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Register a new userInvestigating Disk-Halo Flows and Accretion: A Kinematic and Morphological Analysis of Extraplanar HI in NGC 3044 and NGC 4302
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To further understand the origins of and physical processes operating in extra-planar gas, we present observations and kinematic models of HI in the two nearby, edge-on spiral galaxies NGC 3044 and NGC 4302. We model NGC 3044 as a single, thick disk. Substantial amounts of extra-planar HI are also detected. We detect a decrease in rotation speed with height (a lag) that shallows radially, reaching zero at approximately R25. The large-scale kinematic asymmetry of the approaching and receding halves suggests a recent disturbance. The kinematics and morphology of NGC 4302, a Virgo Cluster member, are greatly disturbed. We model NGC 4302 as a combination of a thin disk and a second, thicker disk, the latter having a hole near the center. We detect lagging extra-planar gas, with indications of shallowing in the receding half, although its characteristics are difficult to constrain. A bridge is detected between NGC 4302 and its companion, NGC 4298. We explore trends involving the extra-planar HI kinematics of these galaxies, as well as galaxies throughout the literature, as well as possible connections between lag properties with star formation and environment. Measured lags are found to be significantly steeper than those modeled by purely ballistic effects, indicating additional factors. Radial shallowing of extra-planar lags is typical and occurs between 0.5R25 and R25, suggesting internal processes are important in dictating extra-planar kinematics.
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We present a kinematic study of ionised extraplanar gas in two low-inclination late-type galaxies (NGC 3982 and NGC 4152) using integral field spectroscopy data from the DiskMass H$alpha$ sample. We first isolate the extraplanar gas emission by masking the H$alpha$ flux from the regularly rotating disc. The extraplanar gas emission is then modelled in the three-dimensional position-velocity domain using a parametric model described by three structural and four kinematic parameters. Best-fit values for the model are determined via a Bayesian MCMC approach. The reliability and accuracy of our modelling method are carefully determined via tests using mock data. We detect ionised extraplanar gas in both galaxies, with scale heights $0.83^{+0.27}_{-0.40},mathrm{kpc}$ (NGC 3982) and $1.87^{+0.43}_{-0.56},mathrm{kpc}$ (NGC 4152) and flux fraction between the extraplanar gas and the regularly rotating gas within the disc of 27% and 15% respectively, consistent with previous determinations in other systems. We find lagging rotation of the ionized extraplanar gas in both galaxies, with vertical rotational gradients $-22.24^{+6.60}_{-13.13} ,mathrm{km,s^{-1},kpc^{-1}}$ and $-11.18^{+3.49}_{-4.06},mathrm{km,s^{-1},kpc^{-1}}$, respectively, and weak evidence for vertical and radial inflow in both galaxies. The above results are similar to the kinematics of the neutral extraplanar gas found in several galaxies, though this is the first time that 3D kinematic modelling of ionised extraplanar gas has been carried out. Our results are broadly consistent with a galactic fountain origin combined with gas accretion. However, a dynamical model is required to better understand the formation of ionised extraplanar gas.
Gas infall and outflow are critical for determining the star formation rate and chemical evolution of galaxies but direct measurements of gas flows are diffcult to make. Young massive stars and HII regions in the halos of galaxies are potential tracers for accretion and/or outflows of gas. Gas phase abundances of three HII regions in the lower halos of the edge-on galaxies NGC 3628 and NGC 4522 are determined by analysing optical long-slit spectra. The observed regions have projected distances to the midplane of their host from 1.4 to 3 kpc. With the measured flux densities of the optical nebular emission lines, we derive the oxygen abundance 12 + log(O/H) for the three extraplanar HII regions. The analysis is based on one theoretical and two empirical strong-line calibration methods. The resulting oxygen abundances of the extraplanar HII regions are comparable to the disk HII regions in one case and a little lower in the other case. Since our results depend on the accuracy of the metallicity determinations, we critically discuss the difference of the calibration methods we applied and confirm previously noted offsets. From our measurements, we argue that these three extraplanar HII regions were formed in the disk or at least from disk material. We discuss the processes that could transport disk material into the lower halo of these systems and conclude that gravitational interaction with a companion galaxy is most likely for NGC 3628 while ram pressure is favoured in the case of NGC 4522.
We present a UV study of 3 extended UV (XUV) galaxies that we have observed with the UVIT and the GMRT. XUV galaxies show filamentary or diffuse star formation well beyond their optical disks, in regions where the disk surface density lies below the threshold for star formation. GALEX observations found that surprisingly 30% of all the nearby spiral galaxies have XUV disks. The XUV galaxies can be broadly classified as type 1 and type 2 XUV disks. The type 1 XUV disks have star formation that is linked to that in their main disk, and the UV emission appears as extended, filamentary spiral arms. The UV luminosity is associated with compact star forming regions along the extended spiral arms. The star formation is probably driven by slow gas accretion from nearby galaxies or the intergalactic medium (IGM). But the type 2 XUV disks have star formation associated with an outer low luminosity stellar disk that is often truncated near the optical radius of the galaxy. The nature of the stellar disks in type 2 XUV disks are similar to that of the diffuse stellar disks of low surface brightness galaxies. The star formation in type 2 XUV disks is thought to be due to rapid gas accretion or gas infall from nearby high velocity clouds (HVCs), interacting galaxies or the IGM. In this paper we investigate the star formation properties of the XUV regions of two type 2 galaxies and one mixed XUV type galaxy and compare them with the neutral hydrogen (HI) emission in their disks. We present preliminary results of our UVIT (FUV and NUV) observations of NGC 2541, NGC 5832 and ESO406-042, as well as GMRT observations of their HI emission. We describe the UV emission morphology, estimate the star formation rates and compare it with the HI distribution in these type 2 and mixed XUV galaxies.
The interstellar thick disks of galaxies serve as the interface between the thin star-forming disk, where feedback-driven outflows originate, and the distant halo, the repository for accreted gas. We present optical emission line spectroscopy of a luminous thick disk H II region located at $z = 860$ pc above the plane of the spiral galaxy NGC 4013 taken with the Multi-Object Double Spectrograph on the Large Binocular Telescope. This nebula, with an H$alpha$ luminosity $sim4-7$ times that of the Orion nebula, surrounds a luminous cluster of young, hot stars that ionize the surrounding interstellar gas of the thick disk, providing a measure of the properties of that gas. We demonstrate that strong emission line methods can provide accurate measures of relative abundances between pairs of H II regions. From our emission line spectroscopy, we show that the metal content of the thick disk H II region is a factor of $approx2$ lower than gas in H II regions at the midplane of this galaxy (with the relative abundance of O in the thick disk lower by $-0.32pm 0.09$ dex). This implies incomplete mixing of material in the thick disk on small scales (100s of parsecs) and that there is accretion of low-metallicity gas through the thick disks of spirals. The inclusion of low-metallicity gas this close to the plane of NGC 4013 is reminiscent of the recently-proposed fountain-driven accretion models.
The low spin rates measured for solar-type stars at an age of a few Myr (~10% of the break-up velocity) indicate that some mechanism of angular momentum regulation must be at play in the early pre-main sequence. We characterize the rotation properties for members of the region NGC 2264 (~3 Myr), and investigate the accretion-rotation connection at an age where about 50% of the stars have already lost their disks. We examined a sample of 500 cluster members whose photometric variations were monitored in the optical for 38 consecutive days with CoRoT. Light curves were analyzed for periodicity using the Lomb-Scargle periodogram, the autocorrelation function and the string-length method. The period distribution obtained for the cluster consists of a smooth distribution centered around P=5.2 d with two peaks at P=1-2 d and 3-4 d. A separate analysis of CTTS and WTTS indicates that the P=1-2 d peak is associated with the latter, while both groups contribute to the P=3-4 d peak. The comparison between CTTS and WTTS supports the idea of a rotation-accretion connection: their respective rotational properties are statistically different, and CTTS rotate on average more slowly than WTTS. We also observe a clear dearth of fast rotators with strong accretion signatures (large UV flux excess). This is consistent with earlier findings that fast rotators in young star clusters are typically devoid of dusty disks. Our sample shows some evidence of a mass dependence in the rotation properties of NGC 2264 members, lower-mass stars spinning on average faster. This study confirms that disks influence the rotational evolution of young stars. The idea of disk-locking may be consistent with the picture of rotation and rotation-accretion connection that we observe for the NGC 2264 cluster. However, the origin of the several substructures that we observe in the period distribution deserves further investigation.
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