HI Bridge Research Articles

Faraday rotation study of NGC 612 (PKS 0131−36): a hybrid radio source and its magnetized circumgalactic environment

We present a polarization and Faraday rotation study of the hybrid morphology radio galaxy NGC 612 (PKS 0131-36), using Australian Telescope Compact Array observations from 1 to 3 GHz. In general, the results are consistent with an external Faraday screen close to the radio source. In the eastern FRII lobe, the RM of the hotspot increases in magnitude towards the leading edge, as well as changing sign (compared to the rest of the lobe). The Faraday depolarization is also ~3 times larger at the hotspot than elsewhere. A plausible explanation for this is significant compression of ambient magnetised gas by the bow shock produced by the advancing hotspot. The western FRI lobe also exhibits some evidence of interaction with local magnetised gas, as a transverse band of high RM coincides with a distinct bend in the lobe. Previous observations of NGC 612 revealed an HI bridge of tidal debris along the direction of the eastern lobe towards the gas-rich companion NGC 619. We find no clear evidence that ionised gas associated with this bridge is either mixing with or lies in the foreground of the radio source. This is consistent with the absence of HI absorption against the hotspot, and indicates that the tidal debris must lie mostly behind the eastern lobe.

Comparing the star formation history of three nearby disk galaxies

AbstractUnderstanding the effect of environment on galaxy formation and evolution is one of the hot topics in extragalactic astronomy. Here we constructed a chemical evolution model of disk galaxies. By comparing the model predictions with the observed profiles, we investigated the star formation history of M33, NGC 300 and NGC 2403. We found that M33 has much longer infall timescale than NGC 300 and NGC 2403, and the star formation process of M33 is still active at later phase. Our results suggested that the cold gas supply of M33 is sufficient in the present-day, which may originate from the HI bridge between M33 and M31. In other words, we argue that the local environment plays an important role on the star formation history of a galaxy, at least for M33.

Revealing H i gas in emission and absorption on pc to kpc scales in a galaxy at z ∼ 0.017

We present a detailed study of the quasar-galaxy pair: J1243+4043 - UGC07904. The sight line of the background quasar ( $z_q$= 1.5266) passes through a region of the galaxy ($z_g$=0.0169) at an impact parameter of 6.9 kpc with high metallicity (0.5 Z$_\odot$) and negligible dust extinction. We detect HI 21-cm absorption from the foreground galaxy at arcsecond and milliarcsecond scales. For typical cold neutral medium (CNM) temperatures in the Milky Way, this 21-cm absorber can be classified as a damped Ly$\alpha$ absorber (DLA). We infer the harmonic mean spin temperature of the gas to be $\sim$400 K and for a simple two-phase medium we estimate the CNM-fraction to be $f_{\rm CNM}$ = 0.27. This is remarkably consistent with the CNM fraction observed in the Galaxy and less than that of high-redshift DLAs. The quasar exhibits a core-jet morphology on milliarcsecond scales, corresponding to an overall extent of $\sim$9 pc at $z_g$. We show that the size of CNM absorbing clouds associated with the foreground galaxy is $>$5 pc and they may be part of cold gas structures that extend beyond $\sim$35 pc. Interestingly, the rotation measure of quasar J1243+4043 is higher than any other source in samples of quasars with high-$z$ DLAs. However, we do not find any detectable differences in RMs and polarization fraction of sight lines with or without high-$z$ ($z\ge2$) DLAs or low-$z$ ($z\le0.3$) 21-cm absorbers. Finally, the foreground galaxy UGC07904 is also part of a galaxy group. We serendipitously detect HI 21-cm emission from four members of the group, and a $\sim$80 kpc long HI bridge connecting two of the other members. The latter, together with the properties of the group members, suggests that the group is a highly interactive environment.

Atomic hydrogen bridge fueling NGC 4418 with gas from VV 655

The galaxy NGC 4418 harbours a compact ($<20$ pc) core with a very high bolometric luminosity ($\sim10^{11}$L$_\odot$). As most of the galaxy's energy output comes from this small region, it is of interest to determine what fuels this intense activity. An interaction with VV 655 has been proposed, where gas aquired by NGC 4418 could trigger intense star formation and/or black hole accretion in the centre. We aim to constrain the interaction hypothesis by studying neutral hydrogen structures around the two galaxies. We present observations at 1.4 GHz with the Very Large Array of radio continuum as well as emission and absorption from atomic hydrogen. Gaussian distributions are fitted to observed HI emission and absorption spectra. An atomic HI bridge is seen in emission, connecting NGC 4418 to VV 655. While NGC 4418 is bright in continuum emission and seen in HI absorption, VV 655 is barely detected in the continuum but show bright HI emission (M$_\mathrm{HI}\sim10^9$ M$_\odot$). We estimate SFRs from 1.4 GHz of 3.2 M$_\odot$ yr$^{-1}$ and 0.13 M$_\odot$ yr$^{-1}$ for NGC 4418 and VV 655 respectively. Systemic HI velocities of 2202$\pm$20 km s$^{-1}$ (emission) and 2105.4$\pm$10 km s$^{-1}$ (absorption) are measured for VV 655 and NGC 4418 respectively. Redshifted HI absorption is seen towards NGC 4418, suggesting gas infall. Blueshifted HI-emission is seen north-west of NGC 4418, which we interpret as a continuation of the outflow previously discussed by Sakamoto et al. (2013). The morphology and velocity structure seen in HI is consistent with an interaction scenario, where gas was transferred from VV 655 to NGC 4418, and may fuel the activity in the centre.

The ‘shook up’ galaxy NGC 3079: the complex interplay between H i, activity and environment

We present deep neutral hydrogen (HI) observations of the starburst/Seyfert galaxy NGC 3079 and its environment, obtained with the Westerbork Synthesis Radio Telescope. Our observations reveal previously unknown components, both in HI emission and in absorption, that show that NGC 3079 is going through a hectic phase in its evolution. The HI disk appears much more extended than previously observed and is morphologically and kinematically lopsided on all scales with evidence for strong non-circular motions in the central regions. Our data reveal prominent gas streams encircling the entire galaxy suggesting strong interaction with its neighbours. A 33-kpc long HI bridge is detected between NGC 3079 and MCG 9-17-9, likely caused by ram-pressure stripping of MGC 9-17-9 by the halo of hot gas of NGC 3079. The cometary HI tail of the companion NGC 3073, earlier discovered by Irwin et al., extends about twice as long in our data, while a shorter, second tail is also found. This tail is likely caused by ram-pressure stripping by the strong, starburst driven wind coming from NGC 3079. We also detect, in absorption, a nuclear HI outflow extending to velocities well outside what expected for gravitational motion. This is likely an atomic counterpart of the well-studied outflow of ionised gas present in this galaxy. This may indicate that also large amounts of cold gas are blown out of NGC 3079 by the starburst/AGN. Our estimates of the jet energy and kinetic power suggest that both the AGN and the starburst in NGC 3079 are powerful enough to drive the atomic outflow.

AGC 226067: A possible interacting low-mass system

We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source AGC 226067. Originally identified as an ultra-compact high velocity cloud and candidate Local Group galaxy, AGC 226067 is spatially and kinematically coincident with the Virgo cluster, and the identification by multiple groups of an optical counterpart with no resolved stars supports the interpretation that this systems lies at the Virgo distance (D=17 Mpc). The combined observations reveal that the system consists of multiple components: a central HI source associated with the optical counterpart (AGC 226067), a smaller HI-only component (AGC 229490), a second optical component (AGC 229491), and extended low surface brightness HI. Only ~1/4 of the single-dish HI emission is associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x 10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g = 3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC 226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC 229491 are separated by only 0.5'. The current data do not resolve the HI in AGC 229490 and its origin is unclear. We discuss possible scenarios for this system of objects: an interacting system of dwarf galaxies, accretion of material onto AGC 226067, or stripping of material from AGC 226067.

DETERMINING THE NATURE OF THE EXTENDED H I STRUCTURE AROUND LITTLE THINGS DWARF GALAXY NGC 1569

This work presents an extended, neutral Hydrogen emission map around Magellanic-type dwarf irregular galaxy (dIm) NGC 1569. In the Spring of 2010, the Robert C. Byrd Green Bank Telescope (GBT) was used to map a 9 degree x 2 degree region in HI line emission that includes NGC 1569 and IC 342 as well as two other dwarf galaxies. The primary objective for these observations was to search for structures potentially connecting NGC 1569 with IC 342 group members in order to trace previous interactions and thus, provide an explanation for the starburst and peculiar kinematics prevalent in NGC 1569. A large, half-degree diameter HI cloud was detected that shares the same position and velocity as NGC 1569. Also, two long structures were discovered that are reminiscent of intergalactic filaments extending out in a v-shaped manner from NGC 1569 toward UGCA 92, a nearby dwarf galaxy. These filamentary structures extend for about 1.5 degrees, which is 77 kpc at NGC 1569. There is a continuous velocity succession with the 0.5 degree HI cloud, filaments, and main body of the galaxy. The 0.5 degree HI cloud and filamentary structures may be foreground Milky Way, but are suggestive as possible remnants of an interaction between NGC 1569 and UGCA 92. The data also show two tidal tails extending from UGCA 86 and IC 342, respectively. These structures may be part of a continuous HI bridge but more data are needed to determine if this is the case.

A dynamical model for the Taffy galaxies UGC 12914/5

The spectacular head-on collision of the two gas-rich galaxies of the Taffy system, UGC 12914/15, gives us a unique opportunity to study the consequences of a direct ISM-ISM collision. To interpret existing multi-wavelength observations, we made dynamical simulations of the Taffy system including a sticky particle component. To compare simulation snapshots to HI and CO observations, we assume that the molecular fraction of the gas depends on the square root of the gas volume density. For the comparison of our simulations with observations of polarized radio continuum emission, we calculated the evolution of the 3D large-scale magnetic field for our simulations. The induction equations including the time-dependent gas-velocity fields from the dynamical model were solved for this purpose. Our simulations reproduce the stellar distribution of the primary galaxy, UGC 12914, the prominent HI and CO gas bridge, the offset between the CO and HI emission in the bridge, the bridge isovelocity vectors parallel to the bridge, the HI double-line profiles in the bridge region, the large line-widths (~200 km/s) in the bridge region, the high field strength of the bridge large-scale regular magnetic field, the projected magnetic field vectors parallel to the bridge and the strong total power radio continuum emission from the bridge. The stellar distribution of the secondary model galaxy is more perturbed than observed. The observed distortion of the HI envelope of the Taffy system is not reproduced by our simulations which use initially symmetric gas disks. The model allows us to define the bridge region in three dimensions. We estimate the total bridge gas mass (HI, warm and cold H2) to be 5 to 6 10^9 M_sun, with a molecular fraction M_H2/M_HI of about unity (abrigded).

THE NEUTRAL HYDROGEN BRIDGE BETWEEN M31 AND M33

The Green Bank Telescope has been used to search for 21cm HI emission over a large area between the galaxies M31 and M33 in an attempt to confirm at 9.1 arcmin angular resolution the detection by Braun and Thilker (2004) of a very extensive neutral gas "bridge" between the two systems at the level NHI approximately 10^{17} cm^{-2}. We detect HI emission at several locations up to 120 kpc in projected distance from M31, at least half the distance to M33, with velocities similar to that of the galaxies, confirming the essence of the Braun and Thilker discovery. The HI does not appear to be associated with the extraplanar high-velocity clouds of either galaxy. In two places we measure NHI > 3 x 10^{18} cm^{-2}, indicative of concentrations of HI with ~10^5 solar masses on scales <2 kpc, but over most of the field we have only 5sigma upper limits of NHI <= 1.4 x 10^{18} cm^{-2}. In very deep measurements in two directions HI lines were detected at a few 10^{17} cm^{-2}. The absence of emission at another location to a 5sigma limit NHI <= 1.5 x 10^{17} cm^{-2} suggests that the HI bridge is either patchy or confined to within ~125 kpc of M31. The measurements also cover two of M31's dwarf galaxies, And II and And XV, but in neither case is there evidence for associated HI at the 5sigma level of 1.4 x 10^4 solar masses of HI for And II, and 9.3 x 10^3 solar masses for And XV.

H i in very metal-poor galaxies: the SBS 0335-052 system

We present Giant Metrewave Radio Telescope (GMRT), HI 21cm observations of SBS 0335-052E and SBS 0335-052W, a close pair of dwarf galaxies, which are further unusual in being the most metal-poor star-forming galaxies known. We present images at several angular resolutions, ranging from ~40 to 4 arcsec. These images show that SBS 0335-052 is a strongly interacting system, with a faint diffuse HI bridge seen at low resolution, and elongated tails seen at the higher resolutions. The overall morphology suggests that the pair represents a major merger of extremely gas-rich galaxies. The low-resolution velocity field is dominated by the velocity difference between the two galaxies and the velocity gradient along the tidal features. However, for SBS 0335-052W at least, at high angular resolution, one sees a central velocity field that could be associated with the spin of the original undisturbed disc. The highest angular resolution HI images show that the ionized superbubble, identified by Thuan, Izotov & Lipovetsky (1997), in the Hubble Space Telescope (HST) images of SBS 0335-052E, is extended along one of the diffuse tidal features, and that there is a high-density HI clump at the other end of the superbubble. The star formation in SBS 0335-052E occurs mainly in a group of superstar clusters (SSCs) with a clear age gradient; the age decreases as one approaches the dense HI clump. We suggest that this propagating star formation is driven by the superbubble expanding into a medium with a tidally-produced density gradient. The high pressures associated with the compressed material would also naturally explain why current star formation is mainly concentrated in superstar clusters.

HI distribution and kinematics of NGC 1569

We present WSRT observations of high sensitivity and resolution of the neutral hydrogen in the starburst dwarf galaxy NGC 1569. Assuming a distance of 2.2 Mpc, we find a total HI mass of to be distributed in the form of a dense, clumpy ridge surrounded by more extended diffuse HI containing a few additional discrete features, such as a Western HI Arm and an HI bridge reaching out to a small counterrotating companion cloud. About 10% by mass of all HI in NGC 1569 is at unusually high velocities. Some of this HI may be associated with the mass outflow evident from Hα measurements, but some may also be associated with NGC 1569's HI companion and intervening HI bridge, in which case, infall rather than outflow might be the cause of the discrepant velocities. No indication of a large bubble structure was found in position-velocity maps of the high-velocity HI. The galaxy as a whole is in modest overall rotation, but the HI gas lacks any sign of rotation within (0.6 kpc) from the center, i.e. over most of the optical galaxy. Here, turbulent motions resulting from the starburst appear to dominate over rotation. In the outer disk, the rotational velocities reach a maximum of , but turbulent motion remains significant. Thus, starburst effects are still noticeable in the outer HI disk, although they are no longer dominant beyond 0.6 kpc. Even excluding the most extreme high-velocity HI clouds, NGC 1569 still has an unusually high mean HI velocity dispersion of , more than double that of other dwarf galaxies.

Gas dynamics in the barred Seyfert galaxy NGC 4151 -- II. High-resolution HI study

We present sensitive, high angular resolution (6 ′′ × 5 ′′ ) λ21-cm observations of the neutral hydrogen in the nearby barred Seyfert galaxy, NGC4151. These HI observations, obtained using the VLA in B-configuration, are the highest resolution to date of this galaxy, and reveal hitherto unprecedented detail in the distribution and kinematics of the HI on sub-kiloparsec scales. A complete analysis and discussion of the HI data are presented and the global properties of the galaxy are related to the bar dynamics presented in Paper I. HI absorption, consistent with previous studies, is detected against the radio continuum nucleus and shows two components – a deep absorption component, centred at 987 ± 1 km s −1 and width 87 ± 3 km s −1 , and a weaker component, redshifted to 1096 ± 6 km s −1 with a width of 35 ± 15 km s −1 . An alternative fit is also presented. In addition to the absorption, a high velocity cloud is detected in emission, coincident with the nucleus. This cloud is red-shifted by 260 km s −1 from systemic, has an HI mass of 2.3 × 10 7 M⊙, and corresponds to outflow on the far side of the nucleus. Contrary to previous studies, no HI bridge is detected reaching from the shocks directly across the nucleus. Instead, the gas streams from the shocks onto smaller orbits and forms fingers of HI which wind around the nucleus, consistent with predictions from general numerical simulations of bars. These fingers correspond closely with dust arcs seen in optical studies and resemble nuclear features seen by others in weak barred galaxies such as M100. A new rotation curve is presented, extending to within 8 ′′ of the nucleus and showing a turnover at a radius of �35 ′′ , which was previously undetected in lower resolution studies. The corresponding resonance curve and the properties of the shocks (Paper I) yield a bar pattern speed of 24.5 ± 3.7 km s −1 and one Inner Lindblad Resonance (ILR) at a radius of 2.8 ±0.6 kpc. Our observations, however, do not rule out the possibility of an Inner ILR.

Magnetic Field Structure in the Small Magellanic Cloud

The present status of an ongoing observational program to study the magnetic field structure of the SMC by means of optical polarimetry is presented. Being the SMC more distant than the LMC, the observed optical polarization would mainly originate in the distorted outer layers in the front side of the SMC. Highly sensitive data obtained up to now show a magnetic field aligned with the HI bridge connecting the SMC with the LMC, revealing one of the few detected intergalactic magnetic fields associated with interacting galaxies.

HI Profiles in the Bridge Region of the Megallanic Clouds

AbstractTwo hundred and seventeen HI profiles at positions approximately 1 ° apart in the bridge region between the Small and Large Magellanic Clouds have been observed with a 15’ arc beam. Diagrams of all the profiles, lists of column densities and average radial velocities are given, together with details of the extensive Gaussian analysis needed to account for the components.It is shown that the bridge region is most complex. (a) Two radial velocity groups, +214 and +238 km s-1, represent the actual HI bridge between the two galaxies, (b) Three other components, at mean radial velocities of +155, +177 and +195 km s-1 are seen to be integral parts of the SMC, stretching east to R. A. ~ 04h. (c) A further three components in groups at mean radial velocities +253, +272 and +293 km s-1 appear to be extensions of HI from the main body of the LMC.Nine sets of five closely spaced observations in the lower Magellanic Stream and in the bridge region at high sensitivity supply further information about the region.

A blue stellar population in the HI bridge between the two Magellanic Clouds

Radio observations have shown1 that a common HI cloud surrounds the Magellanic Cloud system. However, no stellar link between the two galaxies has previously been found. The furthest known stellar extension eastward of the Small Magellanic Cloud (SMC) is the blue association of stars found by Kunkel2, which lie close to the extreme tip of the ‘wing’ of the SMC. The SMC wing was discovered by Shapley3, who described it as “a large cloud of faint stars extending eastward from the SMC toward the LMC [Large Magellanic Cloud]”. More recently, Westerlund4 described the brighter (V<20.0) stellar population of the wing as being a mixture of extreme Population I, OB associations, HII regions and both blue and yellow supergrants. The rest of the Magellanic system shows a strong correlation between HI density and the presence of stellar condensations, and this has led to speculation that stellar associations may be present in the HI extension of the optical wing (the HI bridge indicated in Fig. 1) and also independently in regions of high HI density in the Magellanic stream5. Deep Hα photography6 shows weak diffuse emission between the Magellanic Clouds., supporting the possible existence of an optical counterpart to the HI bridge, but star counts near the eastern tip of the wing7 failed to reveal any link and indeed corroborated earlier work on the structure of the wing8. We now report the discovery of several hundred blue main-sequence stars in an area reaching 9° east of the SMC, coinciding with the HI ridge extending from the wing. These stare lie midway between the two Clouds and are more than 2 kpc further east than what was believed to be the eastern tip of the SMC wing. The apparent magnitudes of the brightest main-sequence stars suggest an age of ∼100 Myr, similar to ages of associations in the wing.

Evidence for Hα emission associated with the HI bridge connecting the Small and Large Magellanic Clouds

The bridge of matter connecting the Small Magellanic Cloud (SMC) with the Large Magellanic Cloud (LMC) was first detected thanks to radio observations, and detailed maps of this H I bridge are now available1. Observations of the Magellanic system at Hα wavelength reported by Johnson et al.2 in 1982 seem to show a diffuse Hα emission associated with the H I bridge. As they suggested, it is necessary to confirm this association by obtaining Hα profiles in the direction of this bridge. We did this in February 1984 with the help of a scanning Perot–Fabry interferometer and a photon-counting system at the 1.5-m European Space Observatory (ESO) telescope. The choice of the area to be observed was based on recent observations3 with the Very Wide Field Camera (VWFC) on board Spacelab 1 (November 1983) showing an extension of the SMC towards the LMC at wavelengths of 1,930 and 1,650 A. Detection of Hα emission would check that the ultraviolet emission was emanating from the bridge connecting the two galaxies by showing that the cloud of hot stars giving rise to this ultraviolet emission was ionizing part of the H I bridge.

Interacting Galaxies: The Kinematics of NGC 4038/39 and the HI Bridge Between M81 and NGC 3077

Computer simulations of galaxy encounters (Toomre 1974 and references therein) have strongly supported the idea that the luminous bridges and tails in many multiple galaxies are a result of gravitational interaction. The need for an observational test of the kinematics of these models has prompted observations of NGC 4038/39 (the “antennae”) in the 21 cm line of neutral hydrogen (HI) with the Westerbork Synthesis Radio Telescope (WSRT) at an angular resolution of 100″×150″ (α×δ). A detailed description of the observations and data reduction is given elsewhere (van der Hulst, 1977). Neutral hydrogen is definitely associated with the tails as was already indicated by the filled aperture observations of Huchtmeier and Bohnenstengel (1975) at the lower resolution of 8′.7. The distribution of HI column density is shown in Figure 1 superposed on a print of a plate obtained by F. Schweizer with the 4 m telescope at Cerro Tololo. Owing to the low declination of the object (−19°) the WSRT map is affected by several sidelobes; the main effect is an attenuation of extended east-west structures, for which reason the HI along the southern tail in Fig. 1 shows a gap. Computer experiments and a comparison with the filled aperture observations show that the WSRT data are entirely consistent with an HI distribution which is uniform along this tail and increases with about a factor 2 at the extreme end, where the tail is very blue and small HII regions are present (Schweizer, elsewhere in this volume). The tails contain 2.9 × 109M⊙of HI (assuming a distance of 20 Mpc), 85% of which is in the southern and more prominent tail. About 1.1 × 109M⊙of HI is associated with the galaxies which are not spatially resolved at the present angular resolution.