We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line spectroscopy and integral-field unit observations of the intergalactic medium (IGM) in the Stephans Quintet (SQ) galaxy group. SQ hosts a 30 kpc long shocked ridge triggered b
y a galaxy collision at a relative velocity of 1000 km/s, where large amounts of cold (10-100 K) and warm (100-5000 K) molecular gas coexist with a hot plasma. COS spectroscopy along five lines-of-sight, probing 1 kpc-diameter regions in the IGM, reveals very broad (~2000 km/s) and powerful Ly$alpha$ line emission with complex line shapes. These Lyman-alpha line profiles are often similar to, or sometimes much broader than line profiles obtained in H$beta$, [CII], and CO (1-0) emission along the same lines-of-sight. In these cases, the breadth of the Ly$alpha$ emission, compared with H$beta$, implies resonance scattering. Line ratios of Ly$alpha$/H$beta$ for the two COS pointings closest to the center of the shocked ridge are close to the Case B recombination value, suggesting that at these positions Ly$alpha$ photons escape through scattering in a low density medium free of dust. Some Ly$alpha$ spectra show suppressed velocity components compared with [CII] and H$beta$, implying that some of the Ly$alpha$ photons are absorbed. Scattering indicates that the neutral gas of the IGM is clumpy, with multiple clumps along a given line of sight. Remarkably, over more than four orders of magnitude in temperature, the powers radiated by the multi-phase IGM in X-rays, Ly$alpha$, H$_2$, [CII] are comparable within a factor of a few. We suggest that both shocks and mixing layers co-exist and contribute to the energy dissipation associated with a turbulent energy cascade. This may be important for the cooling of gas at higher redshifts, where the metal content is lower than in this local system, and a high amplitude of turbulence more common.
The Sun is embedded in the so-called Local Bubble (LB) -- a cavity of hot plasma created by supernova explosions and surrounded by a shell of cold, dusty gas. Knowing the local distortion of the Galactic magnetic field associated with the LB is criti
cal for the modeling of interstellar polarization data at high Galactic latitudes. In this his paper, we relate the structure of the Galactic magnetic field on the LB scale to three-dimensional (3D) maps of the local interstellar medium (ISM). First, we extracted the geometry of the LB shell, its inner surface, in particular from 3D dust extinction maps of the local ISM. We expanded the shell inner surface in spherical harmonics, up to a variable maximum multipole degree, which enabled us to control the level of complexity for the modeled surface. Next, we applied an analytical model for the ordered magnetic field in the shell to the modeled shell surface. This magnetic field model was successfully fitted to the textit{Planck} 353~GHz dust polarized emission maps over the Galactic polar caps. For each polar cap, the direction of the mean magnetic field derived from dust polarization (together with the prior that the field points toward longitude $90^circ pm 90^circ$) is found to be consistent with the Faraday spectra of the nearby diffuse synchrotron emission. Our work presents a new approach to modeling the local structure of the Galactic magnetic field. We expect our methodology and our results to be useful both in modeling the local ISM as traced by its different components and in modeling the dust polarized emission, which is a long-awaited input for studies of the polarized foregrounds for cosmic microwave background.
The magnetic field in the local interstellar medium does not follow the large-scale Galactic magnetic field. The local magnetic field has probably been distorted by the Local Bubble, a cavity of hot ionized gas extending all around the Sun and surrou
nded by a shell of cold neutral gas and dust. However, so far no conclusive association between the local magnetic field and the Local Bubble has been established. Here we develop an analytical model for the magnetic field in the shell of the Local Bubble, which we represent as an inclined spheroid, off-centred from the Sun. We fit the model to Planck dust polarized emission observations within 30 deg of the Galactic poles. We find a solution that is consistent with a highly deformed magnetic field, with significantly different directions towards the north and south Galactic poles. This work sets a methodological framework for modelling the three-dimensional (3D) structure of the magnetic field in the local interstellar medium, which is a most awaited input for large-scale Galactic magnetic field models.
Within four nearby (d < 160 pc) molecular clouds, we statistically evaluate the structure of the interstellar magnetic field, projected on the plane of the sky and integrated along the line of sight, as inferred from the polarized thermal emission of
Galactic dust observed by Planck at 353 GHz and from the optical and NIR polarization of background starlight. We compare the dispersion of the field orientation directly in vicinities with an area equivalent to that subtended by the Planck effective beam at 353 GHz (10) and using the second-order structure functions of the field orientation angles. We find that the average dispersion of the starlight-inferred field orientations within 10-diameter vicinities is less than 20 deg, and that at these scales the mean field orientation is on average within 5 deg of that inferred from the submillimetre polarization observations in the considered regions. We also find that the dispersion of starlight polarization orientations and the polarization fractions within these vicinities are well reproduced by a Gaussian model of the turbulent structure of the magnetic field, in agreement with the findings reported by the Planck collaboration at scales greater than 10 and for comparable column densities. At scales greater than 10, we find differences of up to 14.7 deg between the second-order structure functions obtained from starlight and submillimetre polarization observations in the same positions in the plane of the sky, but comparison with a Gaussian model of the turbulent structure of the magnetic field indicates that these differences are small and are consistent with the difference in angular resolution between both techniques.
We detect bright [CII]158$mu$m line emission from the radio galaxy 3C 326N at z=0.09, which shows weak star formation ($SFR<0.07$M$_{odot}$~yr$^{-1}$) despite having strong H$_2$ line emission and $2times 10^9$M$_{odot}$ of molecular gas. The [CII] l
ine is twice as strong as the 0-0S(1) 17$mu$m H$_2$ line, and both lines are much in excess what is expected from UV heating. We combine infrared Spitzer and Herschel data with gas and dust modeling to infer the gas physical conditions. The [CII] line traces 30 to 50% of the molecular gas mass, which is warm (70<T<100K) and at moderate densities $700<n_{H}<3000$cm$^{-3}$. The [CII] line is broad with a blue-shifted wing, and likely to be shaped by a combination of rotation, outflowing gas, and turbulence. It matches the near-infrared H$_2$ and the Na D optical absorption lines. If the wing is interpreted as an outflow, the mass loss rate would be larger than 20M$_{odot}$/yr, and the depletion timescale shorter than the orbital timescale ($10^8$yr). These outflow rates may be over-estimated because the stochastic injection of turbulence on galactic scales can contribute to the skewness of the line profile and mimic outflowing gas. We argue that the dissipation of turbulence is the main heating process of this gas. Cosmic rays can also contribute to the heating but they require an average gas density larger than the observational constraints. We show that strong turbulent support maintains a high gas vertical scale height (0.3-4kpc) in the disk and can inhibit the formation of gravitationally-bound structures at all scales, offering a natural explanation for the weakness of star formation in 3C 326N. 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).
We present the first Herschel spectroscopic detections of the [OI]63 and [CII]158 micron fine-structure transitions, and a single para-H2O line from the 35 x 15 kpc^2 shocked intergalactic filament in Stephans Quintet. The filament is believed to hav
e been formed when a high-speed intruder to the group collided with clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (> 1000 km s^-1) luminous [CII] line profiles, as well as fainter [OI]63micron emission. SPIRE FTS observations reveal water emission from the p-H2O (111-000) transition at several positions in the filament, but no other molecular lines. The H2O line is narrow, and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [CII]/PAH{tot) and [CII]/FIR ratios are too large to be explained by normal photo-electric heating in PDRs. HII region excitation or X-ray/Cosmic Ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [CII], [OI] and warm H2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the IGM is dissipated to small scales and low-velocities, via shocks and turbulent eddies. Low-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H2 ratios observed. The discovery that [CII] emission can be enhanced, in large-scale turbulent regions in collisional environments has implications for the interpretation of [CII] emission in high-z galaxies.
The Spitzer GLIMPSE and MIPSGAL surveys have revealed a wealth of details of the Galactic plane. We use them to study the energetics and dust properties of M16, one of the best known SFR. We present MIPSGAL observations of M16 at 24 and 70 $mu$m and
combine them with previous IR data. The MIR image shows a shell inside the molecular borders of the nebula. The morphologies at 24 and 70 $mu$m are different, and its color ratio is unusually warm. The FIR image resembles the one at 8 $mu$m that enhances the molecular cloud. We measure IR SEDs within the shell and the PDRs. We use the DUSTEM model to fit the SEDs and constrain dust temperature, dust size distribution, and ISRF intensity relative to that provided by the star cluster NGC6611. Within the PDRs, the dust temperature, the dust size distribution, and the ISRF intensity are in agreement with expectations. Within the shell, the dust is hotter and an ISRF larger than that provided by NGC6611 is required. We quantify two solutions. (1) The size distribution of the dust in the shell is not that of interstellar dust. (2) The dust emission arises from a hot plasma where UV and collisions with electrons contribute to the heating. We suggest two interpretations for the shell. (1) The shell matter is supplied by photo-evaporative flows arising from dense gas exposed to ionized radiation. The flows renew the shell matter as it is pushed by the stellar winds. Within this scenario, we conclude that massive SFR such as M16 have a major impact on the carbon dust size distribution. The grinding of the carbon dust could result from shattering in collisions within shocks driven by the interaction between the winds and the shell. (2) We consider a scenario where the shell is a SNR. We would be witnessing a specific time in the evolution of the SNR where the plasma pressure and temperature would be such that the SNR cools through dust emission.
We have studied the molecular hydrogen energetics of the edge-on spiral galaxy NGC,891, using a 34-position map in the lowest three pure rotational H$_2$ lines observed with the Spitzer Infrared Spectrograph. The S(0), S(1), and S(2) lines are bright
with an extinction corrected total luminosity of $sim2.8 times 10^{7}$ L$_{odot}$, or 0.09% of the total-infrared luminosity of NGC,891. The H$_2$ line ratios are nearly constant along the plane of the galaxy -- we do not observe the previously reported strong drop-off in the S(1)/S(0) line intensity ratio in the outer regions of the galaxy, so we find no evidence for the very massive cold CO-free molecular clouds invoked to explain the past observations. The H$_2$ level excitation temperatures increase monotonically indicating more than one component to the emitting gas. More than 99% of the mass is in the lowest excitation (T$_{ex}$ $sim$125 K) ``warm component. In the inner galaxy, the warm H$_2$ emitting gas is $sim$15% of the CO(1-0)-traced cool molecular gas, while in the outer regions the fraction is twice as high. This large mass of warm gas is heated by a combination of the far-UV photons from stars in photo-dissociation regions (PDRs) and the dissipation of turbulent kinetic energy. Including the observed far-infrared [OI] and [CII] fine-structure line emission and far-infrared continuum emission in a self-consistent manner to constrain the PDR models, we find essentially all of the S(0) and most (70%) of the S(1) line arises from low excitation PDRs, while most (80%) of the S(2) and the remainder of the S(1) line emission arises from low velocity microturbulent dissipation.
We present results from the mid-infrared spectral mapping of Stephans Quintet using the Spitzer Space Telescope. A 1000 km/s collision has produced a group-wide shock and for the first time the large-scale distribution of warm molecular hydrogen emis
sion is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 $times10^{8}$ M$_{odot}$ of warm H$_2$ spread over $sim$ 480 kpc$^2$ and additionally report the discovery of a second major shock-excited H$_2$ feature. This brings the total H$_2$ line luminosity of the group in excess of 10$^42$ erg/s. In the main shock, the H$_2$ line luminosity exceeds, by a factor of three, the X-ray luminosity from the hot shocked gas, confirming that the H$_2$-cooling pathway dominates over the X-ray. [Si II]34.82$mu$m emission, detected at a luminosity of 1/10th of that of the H$_2$, appears to trace the group-wide shock closely and in addition, we detect weak [FeII]25.99$mu$m emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 < $V_{s}$ < 300 km/s) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, PAH and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H$_{2}$ power is driven by turbulent energy transfer from motions in a post-shocked layer. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.
Context. The Spitzer Space Telescope has detected a powerful (L(H2)~10^41 erg s-1) mid-infrared H2 emission towards the galaxy-wide collision in the Stephans Quintet (SQ) galaxy group. This discovery was followed by the detection of more distant H2-l
uminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations set molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed to be multiphase, with H2 gas coexisting with a hot (~ 5 10^6 K), X-ray emitting plasma. The surface brightness of H2 lines exceeds that of the X-rays and the 0-0 S(1) H2 linewidth is ~ 900 km s-1, of the same order of the collision velocity. These observations raise three questions we propose to answer: (i) Why H2 is present in the postshock gas ? (ii) How can we account for the H2 excitation ? (iii) Why H2 is a dominant coolant ? Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H2 formation and excitation in the postshock medium. Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume filling intercloud gas. This produces a ~ 5 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is smaller in clouds. We show that gas heated to temperatures less than 10^6 K cools, keeps its dust content and becomes H2 within the SQ collision age (~ 5 10^6 years). (ii) Since the bulk kinetic energy of the H2 gas is the dominant energy reservoir, we consider that the H2 emission is powered by the dissipation of kinetic turbulent energy. (Abridged)
