ABSTRACT
We study the gas kinematics of a sample of six isolated gas-rich low surface brightness galaxies, of the class called ultra-diffuse galaxies (UDGs). These galaxies have recently been shown ...to be outliers from the baryonic Tully–Fisher relation (BTFR), as they rotate much slower than expected given their baryonic mass, and to have a baryon fraction similar to the cosmological mean. By means of a 3D kinematic modelling fitting technique, we show that the H i in our UDGs is distributed in ‘thin’ regularly rotating discs and we determine their rotation velocity and gas velocity dispersion. We revisit the BTFR adding galaxies from other studies. We find a previously unknown trend between the deviation from the BTFR and the exponential disc scale length valid for dwarf galaxies with circular speeds ≲ 45 km s−1, with our UDGs being at the extreme end. Based on our findings, we suggest that the high baryon fractions of our UDGs may originate due to the fact that they have experienced weak stellar feedback, likely due to their low star formation rate surface densities, and as a result they did not eject significant amounts of gas out of their discs. At the same time, we find indications that our UDGs may have higher-than-average stellar specific angular momentum, which can explain their large optical scale lengths.
Abstract
Galaxies inhabit a wide range of environments and therefore are affected by different physical mechanisms. Spatially resolved maps combined with the knowledge of the hosting environment are ...very powerful for classifying galaxies by physical process. In the context of the GAs Stripping Phenomena in galaxies (GASP), we present a study of 27 non-cluster galaxies: 24 of them were selected for showing asymmetries and disturbances in the optical morphology, suggestive of gas stripping; 3 of them are passive galaxies and were included to characterize the final stages of galaxy evolution. We therefore provide a panorama of the different processes taking place in low-density environments. The analysis of VLT/MUSE data allows us to separate galaxies into the following categories: galaxy–galaxy interactions (2 galaxies), mergers (6), ram pressure stripping (4), cosmic web stripping (2), cosmic web enhancement (5), gas accretion (3), and starvation (3). In one galaxy we identify the combination of merger and ram pressure stripping. Only 6/27 of these galaxies have just a tentative classification. We then investigate where these galaxies are located on scaling relations determined for a sample of undisturbed galaxies. Our analysis shows the successes and limitations of a visual optical selection in identifying the processes that deplete galaxies of their gas content and probes the power of IFU data in pinning down the acting mechanism.
Abstract
Cluster galaxies are subject to the ram pressure exerted by the intracluster medium, which can perturb or even strip away their gas while leaving the stars undisturbed. We model the ...distribution and kinematics of the stars and the molecular gas in four late-type cluster galaxies (JO201, JO204, JO206, and JW100), which show tails of atomic and ionized gas indicative of ongoing ram pressure stripping. We analyze MUSE@VLT data and CO data from the Atacama Large Millimeter Array searching for signatures of radial gas flows, ram pressure stripping, and other perturbations. We find that all galaxies, with the possible exception of JW100, host stellar bars. Signatures of ram pressure are found in JO201 and JO206, which also shows clear indications of ongoing stripping in the molecular disk outskirts. The stripping affects the whole molecular gas disk of JW100. The molecular gas kinematics in JO204 is instead dominated by rotation rather than ram pressure. We also find indications of enhanced turbulence of the molecular gas compared to field galaxies. Large-scale radial flows of molecular gas are present in JO204 and JW100, but more uncertain in JO201 and JO206. We show that our sample follows the molecular gas mass–size relation, confirming that it is essentially independent of environment even for the most extreme cases of stripping. Our findings are consistent with the molecular gas being affected by the ram pressure on different timescales and less severely than the atomic and ionized gas phases, likely because the molecular gas is denser and more gravitationally bound to the galaxy.
Abstract
Cluster galaxies are affected by the surrounding environment, which influences, in particular, their gas, stellar content, and morphology. In particular, the ram pressure exerted by the ...intracluster medium promotes the formation of multiphase tails of stripped gas detectable both at optical wavelengths and in the submillimeter and radio regimes, tracing the cold molecular and atomic gas components, respectively. In this work we analyze a sample of 16 galaxies belonging to clusters at redshift ∼0.05 showing evidence of an asymmetric H
i
morphology (based on MeerKAT observations) with and without a star-forming tail. To this sample we add three galaxies with evidence of a star-forming tail and no H
i
detection. Here we present the galaxies’ H
2
gas content from APEX observations of the CO (2–1) emission. We find that in most galaxies with a star-forming tail the H
2
global content is enhanced with respect to undisturbed field galaxies with similar stellar masses, suggesting an evolutionary path driven by the ram pressure stripping. As galaxies enter into the clusters, their H
i
is displaced but also partially converted into H
2
, so that they are H
2
enriched when they pass close to the pericenter, that is, when they also develop the star-forming tails that are well visible in UV or
B
broad bands and in H
α
emission. An inspection of the phase-space diagram for our sample suggests an anticorrelation between the H
i
and H
2
gas phases as galaxies fall into the cluster potential. This peculiar behavior is a key signature of the ram pressure stripping in action.
Abstract
We characterize the morphological properties of a statistically relevant sample of H
α
and UV young star-forming clumps and optical complexes, observed with the Hubble Space Telescope in six ...galaxies of the GASP sample undergoing ram pressure stripping. The catalogs comprise 2406 (323 in the tails) H
α
clumps, 3750 (899) UV clumps, and 424 tail optical complexes. About 15%–20% of the clumps and 50% of the complexes are resolved in size. We find that more than half of the complexes contain no H
α
clumps, while most of them contain at least one UV clump. The clump number and size increase with the complex size, while the median complex filling factor is larger for UV clumps (0.27) than that for H
α
clumps (0.10) and does not correlate with almost any morphological property. This suggests that the clumps' number and size grow with the complex keeping the filling factor constant. When studying the position of the clumps inside their complexes, H
α
clumps, and UV clumps to a lesser extent, show a displacement from the complex center of 0.1–1 kpc, and in ∼60% of the cases, they are displaced away from the galactic disk. This is in accordance with the fireball configuration, already observed in the tails of stripped galaxies. Finally, the filling factor and the clump radius increase with the distance from the galactic disk, suggesting that the reciprocal displacement of the different stellar generations increases as a consequence of the velocity gradient caused by ram pressure.
Star formation (SF) laws are fundamental relations between the gas content of a galaxy and its star formation rate (SFR) and play key roles in galaxy evolution models. In this paper, we present new ...empirical SF laws of disc galaxies based on volume densities. Following the assumption of hydrostatic equilibrium, we calculated the radial growth of the thickness of the gaseous discs in the combined gravitational potential of dark matter, stars, and gas for 12 nearby star-forming galaxies. This allowed us to convert the observed surface densities of gas and SFR into the deprojected volume densities. We found a tight correlation with slope in the range 1.3–1.9 between the volume densities of gas (HI+H2) and the SFR with a significantly smaller scatter than the surface-based (Kennicutt) law and no change in the slope over five orders of magnitude. This indicates that taking into account the radial increase of the thickness of galaxy discs is crucial to reconstruct their three-dimensional density profiles, in particular in their outskirts. Moreover, our result suggests that the break in the slope seen in the Kennicutt law is due to disc flaring rather than to a drop of the SF efficiency at low surface densities. Surprisingly, we discovered an unexpected correlation between the volume densities of HI and SFR, indicating that the atomic gas is a good tracer of the cold star-forming gas, especially in low density HI-dominated environments.
The local gravitational instability of rotating discs is believed to be an important mechanism in different astrophysical processes, including the formation of gas and stellar clumps in galaxies. We ...aim to study the local gravitational instability of two-component thick discs in three dimensions. We use as a starting point a recently proposed analytic three-dimensional (3D) instability criterion for discs with non-negligible thickness that takes the form $ where Qthreed is a 3D version of the classical 2D Toomre $Q$ parameter for razor-thin discs. Here, we extend the 3D stability analysis to two-component discs, considering first the influence on Qthreed of a second unresponsive component, and then the case in which both components are responsive. We present the application to two-component discs with isothermal vertical distributions, which can represent for instance, galactic discs with both stellar and gaseous components. Finally, we relax the assumption of vertical isothermal distribution, by studying one-component self-gravitating discs with polytropic vertical distributions for a range of values of the polytropic index corresponding to convectively stable configurations We find that $ where Qthreed can be computed from observationally inferred quantities, is a robust indicator of local gravitational instability, depending only weakly on the presence of a second component and on the vertical gradient of temperature or velocity dispersion. We derive a sufficient condition for local gravitational instability in the midplane of two-component discs, which can be employed when both components have $ Qthreed>1$.
In the last decades, much effort has been put into finding the star formation law, which could unequivocally link the gas and the star formation rate (SFR) densities measured on a sub-kiloparsec ...scale in star-forming galaxies. The conventional approach of using the observed surface densities to infer star formation laws has however revealed a major and well-known issue, as such relations are valid for the high-density regions of galaxies but break down in low-density and HI-dominated environments. Recently, an empirical correlation between the total gas (HI+H
2
) and the SFR volume densities was obtained for a sample of nearby disc galaxies and for the Milky Way. This volumetric star formation (VSF) law is a single power-law with no break and a smaller intrinsic scatter with respect to the star formation laws based on the surface density. In this work, we explore the VSF law in the regime of dwarf galaxies in order to test its validity in HI-dominated, low-density, and low-metallicity environments. In addition, we assess this relation in the outskirts of spiral galaxies, which are low-density and HI-dominated regions similar to dwarf galaxies. Remarkably, we find that the VSF law, namely
ρ
SFR
∝
ρ
gas
α
with
α
≈ 2, is valid for both these regimes. This result indicates that the VSF law, which holds unbroken for a wide range of gas (≈3 dex) and SFR (≈6 dex) volume densities, is the empirical relation with the smallest intrinsic scatter and is likely more fundamental than surface-based star formation laws.
It is widely known that the gas in galaxy discs is highly turbulent, but there is much debate on which mechanism can energetically maintain this turbulence. Among the possible candidates, supernova ...(SN) explosions are likely the primary drivers but doubts remain on whether they can be sufficient in regions of moderate star formation activity, in particular in the outer parts of discs. Thus, a number of alternative mechanisms have been proposed. In this paper, we measure the SN efficiency
η
, namely the fraction of the total SN energy needed to sustain turbulence in galaxies, and verify that SNe can indeed be the sole driving mechanism. The key novelty of our approach is that we take into account the increased turbulence dissipation timescale associated with the flaring in outer regions of gaseous discs. We analyse the distribution and kinematics of HI and CO in ten nearby star-forming galaxies to obtain the radial profiles of the kinetic energy per unit area for both the atomic gas and the molecular gas. We use a theoretical model to reproduce the observed energy with the sum of turbulent energy from SNe, as inferred from the observed star formation rate (SFR) surface density, and the gas thermal energy. For the atomic gas, we explore two extreme cases in which the atomic gas is made either of cold neutral medium or warm neutral medium, and the more realistic scenario with a mixture of the two phases. We find that the observed kinetic energy is remarkably well reproduced by our model across the whole extent of the galactic discs, assuming
η
constant with the galactocentric radius. Taking into account the uncertainties on the SFR surface density and on the atomic gas phase, we obtain that the median SN efficiencies for our sample of galaxies are ⟨
η
atom
⟩ = 0.015
−0.008
+0.018
for the atomic gas and ⟨
η
mol
⟩ = 0.003
−0.002
+0.006
for the molecular gas. We conclude that SNe alone can sustain gas turbulence in nearby galaxies with only few percent of their energy and that there is essentially no need for any further source of energy.
In the last decades, much effort has been put into finding the star formation law, which could unequivocally link the gas and the star formation rate (SFR) densities measured on a sub-kiloparsec ...scale in star-forming galaxies. The conventional approach of using the observed surface densities to infer star formation laws has however revealed a major and well-known issue, as such relations are valid for the high-density regions of galaxies but break down in low-density and HI-dominated environments. Recently, an empirical correlation between the total gas (HI+H2) and the SFR volume densities was obtained for a sample of nearby disc galaxies and for the Milky Way. This volumetric star formation (VSF) law is a single power-law with no break and a smaller intrinsic scatter with respect to the star formation laws based on the surface density. In this work, we explore the VSF law in the regime of dwarf galaxies in order to test its validity in HI-dominated, low-density, and low-metallicity environments. In addition, we assess this relation in the outskirts of spiral galaxies, which are low-density and HI-dominated regions similar to dwarf galaxies. Remarkably, we find that the VSF law, namely ρSFR ∝ ρgasα with α ≈ 2, is valid for both these regimes. This result indicates that the VSF law, which holds unbroken for a wide range of gas (≈3 dex) and SFR (≈6 dex) volume densities, is the empirical relation with the smallest intrinsic scatter and is likely more fundamental than surface-based star formation laws.
PDF
