IC 3418: STAR FORMATION IN A TURBULENT WAKE (original) (raw)
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The Astrophysical Journal, 2013
We report the detection of an X-ray absorption feature near the galaxy M86 in the Virgo cluster. The absorber has a column density of 2 − 3 × 10 20 cm −2 , and its position coincides with the peak of an intracluster H i cloud which was removed from the galaxy NGC 4388 presumably by ram pressure. These results indicate that the H i cloud is located in front of M86 along the line-of-sight, and suggest that the stripping was primarily created by an interaction between NGC 4388 and the hot plasmas of the Virgo cluster, not the M86 halo. By calculating an X-ray temperature map, we further detected an X-ray counterpart of the H i cloud up to ≈ 3 ′ south of M86. It has a temperature of 0.89 keV and a mass of ∼ 4.5×10 8 M ⊙ , exceeding the estimated H i gas mass. The high hot-to-cold gas ratio in the cloud indicates a significant evaporation of the H i gas, probably by thermal conduction from the hotter cluster plasma with a sub-Spitzer rate.
Spectacular tails of ionized gas in the Virgo cluster galaxy NGC 4569
Astronomy & Astrophysics
Using MegaCam at the CFHT, we obtained a deep narrow band Hα+[NII] wide-field image of NGC 4569 (M90), the brightest late-type galaxy in the Virgo cluster. The image reveals the presence of long tails of diffuse ionized gas, without any associated stellar component extending from the disc of the galaxy up to 80 kpc (projected distance) and with a typical surface brightness of a few 10 −18 erg s −1 cm −2 arcsec −2. These features provide direct evidence that NGC 4569 is undergoing a ram-presure stripping event. The image also shows a prominent 8 kpc spur of ionized gas that is associated with the nucleus that spectroscopic data identify as an outflow. With some assumptions on the 3D distribution of the gas, we use the Hα surface brightness of these extended low-surface brightness features to derive the density and the mass of the gas that has been stripped during the interaction of the galaxy with the intracluster medium. The comparison with ad hoc chemo-spectrophotometric models of galaxy evolution indicates that the mass of the Hα emitting gas in the tail is a large fraction of that of the cold phase that has been stripped from the disc, suggesting that the gas is ionized within the tail during the stripping process. The lack of star-forming regions suggests that mechanisms other than photoionization are responsible for the excitation of the gas (shocks, heat conduction, magneto hydrodynamic waves). This analysis indicates that ram pressure stripping is efficient in massive (M star 10 10.5 M) galaxies located in intermediate-mass (10 14 M) clusters under formation. It also shows that the mass of gas expelled by the nuclear outflow is only ∼1% than that removed during the ram pressure stripping event. Together these results indicate that ram pressure stripping, rather than starvation through nuclear feedback, can be the dominant mechanism that is responsible for the quenching of the star formation activity of galaxies in high density environments.
From giant clumps to clouds -- III. The connection between star formation and turbulence in the ISM
arXiv: Astrophysics of Galaxies, 2021
Supersonic gas turbulence is a ubiquitous property of the interstellar medium. The level of turbulence, quantified by the gas velocity dispersion (g), is observed to increase with the star formation rate (SFR) rate of a galaxy, but it is yet not established whether this trend is driven by stellar feedback or gravitational instabilities. In this work we carry out hydrodynamical simulations of entire disc galaxies, with different gas fractions, to understand the origins of the SFRg relation. We show that disc galaxies reach the same levels of turbulence regardless of the presence of stellar feedback processes, and argue that this is an outcome of the way disc galaxies regulate their gravitational stability. The simulations match the SFRg relation up to SFRs of the order of tens of M yr −1 and g ∼ 50 km s −1 in neutral hydrogen and molecular gas, but fail to reach the very large values (> 100 km s −1) reported in the literature for rapidly star forming galaxies. We demonstrate that such high values of g can be explained by 1) insufficient beam smearing corrections in observations, and 2) stellar feedback being coupled to the ionised gas phase traced by recombination lines. Given that the observed SFRg relation is composed of highly heterogeneous data, with g at high SFRs almost exclusively being derived from H observations of high redshift galaxies with complex morphologies, we caution against analytical models that attempt explain the SFRg relation without accounting for these effects.
Spectacular tails of ionised gas in the Virgo cluster galaxy NGC 4569
2016
We obtained using MegaCam at the CFHT a deep narrow band Halpha+[NII] wide field image of NGC 4569, the brightest late-type galaxy in the Virgo cluster. The image reveals the presence of long tails of diffuse ionised gas without any associated stellar component extending from the disc of the galaxy up to ~ 80 kpc (projected distance) with a typical surface brightness of a few 10^-18 erg s-1 cm-2 arcsec-2. These features provide direct evidence that NGC 4569 is undergoing a ram presure stripping event. The image also shows a prominent 8 kpc spur of ionised gas associated to the nucleus that spectroscopic data identify as an outflow. With some assumptions on the 3D distribution of the gas, we use the Halpha surface brightness of these extended low surface brightness features to derive the density and the mass of the gas stripped during the interaction of the galaxy with the ICM. The comparison with ad-hoc chemo-spectrophotometric models of galaxy evolution indicates that the mass of t...
2014
We present an analytical model of the relation between the surface density of gas and star formation rate in galaxies and clouds, as a function of the presence of supersonic turbulence and the associated structure of the interstellar medium. The model predicts a power-law relation of index 3/2, flattened under the effects of stellar feedback at high densities or in very turbulent media, and a break at low surface densities when ISM turbulence becomes too weak to induce strong compression. This model explains the diversity of star formation laws and thresholds observed in nearby spirals and their resolved regions, the Small Magellanic Cloud, high-redshift disks and starbursting mergers, as well as Galactic molecular clouds. While other models have proposed interstellar dust content and molecule formation to be key ingredients to the observed variations of the star formation efficiency, we demonstrate instead that these variations can be explained by interstellar medium turbulence and structure in various types of galaxies.
Giant Molecular Clouds and Star Formation in the Tidal Molecular Arm of NGC 4039
The Astrophysical Journal, 2012
The properties of tidally induced arms provide a means to study molecular cloud formation and the subsequent star formation under environmental conditions which in principle are different from quasi stationary spiral arms. We report the properties of a newly discovered molecular gas arm of likely tidal origin at the south of NGC 4039 and the overlap region in the Antennae galaxies, with a resolution of 1. 68 × 0. 85, using the Atacama Large Millimeter/submillimeter Array science verification CO(2-1) data. The arm extends 3.4 kpc (34 ) and is characterized by widths of 200 pc (2 ) and velocity widths of typically ∆V 10-20 km s −1 . About 10 clumps are strung out along this structure, most of them unresolved, with average surface densities of Σ gas 10-100 M pc −2 , and masses of (1-8)×10 6 M . These structures resemble the morphology of beads on a string, with an almost equidistant separation between the beads of about 350 pc, which may represent a characteristic separation scale for giant molecular associations. We find that the star formation efficiency at a resolution of 6 (600 pc) is in general a factor of 10 higher than in disk galaxies and other tidal arms and bridges. This arm is linked, based on the distribution and kinematics, to the base of the western spiral arm of NGC 4039, but its morphology is different to that predicted by high-resolution simulations of the Antennae galaxies.
Monthly Notices of the Royal Astronomical Society, 2010
We have used new deep observations of the Coma cluster from Galaxy Evolution Explorer to visually identify 13 star-forming galaxies with asymmetric morphologies in the ultraviolet. Aided by wide-field optical broad-band and Hα imaging, we interpret the asymmetric features as being due to star formation within gas stripped from the galaxies by interaction with the cluster environment. The selected objects display a range of structures from broad fan-shaped systems of filaments and knots ("jellyfish") to narrower and smoother tails extending up to 100 kpc in length. Some of the features have been discussed previously in the literature, while others are newly identified here. We assess the ensemble properties of the sample. The candidate stripping events are located closer to the cluster centre than other star-forming galaxies; their radial distribution is more similar to that of all cluster members, dominated by passive galaxies. The fraction of blue galaxies which are undergoing stripping falls from 40 per cent in the central 500 kpc, to less than 5 per cent beyond 1 Mpc. We find that tails pointing away from (i.e. galaxies moving towards) the cluster centre are strongly favoured (11/13 cases). From the small number of "outgoing" galaxies with stripping signatures, we conclude that the stripping events occur primarily on first passage towards the cluster centre, and are short-lived compared to the cluster crossing time. Using galaxy infall trajectories extracted from a cosmological simulation, we find that the observed fraction of blue galaxies undergoing stripping can be reproduced if the events are triggered at a threshold radius of ∼1 Mpc and detectable for ∼500 Myr. Hubble Space Telescope images are available for two galaxies from our sample and reveal compact blue knots coincident with UV and Hα emission, apparently forming stars within the stripped material. Our results confirm that stripping of gas from infalling galaxies, and associated star formation in the stripped material, is a widespread phenomenon in rich clusters. Deep UV imaging of additional clusters is a promising route to constructing a statistically powerful sample of stripping events and constraining models for the truncation of star formation in clusters.
Exceptional AGN-driven turbulence inhibits star formation in the 3C 326N radio galaxy
Astronomy & Astrophysics, 2015
We detect bright [Cii]λ158 µm line emission from the radio galaxy 3C 326N at z = 0.09, which shows no sign of on-going or recent star formation (SFR < 0.07 M ⊙ yr −1 ) despite having strong H 2 line emission and a substantial amount of molecular gas (2 × 10 9 M ⊙ , inferred from the modeling of the far infrared (FIR) dust emission and the CO(1-0) line emission). The [Cii] line is twice as strong as the 0-0S(1) 17µm H 2 line, and both lines are much in excess what is expected from UV heating. We combine infrared Spitzer and Herschel photometry and line spectroscopy with gas and dust modeling to infer the physical conditions in the [Cii]-emitting gas. The [Cii] line, like rotational H 2 emission, traces a significant fraction (30 to 50%) of the total molecular gas mass. This gas is warm (70 < T < 100 K) and at moderate densities 700 < n H < 3000 cm −3 , constrained by both the observed [Cii]-to-[Oi] and [Cii]-to-FIR ratios. The [Cii] line is broad, asymmetric, with a red-shifted core component (FWHM = 390 km s −1 ) and a very broad blue-shifted wing (FWHM = 810 km s −1 ). The line profile of [Cii] is similar to the profiles of the near-infrared H 2 lines and the Na D optical absorption lines, and is likely to be shaped by a combination of rotation, outflowing gas, and turbulence. If the line wing is interpreted as an outflow, the mass loss rate would be larger than 20 M ⊙ yr −1 , and the depletion timescale of the order of the orbital timescale (≈ 3 × 10 7 yr). If true, we are observing this object at a very specific and brief time in its evolution, assuming that the disk is not replenished. Although there are evidence for an outflow in this source, we caution that the outflow rates may be over-estimated because the stochastic injection of turbulent energy on galactic scales can create short-lived, large velocity increments that contribute to the skewness of the line profile and mimic outflowing gas. The gas physical conditions raise the issue of the heating mechanism of the warm gas, and we show that the dissipation of turbulent energy is the main heating process. Cosmic rays can also contribute to the heating, but cannot be the dominant heating source because it requires an average gas density larger than the observational constraints. After subtraction of the contribution of the disk rotation, we estimate the turbulent velocity dispersion of the molecular gas to be 120 < σ turb < 330 km s −1 , which corresponds to a turbulent heating rate higher than the gas cooling rate computed from the line emission. The dissipation timescale of the turbulent energy (2 × 10 7 − 10 8 yrs) is comparable or larger than the jet lifetime or the dynamical timescale of the outflow, which means that turbulence can be sustained during the quiescent phases when the radio jet is shut off. The strong turbulent support maintains a very high gas scale height (0.3 to 4 kpc) in the disk. The cascade of turbulent energy can inhibit the formation of gravitationally-bound structures at all scales, which offers a natural explanation for the lack of on-going star formation in 3C 326N, despite it having sufficient molecular gas to form stars at a rate of a few solar mass per year. To conclude, the bright [Cii] line indicates that strong AGN jet-driven turbulence may play a key role in enhancing the amount of molecular gas (positive feedback) but yet can prevent star formation on galactic scales (negative feedback).
Ionized gas kinematics and massive star formation in NGC�1530
Astronomy and Astrophysics, 2004
We present emission line mapping of the strongly barred galaxy NGC 1530 obtained using Fabry-Pérot interferometry in Hα, at significantly enhanced angular resolution compared with previously published studies. The main point of the work is to examine in detail the non-circular components of the velocity field of the gas, presumably induced by the strongly non-axisymmetric gravitational potential of the bar. To do this we first derive a model rotation curve making minimum assumptions about kinematic symmetry, and go on to measure the non-circular component of the full radial velocity field. This clearly reveals the streaming motions associated with the spiral density wave producing the arms, and the quasi-elliptical motions with speeds of order 100 km s −1 aligned with the bar. It also shows in some detail how these flows swing in towards and around the nucleus as they cross a circumnuclear resonance, from the dominant "x1 orbits" outside the resonance to "x2 orbits" within it. Comparing cross-sections of this residual velocity map along and across the bar with the surface brightness map in Hα indicates a systematic offset between regions of high non-circular velocity and massive star formation. To investigate further we produce maps of velocity gradient along and across the bar. These illustrate very nicely the shear compression of the gas, revealed by the location of the dust lanes along loci of maximum velocity gradient perpendicular to the bar. They also show clearly how shear, seen in our data as velocity gradient perpendicular to the flow, acts to inhibit massive star formation, whereas shocks, seen as strong velocity gradients along the flow vector, act to enhance it. Although the inhibiting effect of gas shear flow on star formation has long been predicted, this is the clearest observational illustration so far of the effect, thanks to the strong shock-induced counterflow system in the bar. It is also the clearest evidence that dust picks out shock-induced inflow along bars. These observations should be of considerable interest to those modelling massive star formation in general.
The Astrophysical Journal, 2005
We derive an analytic prediction for the star formation rate in environments ranging from normal galactic disks to starbursts and ULIRGs in terms of the observables of those systems. Our calculation is based on three premises: (1) star formation occurs in virialized molecular clouds that are supersonically turbulent; (2) the density distribution within these clouds is lognormal, as expected for supersonic isothermal turbulence; (3) stars form in any sub-region of a cloud that is so overdense that its gravitational potential energy exceeds the energy in turbulent motions. We show that a theory based on this model is consistent with simulations and with the observed star formation rate in the Milky Way. We use our theory to derive the Kennicutt-Schmidt Law from first principles, and make other predictions that can be tested by future observations. We also provide an algorithm for estimating the star formation rate that is suitable for inclusion in numerical simulations.