-dynamical simulations. From the analysis of archival 21-cm observations, we find that the young stellar ‘bar’ coincides with an H i high-density region proposed by previous studies to be a purely gaseous bar; we furthermore confirm radial motions of a few km s−1, compatible with an inflow/outflow, and derive a steeply rising rotation curve and high H i surface density at the centre, indicating a very concentrated mass distribution. We propose that the peculiar properties of the stellar and gaseous components of NGC 3741 may be explained by a recent merger or ongoing gas accretion from the intergalactic medium, which caused gas inflows towards the galaxy centre and triggered star formation a few hundred Myr ago. This event may explain the young and extended stellar features, the bar-like structure, the very extended H i disc and the central H i spiral arms. The high central H i density and the steeply rising rotation curve suggest that NGC 3741 may be the progenitor or the descendant of a starburst dwarf.
Local gravitational instability (LGI) is considered crucial for regulating star formation and gas turbulence in galaxy discs, especially at high redshift. Instability criteria usually assume ...infinitesimally thin discs or rely on approximations to include the stabilising effect of the gas disc thickness. We test a new 3D instability criterion for rotating gas discs that are vertically stratified in an external potential. This criterion reads Q 3D < 1, where Q 3D is the 3D analogue of the Toomre parameter Q . The advantage of Q 3D is that it allows us to study LGI in and above the galaxy midplane in a rigorous and self-consistent way. We apply the criterion to a sample of 44 star-forming galaxies at 0 ≲ z ≲ 5 hosting rotating discs of cold gas. The sample is representative of galaxies on the main sequence at z ≈ 0 and includes massive star-forming and starburst galaxies at 1 ≲ z ≲ 5. For each galaxy, we first apply the Toomre criterion for infinitesimally thin discs, finding ten unstable systems. We then obtain maps of Q 3D from a 3D model of the gas disc derived in the combined potential of dark matter, stars and the gas itself. According to the 3D criterion, two galaxies with Q < 1 show no evidence of instability and the unstable regions that are 20% smaller than those where Q < 1. No unstable disc is found at 0 ≲ z ≲ 1, while ≈60% of the systems at 2 ≲ z ≲ 5 are locally unstable. In these latter, a relatively small fraction of the total gas (≈30%) is potentially affected by the instability. Our results disfavour LGI as the main regulator of star formation and turbulence in moderately star-forming galaxies in the present-day Universe. LGI likely becomes important at high redshift, but the input by other mechanisms seems required in a significant portion of the disc. We also estimate the expected mass of clumps in the unstable regions, offering testable predictions for observations.
Aims. In this work, we aim to characterize the stellar populations of star-forming regions detached from the stellar disks of galaxies undergoing ram-pressure stripping. Methods. Using images ...collected with the WFC3 camera on board of the Hubble Space Telescope, we detected stellar clumps in continuum-subtracted H α and the ultraviolet ( F 275 W filter); such clumps are often embedded in larger regions (star-forming complexes) detected in the optical ( F 606 W filter). Our sample includes 347 H α clumps, 851 F 275 W clumps, and 296 star-forming complexes. We modeled the photometry of these objects in five bands using BAGPIPES to obtain their stellar population parameters. Results. The median mass-weighted stellar ages are 27 Myr for H α clumps and 39 Myr for F 275 W clumps and star-forming complexes, but the oldest stars in the complexes can be older than ∼300 Myr which indicates that star formation is sustained for long periods of time. Stellar masses vary from 10 3.5 to 10 7.1 M ⊙ , with star-forming complexes being more massive objects in the sample. Clumps and complexes found further away from the host galaxy are on average younger, less massive, and less obscured by dust. We interpret these trends as due to the effect of ram pressure in different phases of the interstellar medium. H α clumps form a well-defined sequence in the stellar mass–SFR plane with a slope of 0.73. Some F 275 W clumps and star-forming complexes follow the same sequence while others stray away from it and passively age. The difference in mean stellar age between a complex and its youngest embedded clump scales with the distance between the clump and the center of the optical emission of the complex, with the most displaced clumps being hosted by the most elongated complexes. This is consistent with a fireball-like morphology, where star formation proceeds in a small portion of the complex while older stars are left behind producing a linear stellar population gradient. The stellar masses of star-forming complexes are consistent with the ones of globular clusters, but their stellar mass surface densities are lower by 2 dex, and their properties are more consistent with the population of dwarf galaxies in clusters.
Abstract
We present the results of a Very Large Array H
i
imaging survey aimed at understanding why some galaxies develop long extraplanar H
α
tails, becoming extreme jellyfish galaxies. The ...observations are centered on five extreme jellyfish galaxies optically selected from the WINGS and OMEGAWINGS surveys and confirmed to have long H
α
tails through MUSE observations. Each galaxy is located in a different cluster. In the observations, there are in total 88 other spiral galaxies within the field of view (40′ × 40′) and observed bandwidth (6500 km s
−1
). We detect 13 of these 88 spirals, plus one uncataloged spiral, with H
i
masses ranging from 1 to 7 × 10
9
M
⊙
. Many of these detections have extended H
i
disks, two show direct evidence for ram pressure stripping, and others are possibly affected by tidal forces and/or ram pressure stripping. We stack the 75 nondetected spiral galaxies and find an average H
i
mass of 1.9 × 10
8
M
⊙
, which, given their average stellar mass, implies that they are very H
i
deficient. Comparing the extreme jellyfish galaxies to the other disk galaxies, we find that they are at smaller projected distance from the cluster center, and have a higher stellar mass and higher relative velocity than all other H
i
detections and most nondetections. We conclude that the high stellar mass allows extreme jellyfish galaxies to fall deeply into the cluster before being stripped, and the surrounding ICM pressure gives rise to their spectacular star-forming tails.
Star formation laws are empirical relations between the cold gas (HI+H2) content of a galaxy and its star formation rate (SFR), being crucial for any model of galaxy formation and evolution. A well ...known example of such laws is the Schmidt-Kennicutt law, which is based on the projected surface densities. However, it has been long unclear whether a more fundamental relation exists between the intrinsic volume densities. By assuming the vertical hydrostatic equilibrium, we infer radial profiles for the thickness of gaseous discs in a sample of 23 local galaxies, and use these measurements to convert the observed surface densities of the gas and the SFR into the de-projected volume densities. We find a tight correlation linking these quantities, that we call the volumetric star formation law. This relation and its properties have crucial implications for our understanding of the physics of star formation.
HI and CO observations indicate that the cold gas in galaxies is very turbulent. However, the turbulent energy is expected to be quickly dissipated, implying that some energy source is needed to ...explain the observations. The nature of such turbulence was long unclear, as even the main candidate, supernova (SN) feedback, seemed insufficient. Other mechanisms have been proposed, but without reaching a general consensus. The key novelty of our work is considering that the gas disc thickness and flaring increase the dissipation timescale of turbulence, thus reducing the energy injection rate required to sustain it. In excellent agreement with the theoretical expectations, we found that the fraction of the SN energy (a.k.a. SN coupling efficiency) needed to maintain the cold gas turbulence is ∼ 1%, solving a long-standing conundrum.
The gas in the interstellar medium (ISM) of galaxies is supersonically turbulent. Measurements of turbulence typically rely on cold gas emission lines for low-$z$ galaxies and warm ionized gas ...observations for $z > 0$ galaxies. Studies of warm gas kinematics at $z > 0$ conclude that the turbulence strongly evolves as a function of redshift, due to the increasing impact of gas accretion and mergers in the early Universe. However, recent findings suggest potential biases in turbulence measurements derived from ionized gas at high-$z$, impacting our understanding of turbulence origin, ISM physics and disk formation. We investigate the evolution of turbulence using velocity dispersion (sigma ) measurements from cold gas tracers (i.e., CO CI CII ). The initial dataset comprises 17 galaxy disks with high data quality from the ALPAKA sample, supplemented with galaxies from the literature, resulting in a sample of 57 galaxy disks spanning the redshift range $z = 0 - 5$. This extended sample consists of main-sequence and starburst galaxies with stellar masses $ M_ odot $. The comparison with current Halpha kinematic observations and existing models demonstrates that the velocity dispersion inferred from cold gas tracers differ by a factor of $ 3$ from those obtained using emission lines tracing the warm, ionized gas. We show that stellar feedback is the main driver of turbulence measured from cold gas tracers and the physics of turbulence driving does not appear to evolve with time. This is fundamentally different from the conclusions of studies based on warm gas, which had to consider additional turbulence drivers to explain the high values of sigma . We present a model predicting the redshift evolution of turbulence in galaxy disks, attributing the increase of sigma with redshift to the higher energy injected by supernovae due to the elevated star-formation rate in high-$z$ galaxies. This supernova-driven model suggests that turbulence is lower in galaxies with lower stellar mass compared to those with higher stellar mass. Additionally, it forecasts the evolution of sigma in Milky-Way like progenitors.
Guest editorial Brannigan, Michael; Vicentini, Alessandra; Grego, Kim S. ...
Working with older people (Brighton, England),
10/2021, Letnik:
25, Številka:
3
Journal Article
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