© Lloyd

1. Star-forming Galactic Contrails at z=3.2 as a Source of Metal Enrichment and Ionizing Radiation - http://arxiv.org/pdf/1305.5849v1.pdf
2. INTRODUCTION - Long slit, spectroscopic blind surveys [] have the potential to deliver detailed and otherwise unavailable insights into the gas dynamics and, in conjunction with deep, space-based imaging, the star-gas interactions in proto-galactic halos and the intergalactic medium.
3. Several surveys of this kind [] have discovered a distinct subpopulation of [] extended, asymmetric Ly α emitters at z ~ 3, with a comoving space density on the order of 10 − 3 / Mpc 3 and typical observed line fluxes of a few times 10 − 17 erg / cm 2 / s.
4. With a large number of processes capable of producing Ly α radiation, one may expect the emitters to be drawn from a highly inhomogeneous group of objects.
5. However, the selection by Ly α emission is likely to favor galaxies in certain phases of their formation, when the stellar populations and gas dynamics are particularly conducive to the production and escape of Ly α photons.
6. The peculiar spatial distribution and clustering behaviour of Ly α emitters suggests that environmental effects and interactions may play an important role in determining whether a galaxy appears as a Ly α emitter [].
7. Indeed, all four extended emitters described so far in the papers in this series exhibit signs of interactions.
8. The duration of the processes leading to the production of ionizing radiation (e.g., the lifetimes of massive stars, [or] AGN activity), as well as the astrophysical timescales relevant for the emission of Ly α in high redshift gaseous [] halos (recombination- and resonance-line radiative-transfer timescales) tend to be short compared to the dynamical timescales and life times of the general stellar population.
9. **Thus the spectroscopic detection of such a halo amounts to a "snapshot" of a particularly interactive phase in their formation, illuminated by a "flash" of Ly α emission.
10. Among those extended Ly α emitters published to date, the first one showed diffuse stellar features, in addition to a clear detection of the infall of cold gas into an ordinary high redshift galaxy (paper I).
11. Paper II described what may be a Milky-way-sized halo with multiple galaxies hosting disturbed, partly young stellar populations.
12. A thin filament apparently dominated by high equivalent width Ly α emission may reflect recent intra-halo star formation in a tidal tail or in the wake of a satellite galaxy.
13. The third object, revealing the only case in this sample clearly related to non-stellar processes, is an AGN illuminating a satellite galaxy, possibly triggering the formation of very young stars in its halo (paper III).
14. The object examined in the present paper is a large halo surrounded by Ly α filaments illuminated by a group of distorted, mostly blue galaxies.
15. As we shall argue below, the interaction in this case appears to be between the galaxies and a gaseous medium through which they move, and which appears to strip off part of their gas and induce star formation in their wake.
16. INTERPRETATION - As for the nature of the galaxies in the field, most of the objects with a head-tail structure have their heads to the south of the tails (1,2,3,4 and 5), making it less likely that the features have been produced mainly by tidal interactions.
17. Rather, the evidence appears consistent with large scale ram-pressure stripping [] of gas, and recent star-formation in the down-stream ablated tails.
18. Ram pressure stripping has been invoked to explain features seen in several astrophysical environments, including galaxy clusters [], the Milky Way halo [] and in galaxy groups[].
19. So far, there is relatively scant observational evidence for this process at high redshift, with the exception of the tadpole galaxies, a population of galaxies increasingly common with [increasing?] redshift [], that may partly have been shaped by ram-pressure [].
20. Groups of galaxies falling into clusters seem to produce H α morphologies similar to the galaxy contrails observed here [].
21. In the present case, a number of observational details closely resemble hydrodynamic features predicted from simulations of supersonic motions in galaxy clusters
22. Tadpole 1 has a bow shock, presumably because it is on the colder side of the interface between hot and cold gas, and its velocity relative to the intergalactic medium exceeds the sound speed for the colder gas.
23. This condition would easily be satisfied when passing through the general filamentary IGM with even moderate velocity, as the typical temperature at z ~ 3 is only a few times 10^4 K [], corresponding to sound speeds of a few tens of km / s.
24. The tail of tadpole 1 is flaring up and shows a turbulent vortex pattern, consistent with the lower viscosity expected in a colder medium.
25. A particular interesting consequence of ram-stripping is the formation of stars in the stripped gas, which has been the subject of recent observational [] and theoretical study.
26. Several strands of evidence suggest that the current situation is indeed an instance of star formation in galactic wakes: the tails of most of the objects have blue colors, indicative of very young stars.
27. The finding of extended metal line emission far out from the galaxies suggests the presence of a stripped, or re-created, interstellar medium that is being excited by the newly forming stars.
28. The high Ly α equivalent width suggested by the tail of galaxy 4 may be another sign of hot, young stars, as discussed in paper II.
29. The usual condition for ram-pressure stripping to take place is that the ram-pressure on the gas in a galaxy, experienced when passing through the intergalactic medium, needs to exceed the gravitational binding force per surface area, ρv 2 > (π/2) GM (<R) ρ gal (R) / R [].
30. It has often been assumed that ram pressure stripping is most relevant for low redshift, massive clusters.
31. However, the hierarchical nature of structure formation, leading to more compact gravitational potential wells, higher gas densities, and higher interaction rates at z ~ 3 (when compared to the local universe) suggests that one should expect miniature versions of the ram-pressure stripping seen in low redshift clusters to occur among satellites in individual high z galactic halos.
32. With the higher density at high redshift favoring a higher pressure for a given velocity, the in-falling satellites themselves collapse from a denser background as well, so for the effect of ram pressure stripping one would have to look to lower mass, dwarf galaxies, perhaps aided by processes that may lower the binding energy of the gas further.
33. The higher merger rate at high redshift may also work to increase the amount of ram-pressure stripped gas [], as may stellar or galactic outflows, as long as they can offset a significant part of the gravitational binding energy.
34. In addition, new cosmological hydro-simulation techniques suggest more efficient stripping [] and the presence of puffed up, high-angular momentum gas [], "ready to go".
35. Recently it has been argued [] that, in particular, dwarf galaxies do not even require the encounter with fully formed massive halos but can lose gas to ram-pressure stripping in large-scale structure filaments at high redshift when entering terminal nodes like the (future) Local Group pancake.
36. In this case, our spectrograph slit may have intersected a filament of the cosmic web, lit up by the star-formation in the ablated contrails of a swarm of coherently moving galaxies.
37. To attain the high relative velocities required for stripping, the galaxies would have to move highly supersonically with respect to the gas they are plunging into.
38. While we appear to be seeing one galaxy with a bow shock, it is not clear if the velocities of the other objects are high enough for this to work.
39. However, as argued above, the presence of an accretion shock in the terminal node with hot gas on one side may make this scenario consistent with the observations.
40. A variant of this picture may explain the stripping and the apparent gradient in the properties of the tadpoles as a group of galaxies being "hosed down" when obliquely passing an accreting stream of gas.
41. 5.1 Metal enrichment and escape of ionizing radiation from star formation in stripped gas:
42. The existence of such extended structures at high redshift, the relatively large number density of galaxies with tidal or ram-pressure related features (see also the disturbed halos described in paper I and II), and the presence of multiple sites of star-formation in a common gaseous halo or large scale filament suggest that the stripping of gas from galaxies in interactions could be an important contributor to the metal enrichment of the intergalactic medium, analogous to the lower redshift process leading to the enrichment of the gas in galaxy clusters.
43. To explain the finding of metal enrichment in the IGM at large distances from the nearest bright galaxy, galactic winds from Lyman break galaxies have been invoked to drive metal-enriched gas far into intergalactic space [].
44. Among the persistent uncertainties with this scenario is that the actually observed ranges of galactic winds invariably fall short of accounting for the metals seen in QSO absorption systems at large distances from such galaxies.
45. However, if, as we have argued above, ram pressure stripping of in-falling dwarf galaxies and star formation in the stripped wake operate at high redshift, there may be less need to invoke long- range winds from the central galaxy of a halo.
46. In this alternative picture, the ram-pressure that led to the ablation of gas and subsequent star formation may also act to dispel newly formed metal enriched gas from the tails, aided by stellar winds and supernova explosions that would find it much easier to escape from the weakly bound [] star forming regions of galactic wakes.
47. In any case, differential motion between the lost gas and the parent star forming galaxies will distribute the gas spatially over time, and the assumption that this process mostly occurs in in-falling dwarf satellites implies that the gas is automatically reaching distances from any brightest halo galaxy as large as commonly observed (in metal absorption lines[]).
48. Tidal interactions between satellites may lead to a similar result, metals expelled into the gaseous halo of brighter galaxies, that came from the shredded interstellar medium of its satellites or from outflows in tidal dwarfs, and both processes may exist among the extended, asymmetric Ly α emitters in our study.
49. There may be differences between the metallicities of the gas ejected from tidal star-forming regions, and the gas lost by star forming regions in galactic contrails.
50. Stars in the former arise from the relatively metal-rich ISM of the parent galaxy, as would stars in gas ablated by ram-pressure or viscous stripping.
51. Stars forming in turbulent wakes behind the galaxies may feed on the lower metallicity gas in the halo or intergalactic medium as well, which may contribute to the signatures of hot, young stars described in paper II.
52. As argued earlier in paper II, extragalactic star formation in the wakes of stripped galaxies and in tidal tails may facilitate the production and escape of ionizing photons and may have brought about the reionization of the universe at high redshift.
53. Star formation outside of the dense galactic H I cocoons would lead to lines of sight with reduced optical depth for ionizing photons.
54. The presence of young, massive stars in the wakes, with the weak gravitational binding force enabling easy removal of neutral gas by even moderate amounts of stellar winds or supernova outflows would all tend to enhance the escape of ionizing radiation.
55. 6 CONCLUSIONS - We have detected a Ly α emitting halo with several faint filaments stretching over tens of kpc.
56. The filaments correlate with star-forming regions in the form of mostly blue, faint galaxies, several of which have a distinct tadpole shape and blue, partly turbulent tails, with one object showing what appears to be a bow shock.
57. The GOODS-N ACS F435W image reveals many such features criss-crossing an area several times bigger than the visible extent of the Ly α halo.
58. The emission of the central halo and of the filaments is broadly consistent with being powered by stellar photoionization.
59. We detect spatially extended emission lines from gas surrounding the main tadpole, including He II 1640, N V 1240 and probably O III, Al II, and Fe II, suggesting an extended, extragalactic, interstellar medium with current star formation.
60. The tadpole shapes, partial alignment, and the considerable numbers of unusual broad band objects make it unlikely that the features observed are predominantly tidal in origin [].
61. Instead, the galaxies may have experienced stripping of gas when moving relative to the intergalactic or intra-halo medium, with stars forming downstream in the galactic contrails.
62. This process is observationally and theoretically well established in the local universe.
63. Our observations have identified an occurrence of ram-pressure stripping at high red-shift, possibly involving dwarf galaxies interacting with the gas in more massive, individual galactic halos.
64. The filamentary structure trailing behind a galaxy in the z=2.63 halo described in paper II may be another example of this effect.
65. In the present case, the properties of several tadpoles change along their general direction of motion, which may be consistent with these galaxies passing into a hotter gaseous environment, possibly the region behind an accretion shock.
66. Such a stripping scenario may play out on a larger scale when differential motions of galaxies relative to the nodes in the gaseous cosmic web strip galaxies of[] their gas [].
67. As in the case of local clusters, the galactic contrails should be able to release metal-enriched gas, perhaps enhanced by local stellar feedback, more easily than normal galaxies.
68. At the very least these objects should provide a contribution to the intergalactic metal budget of galactic halos.
69. The loss of enriched gas from galactic contrails may suggest a solution to the long-standing puzzle of how the intergalactic medium at large distances from bright galaxies was polluted with metals.
70. Star formation in galactic contrails would involve young stars, surrounded by lower H I gas columns than stars born in ordinary galaxies, and capable of clearing their environment of dense gas, suggesting a way in which galaxies can ionize the intergalactic medium.