Observations and Kinematic Modeling of Neutral Hydrogen in Spiral Galaxies: Implications for Disk-Halo Flows and Accretion

Recent realizations concerning kinematic measurements of extra-planar layers in nearby galaxies may provide important clues to the origin of such layers and thereby the growth and evolution of galaxy disks. In particular, observations have shown a decrease in rotation speed with height (lags) in the extra-planar layers of multiple galaxies, leading to various models which attempt to understand this gradient in terms of disk-halo flows and accretion of primordial gas. In this thesis we present deep observations and detailed kinematic models of neutral hydrogen (HI) in five nearby, edge-on spiral galaxies (NGC 4244, NGC 4565, NGC 4302, NGC 3044 and NGC 4013) observed with the Very Large Array and the Westerbork Synthesis Radio Telescope. These models provide insight concerning both the morphology and kinematics of HI above and within the disk, especially in terms of their lags. Characterization of the magnitude and radial variation of lags aids in determining whether extra-planar HI originates in the plane of the disk (as described in galactic fountain-type models) or is accreted. In these galaxies we find substantial extra- planar Hi is not ubiquitous, while lags appear to be so. Furthermore, the lags we measure are steeper than those produced by purely ballistic effects, indicating that additional physical effects must be at work. In every (with the exception of NGC 4013) galaxy we model, a radial shallowing of the lag is observed, starting at approximately half of R25, and reaching its lowest magnitude near R25. Combining our sample with results from the literature, we see no clear connection between the presence of extra-planar Hi and star formation, nor do we see any connection between lag magnitude and star formation. Our findings provide constraints for theoretical scenarios which we will describe

HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4565

We present 21-cm observations and models of the neutral hydrogen in NGC 4565, a nearby, edge-on spiral galaxy, as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. These models provide insight concerning both the morphology and kinematics of HI above, as well as within, the disk. NGC 4565 exhibits a distinctly warped and asymmetric disk with a flaring layer. Our modeling provides no evidence for a massive, extended HI halo. We see evidence for a bar and associated radial motions. Additionally, there are indications of radial motions within the disk, possibly associated with a ring of higher density. We see a substantial decrease in rotational velocity with height above the plane of the disk (a lag) of -40 +5/-20 km/s/kpc and -30 +5/-30 km s/kpc in the approaching and receding halves, respectively. This lag is only seen within the inner ~4.75' (14.9 kpc) on the approaching half and ~4.25' (13.4 kpc) on the receding, making this a radially shallowing lag, which is now seen in the HI layers of several galaxies. When comparing results for NGC 4565 and those for other galaxies, there are tentative indications of high star formation rate per unit area being associated with the presence of a halo. Finally, HI is found in two companion galaxies, one of which is clearly interacting with NGC 4565.Comment: 17 pages, 16 figures, accepted for publication in the Astrophysical Journal, modified affiliatio

HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4244

We present 21-cm observations and models of the HI kinematics and distribution of NGC 4244, a nearby edge-on Scd galaxy observed as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. Our models give insight into the HI kinematics and distribution with an emphasis on the potential existence of extra-planar gas as well as a negative gradient in rotational velocity with height above the plane of the disk (a lag). Our models yield strong evidence against a significantly extended halo and instead favor a warp component along the line of sight as an explanation for some of the observed thickening of the disk. Based on these models, we detect a lag of -9 +3/-2 km s-1 kpc-1 in the approaching half and -9 +/-2 km s-1 kpc-1 in the receding half. This lag decreases in magnitude to -5+/-2 km s-1 kpc-1 and -4+/-2 km s-1 kpc-1 near a radius of 10 kpc in the approaching and receding halves respectively. Additionally, we detect several distinct morphological and kinematic features including a shell that is probably driven by star formation within the disk.Comment: 18 pages, 14 figures, Full resolution version may be found at: http://www.astron.nl/halogas/papers/NGC4244.Zschaechner.arXiv.p

A low H I column density filament in NGC 2403 : signature of interaction or accretion

Date of acceptance: 12/07/2014Observed H i accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion occurs in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To constrain the presence and properties of such flows, we present deep H i observations obtained with the NRAO Green Bank Telescope of an area measuring 4° × 4° around NGC 2403. These observations, with a 5σ detection limit of 2.4 × 1018 cm-2 over a 20 km s-1 linewidth, reveal a low column density, extended cloud outside the main H i disk, about 17′ (~ 16 kpc or ~ 2 R25) to the NW of the center of the galaxy. The total H i mass of the cloud is 6.3 × 106 M⊙, or 0.15 percent of the total H i mass of NGC 2403. The cloud is associated with an 8 kpc anomalous-velocity H i filament in the inner disk, that was previously observed in deep VLA observations. We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigboring dwarf galaxyPeer reviewe

ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253

We present spatially resolved ($\sim$50 pc) imaging of molecular gas species in the central kiloparsec of the nearby starburst galaxy NGC 253, based on observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA). A total of 50 molecular lines are detected over a 13 GHz bandwidth imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines. Based on the measured high CO/C$^{17}$O isotopic line ratio ($\gtrsim$350), we show that $^{12}$CO(1-0) has moderate optical depths. A comparison of the HCN and HCO$^{+}$ with their $^{13}$C-substituted isotopologues shows that the HCN(1-0) and HCO$^{+}$(1-0) lines have optical depths at least comparable to CO(1-0). H$^{13}$CN/H$^{13}$CO$^{+}$ (and H$^{13}$CN/HN$^{13}$C) line ratios provide tighter constraints on dense gas properties in this starburst. SiO has elevated abundances across the nucleus. HNCO has the most distinctive morphology of all the bright lines, with its global luminosity dominated by the outer parts of the central region. The dramatic variation seen in the HNCO/SiO line ratio suggests that some of the chemical signatures of shocked gas are being erased in the presence of dominating central radiation fields (traced by C$_{2}$H and CN). High density molecular gas tracers (including HCN, HCO$^+$, and CN) are detected at the base of the molecular outflow. We also detect hydrogen $\beta$ recombination lines that, like their $\alpha$ counterparts, show compact, centrally peaked morphologies, distinct from the molecular gas tracers. A number of sulfur based species are mapped (CS, SO, NS, C$_{2}$S, H$_{2}$CS and CH$_{3}$SH) and have morphologies similar to SiO.Comment: 20 pages, 10 figures, accepted to the Astrophysical Journa

High-resolution Observations of Molecular Lines in Arp 220: Kinematics, Morphology, and Limits on the Applicability of the Ammonia Thermometer

We observe Arp 220, the nearest ultra-luminous infrared galaxy, over 4 GHz in the K and Ka bands, providing constraints for the kinematics and morphology, and identifying molecular species on scales resolving both nuclei (0".6 or 230 pc). We detect multiple molecular species, including hydroxyl (OH ^2Π_(3/2)J = 9/2 F= 4-4; 5-5) in both cores, and tentatively detect H_2O(6_(15)-5_(23)) at ~21.84 GHz in both nuclei, indicating the likely presence of maser emission. The observed frequency range also contains metastable ammonia transitions from (J, K) = (1, 1)–(5, 5), as well as the (9, 9) inversion line; together, they are a well-known thermometer of dense molecular gas. Furthermore, the non-metastable (4, 2) and (10, 9), and possibly the (3, 1), lines are also detected. We apply a standard temperature analysis to Arp 220; however, the analysis is complicated in that standard local thermal equilibrium (LTE) assumptions do not hold. There are indications that a substantial fraction of ammonia could be in the non-metastable transitions, as opposed to only the metastable ones. Thus, the non-metastable transitions could be essential to constraining the temperature. We compare all of these data to ALMA observations of this source, confirming the outflow previously observed by other tracers in both nuclei

HALOGAS observations of NGC 5023 and UGC 2082: Modeling of non-cylindrically symmetric gas distributions in edge-on galaxies

In recent years it has become clear that the vertical structure of disk galaxies is a key ingredient for understanding galaxy evolution. In particular, the presence and structure of extra-planar gas has been a focus of research. The Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey aims to provide a census on the rate of cold neutral gas accretion in nearby galaxies as well as a statistically significant set of galaxies that can be investigated for their extra-planar gas properties. In order to better understand the the vertical structure of the neutral hydrogen in the two edge-on HALOGAS galaxies NGC 5023 and UGC 2082 we construct detailed tilted ring models. The addition of distortions resembling arcs or spiral arms significantly improves the fit of the models to these galaxies. In the case of UGC 2082 no vertical gradient in rotational velocity is required in either symmetric models nor non-symmetric models to match the observations. The best fitting model features two arcs of large vertical extent that may be due to accretion. In the case of NGC 5023 a vertical gradient is required in symmetric models (dV/dz =$-14.9\pm3.8$ km s$^{-1}$ kpc$^{-1}$) and its magnitude is significantly lowered when non-symmetric models are considered (dV/dz =$-9.4\pm3.8$ km s$^{-1}$ kpc$^{-1}$). Additionally it is shown that the underlying disk of NGC 5023 can be made symmetric, in all parameters except the warp, in non-symmetric models. In comparison to the "classical" modeling these models fit the data significantly better with a limited addition of free parameters.Comment: 27 Pages, 22 Figures. Accepted for publication in MNRA