arXiv subjects

W. J. G. de Blok

Publications and source records attributed to W. J. G. de Blok.

At least 109 records · Page 6Linked to original sources

arXiv2012

Disk Stability and Neutral Hydrogen as a Tracer of Dark Matter

We derive the projected surface mass distribution Sigma_M for spherically symmetric mass distributions having an arbitrary rotation curve. For a galaxy with a flat rotation curve and an ISM disk having a constant Toomre stability parameter, Q, the ISM surface mass density Sigma_g as well as Sigma_M both fall off as 1/R. We use published data on a sample of 20 well studied galaxies to show that ISM disks do maintain a constant Q over radii usually encompassing more than 50% of the HI mass. The power law slope in Sigma_g covers a range of exponents and is well correlated with the slope in the epicyclic frequency. This implies that the ISM disk is responding to the potential, and hence that secular evolution is important for setting the structure of ISM disks. We show that the gas to total mass ratio should be anti-correlated with the maximum rotational velocity, and that the sample falls on the expected relationship. A very steep fall off in Sigma_g is required at the outermost radii to keep the mass and angular momentum content finite for typical rotation curve shapes, and is observed. The observation that HI traces dark matter over a significant range of radii in galaxies is thus due to the disks stabilising themselves in a normal dark matter dominated potential. This explanation is consistent with the cold dark matter paradigm.

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arXiv2012

Evidence for a clumpy, rotating gas disk in a submillimeter galaxy at z=4

We present Karl G. Jansky Very Large Array (VLA) observations of the CO(2-1) emission in the z=4.05 submillimeter galaxy (SMG) GN20. These high-resolution data allow us to image the molecular gas at 1.3 kpc resolution just 1.6 Gyr after the Big Bang. The data reveal a clumpy, extended gas reservoir, 14 +/- 4 kpc in diameter, in unprecedented detail. A dynamical analysis shows that the data are consistent with a rotating disk of total dynamical mass 5.4 +/- 2.4 X 10^11 M_sun. We use this dynamical mass estimate to constrain the CO-to-H_2 mass conversion factor (alpha_CO), finding alpha_CO=1.1 +/- 0.6 M_sun (K km s^-1 pc^2)^-1. We identify five distinct molecular gas clumps in the disk of GN20 with masses a few percent of the total gas mass, brightness temperatures of 16-31K, and surface densities of >3,200-4,500 X (alpha_CO/0.8) M_sun pc^-2. Virial mass estimates indicate they could be self-gravitating, and we constrain their CO-to-H_2 mass conversion factor to be <0.2-0.7 M_sun (K km s^-1 pc^2)^-1. A multiwavelength comparison demonstrates that the molecular gas is concentrated in a region of the galaxy that is heavily obscured in the rest-frame UV/optical. We investigate the spatially-resolved gas excitation and find that the CO(6-5)/CO(2-1) ratio is constant with radius, consistent with star formation occuring over a large portion of the disk. We discuss the implications of our results in the context of different fueling scenarios for SMGs.

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arXiv2012

The Opacity of Spiral Galaxy Disks IX; Dust and Gas Surface Densities

Our aim is to explore the relation between gas, atomic and molecular, and dust in spiral galaxies. Gas surface densities are from atomic hydrogen and CO line emission maps. To estimate the dust content, we use the disk opacity as inferred from the number of distant galaxies identified in twelve HST/WFPC2 fields of ten nearby spiral galaxies. The observed number of distant galaxies is calibrated for source confusion and crowding with artificial galaxy counts and here we verify our results with sub-mm surface brightnesses from archival Herschel-SPIRE data. We find that the opacity of the spiral disk does not correlate well with the surface density of atomic (Hi) or molecular hydrogen (H2) alone implying that dust is not only associated with the molecular clouds but also the diffuse atomic disk in these galaxies. Our result is a typical dust-to-gas ratio of 0.04, with some evidence that this ratio declines with galactocentric radius, consistent with recent Herschel results. We discuss the possible causes of this high dust-to-gas ratio; an over-estimate of the dust surface-density, an under-estimate of the molecular hydrogen density from CO maps or a combination of both. We note that while our value of the mean dust-to-gas ratio is high, it is consistent with the metallicity at the measured radii if one assumes the Pilyugin & Thuan calibration of gas metallicity.

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arXiv2012

VLA-ANGST: A high-resolution HI Survey of Nearby Dwarf Galaxies

We present the "Very Large Array survey of Advanced Camera for Surveys Nearby Galaxy Survey Treasury galaxies (VLA-ANGST)." VLA-ANGST is a National Radio Astronomy Observatory Large Program consisting of high spectral (0.6-2.6 km/s) and spatial (~6") resolution observations of neutral, atomic hydrogen (HI) emission toward 35 nearby dwarf galaxies from the ANGST survey. ANGST is a systematic HST survey to establish a legacy of uniform multi-color photometry of resolved stars for a volume-limited sample of nearby galaxies (D\lesssim4 Mpc). VLA-ANGST provides VLA HI observations of the sub-sample of ANGST galaxies with recent star formation that are observable from the northern hemisphere and that were not observed in the "The HI Nearby Galaxy Survey" (THINGS). The overarching scientific goal of VLA-ANGST is to investigate fundamental characteristics of the neutral interstellar medium (ISM) of dwarf galaxies. Here we describe the VLA observations, the data reduction, and the final VLA-ANGST data products. We present an atlas of the integrated HI maps, the intensity-weighted velocity fields, the second moment maps as a measure for the velocity dispersion of the HI, individual channel maps, and integrated HI spectra for each VLA-ANGST galaxy. We closely follow the observational setup and data reduction of THINGS to achieve comparable sensitivity and angular resolution. A major difference, however, is the high velocity resolution of the VLA-ANGST observations (0.65 and 1.3km/s for the majority of the galaxies). The VLA-ANGST data products are made publicly available at: https://science.nrao.edu/science/surveys/vla-angst. With available star formation histories from resolved stellar populations and lower resolution ancillary observations from the FIR to the UV, VLA-ANGST will enable detailed studies of the relationship between the ISM and star formation in dwarf galaxies on a ~100 pc scale.

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arXiv2012

The Shapes of the HI Velocity Profiles of the THINGS Galaxies

We analyze the shapes of the HI velocity profiles of The HI Nearby Galaxy Survey (THINGS) to study the phase structure of the neutral interstellar medium (ISM) and its relation to global galaxy properties. We use a method analogous to the stacking method sometimes used in high redshift HI observations to construct high signal-to-noise (S/N) profiles. We call these high S/N profiles super profiles. We analyze and discuss possible systematics that may change the observed shapes of the super profiles. After quantifying these effects and selecting a sub-sample of unaffected galaxies, we find that the super profiles are best described by a narrow and a broad Gaussian component, which are evidence of the presence of the Cold Neutral Medium (CNM) and the Warm Neutral Medium (WNM). The velocity dispersion of the narrow component range from ~3.4 to ~8.6 km/s with an average of 6.5+/-1.5 km/s, whereas that of the broad component range from ~10.1 to ~24.3 km/s with an average of 16.8+/-4.3 km/s. We find that the super profile parameters correlate with star formation indicators such as metallicity, FUV-NUV colors and H_alpha luminosities. The flux ratio between the narrow and broad components tends to be highest for high metallicity, high star formation rate (SFR) galaxies. We show that the narrow component identified in the super profiles is associated with the presence of star formation, and possibly with molecular hydrogen.

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arXiv2012

Isolated and non-isolated dwarfs in terms of modified Newtonian dynamics

Within the framework of modified Newtonian dynamics (MOND) we investigate the kinematics of two dwarf spiral galaxies belonging to very different environments, namely KK 246 in the Local Void and Holmberg II in the M81 group. A mass model of the rotation curve of KK 246 is presented for the first time, and we show that its observed kinematics are consistent with MOND. We re-derive the outer rotation curve of Holmberg II, by modelling its HI data cube, and find that its inclination should be closer to face-on than previously derived. This implies that Holmberg II has a higher rotation velocity in its outer parts, which, although not very precisely constrained, is consistent with the MOND prediction.

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arXiv2012

Low CO Luminosities in Dwarf Galaxies

[Abridged] We present maps of CO 2-1 emission covering the entire star-forming disks of 16 nearby dwarf galaxies observed by the IRAM HERACLES survey. The data have 13 arcsec angular resolution, ~250 pc at our average distance of 4 Mpc, and sample the galaxies by 10-1000 resolution elements. We apply stacking techniques to perform the first sensitive search for CO emission in dwarfs outside the Local Group ranging from single lines-of-sight, stacked over IR-bright regions of embedded star formation, and stacked over the entire galaxy. We detect 5 dwarfs in CO with total luminosities of L_CO = 3-28 1e6 Kkmspc2. The other 11 dwarfs remain undetected in CO even in the stacked data and have L_CO < 0.4-8 1e6 Kkmspc2. We combine our sample of dwarfs with a large literature sample of spirals to study scaling relations of L_CO with M_B and metallicity. We find that dwarfs with metallicities of Z ~ 1/2-1/10 Z_sun have L_CO about 1e2-1e4x smaller than spirals and that their L_CO per unit L_B is 10-100x smaller. A comparison with tracers of star formation (FUV and 24 micron) shows that L_CO per unit SFR is 10-100x smaller in dwarfs. One possible interpretation is that dwarfs form stars much more efficiently, however we argue that the low L_CO/SFR ratio is due to significant changes of the CO-to-H2 conversion factor, alpha_CO, in low metallicity environments. Assuming a constant H2 depletion time of 1.8 Gyr (as found for nearby spirals) implies alpha_CO values for dwarfs with Z ~ 1/2-1/10 Z_sun that are more than 10x higher than those found in solar metallicity spirals. This significant increase of alpha_CO at low metallicity is consistent with previous studies, in particular those which model dust emission to constrain H2 masses. Even though it is difficult to parameterize the metallicity dependence of alpha_CO, our results suggest that CO is increasingly difficult to detect at lower metallicities.

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arXiv2012

Estimating the Star Formation Rate at 1 kpc Scales in Nearby Galaxies

Using combinations of Hα, ultraviolet (UV), and infrared (IR) emission, we estimate the star formation rate (SFR) surface density, Σ_SFR, at 1 kpc resolution for 30 disk galaxies that are targets of the IRAM HERACLES CO survey. We present a new physically-motivated IR spectral energy distribution-based approach to account for possible contributions to 24\mum emission not associated with recent star formation. Considering a variety of "reference" SFRs from the literature, we revisit the calibration of the 24\mum term in hybrid (UV+IR or Hα+IR) tracers. We show that the overall calibration of this term remains uncertain at the factor of two level because of the lack of wide-field, robust reference SFR estimates. Within this uncertainty, published calibrations represent a reasonable starting point for 1 kpc-wide areas of star-forming disk galaxies but we re-derive and refine the calibration of the IR term in these tracers to match our resolution and approach to 24\mum emission. We compare a large suite of Σ_SFR estimates and find that above Σ_SFR \sim 10^-3 M_\odot yr^-1 kpc^-2 the systematic differences among tracers are less than a factor of two across two orders of magnitude dynamic range. We caution that methodology and data both become serious issues below this level. We note from simple model considerations that focusing on a part of a galaxy dominated by a single stellar population the intrinsic uncertainty in Hαand FUV-based SFRs are \sim 0.3 and \sim 0.5 dex.

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arXiv2012

On the Origin of the Supergiant HI Shell and Putative Companion in NGC 6822

We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large HI hole. The resulting deep color magnitude diagrams show that NGC 6822 has formed >50% of its stars in the last ~5 Gyr. The star formation histories of all six positions are similar over the most recent 500 Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming that the lifetime of the structure is longer than 500 Myr; such long-lived structures have now been observed in multiple systems and may be the norm in galaxies with solid-body rotation. The old stellar populations (red giants and red clump stars) of the putative companion are consistent with those of the extended halo of NGC 6822; this argues against the interpretation of this structure as a bona fide interacting companion galaxy and against its being linked to the formation of the HI hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended HI structure in NGC 6822 is a warped disk inclined to the line of sight.

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arXiv2012

A QUMOND galactic N-body code I: Poisson solver and rotation curve fitting

Here we present a new particle-mesh galactic N-body code that uses the full multigrid algorithm for solving the modified Poisson equation of the Quasi Linear formulation of Modified Newtonian Dynamics (QUMOND). A novel approach for handling the boundary conditions using a refinement strategy is implemented and the accuracy of the code is compared with analytical solutions of Kuzmin disks. We then employ the code to compute the predicted rotation curves for a sample of five spiral galaxies from the THINGS sample. We generated static N-body realisations of the galaxies according to their stellar and gaseous surface densities and allowed their distances, mass-to-light ratios (M/L) and both the stellar and gas scale-heights to vary in order to estimate the best fit parameters. We found that NGC 3621, NGC 3521 and DDO 154 are well fit by MOND using expected values of the distance and M/L. NGC 2403 required a moderately larger $M/L$ than expected and NGC 2903 required a substantially larger value. The surprising result was that the scale-height of the dominant baryonic component was well constrained by the rotation curves: the gas scale-height for DDO 154 and the stellar scale-height for the others. In fact, if the suggested stellar scale-height (one-fifth the stellar scale-length) was used in the case of NGC 3621 and NGC 3521 it would not be possible to produce a good fit to the inner rotation curve. For each of the four stellar dominated galaxies, we calculated the vertical velocity dispersions which we found to be, on the whole, quite typical compared with observed stellar vertical velocity dispersions of face on spirals. We conclude that modelling the gas scale-heights of the gas rich dwarf spiral galaxies will be vital in order to make precise conclusions about MOND.

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arXiv2011

CO in GN20: The Nature of a z=4 Submillimeter Galaxy

We present a study of the formation of clustered, massive galaxies at large look--back times, via high resolution spectroscopic imaging of CO in the unique GN20 proto-cluster. The data reveal evidence for rich structure and gas dynamics in unprecedented detail, allowing us to image the molecular gas with a resolution of only 1 kpc just 1.5 Gyr after the Big Bang. These state-of-the-art data give new insight into the detailed physical processes involved in early massive galaxy formation, and they provide a first glimpse of the morphological studies that will become feasible on a regular basis with ALMA.

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arXiv2011

Improved Modeling of the Mass Distribution of Disk Galaxies by the Einasto Halo Model

(Abridged) The analysis of the rotation curves (RCs) of spiral galaxies provides an efficient diagnostic for studying the properties of dark matter halos and their relations with the baryonic material. We have modeled the RCs of galaxies from The HI Nearby Galaxy Survey (THINGS) with the Einasto halo model, which has emerged as the best-fitting model of the halos arising in dissipationless cosmological N-body simulations. We find that the RCs are significantly better fit with the Einasto halo than with either a pseudo-isothermal sphere (Iso) or Navarro-Frenk-White (NFW) halo models. In our best-fit models, the radius of density slope -2 and the density at this radius are highly correlated. The Einasto index, which controls the overall shape of the density profile, is near unity on average for intermediate and low mass halos. This is not in agreement with the predictions from LCDM simulations. The indices of the most massive halos are in rough agreement with those of cosmological simulations and appear correlated with the halo virial mass. We find that a typical Einasto density profile declines more strongly in its outermost parts than any of the Iso or NFW models whereas it is relatively shallow in its innermost regions. The core nature of those regions of halos thus extends the cusp-core controversy found for the NFW model with low surface density galaxies to the Einasto halo with more massive galaxies like those of THINGS. We thus find that the Einasto halo model provides, so far, the best match to the observed RCs, and can therefore be considered as a new standard model for dark matter halos.

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arXiv2011

A Molecular Star Formation Law in the Atomic Gas Dominated Regime in Nearby Galaxies

We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using atomic hydrogen (HI) data, mostly from THINGS, we predict the local mean CO velocity from the mean HI velocity. By renormalizing the CO velocity axis so that zero corresponds to the local mean HI velocity we are able to stack spectra coherently over large regions as function of radius. This enables us to measure CO intensities with high significance as low as Ico = 0.3 K km/s (H2_SD = 1 Msun/pc2), an improvement of about one order of magnitude over previous studies. We detect CO out to radii Rgal = R25 and find the CO radial profile to follow a uniform exponential decline with scale length of 0.2 R25. Comparing our sensitive CO profiles to matched profiles of HI, Halpha, FUV, and IR emission at 24um and 70um, we observe a tight, roughly linear relation between CO and IR intensity that does not show any notable break between regions that are dominated by molecular (H2) gas (H2_SD > HI_SD) and those dominated by atomic gas (H2_SD < HI_SD). We use combinations of FUV+24um and Halpha+24um to estimate the recent star formation rate (SFR) surface density, SFR_SD, and find approximately linear relations between SFR_SD and H2_SD. We interpret this as evidence for stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H2 ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relations between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between HI and H2 depends strongly on the total gas surface density and radius. It must also depend on additional parameters.

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arXiv2011

Quantified HI Morphology II : Lopsidedness and Interaction in WHISP Column Density Maps

Lopsidedness of the gaseous disk of spiral galaxies is a common phenomenon in disk morphology, profile and kinematics. Simultaneously, the asymmetry of a galaxy's stellar disk, in combination with other morphological parameters, has seen extensive use as an indication of recent merger or interaction in galaxy samples. Quantified morphology of stellar spiral disks is one avenue to determine the merger rate over much of the age of the Universe. In this paper, we measure the quantitative morphology parameters for the HI column density maps from the Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies (WHISP). These are Concentration, Asymmetry, Smoothness, Gini, M20, and one addition of our own, the Gini parameter of the second order moment (GM). Our aim is to determine if lopsided or interacting disks can be identified with these parameters. Our sample of 141 HI maps have all previous classifications on their lopsidedness and interaction. We find that the Asymmetry, M20, and our new GM parameter correlate only weakly with the previous morphological lopsidedness quantification. These three parameters may be used to compute a probability that an HI disk is morphologically lopsided but not unequivocally to determine it. However, we do find that that the question whether or not an HI disk is interacting can be settled well using morphological parameters. Parameter cuts from the literature do not translate from ultraviolet to HI directly but new selection criteria using combinations of Asymmetry and M20 or Concentration and M20, work very well. We suggest that future all-sky HI surveys may use these parameters of the column density maps to determine the merger fraction and hence rate in the local Universe with a high degree of accuracy.

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arXiv2011

Quantified HI Morphology V: HI Disks in the Virgo Cluster

We explore the quantified morphology of atomic hydrogen (HI) disks in the Virgo cluster. These galaxies display a wealth of phenomena in their Hi morphology, e.g., tails, truncation and warps. These morphological disturbances are related to the ram-pressure stripping and tidal interaction that galaxies undergo in this dense cluster environment. To quantify the morphological transformation of the HI disks, we compute the morphological parameters of CAS, Gini, and M20 and our own GM for 51 galaxies in 48 HI column density maps from the VIVA project. Some morphological phenomena can be identified in this space of relatively low resolution HI data. Truncation of the HI disk can be cleanly identified via the Concentration parameter (C<1) and Concentration can also be used to identify HI deficient disks (1<C<5). Tidal interaction is typically identified using combinations of these morphological parameters, applied to (optical) images of galaxies. We find that some selection criteria (Gini-M20, Asymmetry, and a modified Concentration-M20) are still applicable for the coarse (~15" FWHM) VIVA HI data. The phenomena of tidal tails can be reasonably well identified using the Gini-M20 criterion (60% of galaxies with tails identified but with as many contaminants). Ram-pressure does move HI disks into and out of most of our interaction criteria: the ram-pressure sequence identified by Vollmer et al. (2009) tracks into and out of some of these criteria (Asymmetry based and the Gini-M20 selections, but not the Concentration-M20 or the GM based ones). Therefore, future searches for interaction using HI morphologies should take ram-pressure into account as a mechanism to disturb HI disks enough to make them appear as gravitationally interacting. One mechanism would be to remove all the HI deficient (C<5) disks from the sample, as these have undergone more than one HI removal mechanism.

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arXiv2011

Quantified HI Morphology I: Multi-Wavelengths Analysis of the THINGS Galaxies

Galaxy evolution is driven to a large extent by interactions and mergers with other galaxies and the gas in galaxies is extremely sensitive to the interactions. One method to measure such interactions uses the quantified morphology of galaxy images. Well-established parameters are Concentration, Asymmetry, Smoothness, Gini, and M20 of a galaxy image. Thus far, the application of this technique has mostly been restricted to restframe ultra-violet and optical images. However, with the new radio observatories being commissioned (MeerKAT, ASKAP, EVLA, WSRT/APERTIF, and ultimately SKA), a new window on the neutral atomic hydrogen gas (HI) morphology of a large numbers of galaxies will open up. The quantified morphology of gas disks of spirals can be an alternative indicator of the level and frequency of interaction. The HI in galaxies is typically spatially more extended and more sensitive to low-mass or weak interactions. In this paper, we explore six morphological parameters calculated over the extent of the stellar (optical) disk and the extent of the gas disk for a range of wavelengths spanning UV, Optical, Near- and Far-Infrared and 21 cm (HI) of 28 galaxies from The HI Nearby Galaxy Survey (THINGS). Though the THINGS sample is small and contains only a single ongoing interaction, it spans both non-interacting and post-interacting galaxies with a wealth of multi-wavelength data. We find that the choice of area for the computation of the morphological parameters is less of an issue than the wavelength at which they are measured. The signal of interaction is as good in the HI as in any of the other wavelengths in which morphology has been used to trace the interaction rate to date, mostly star-formation dominated ones (near- and far-ultraviolet). The Asymmetry and M20 parameters are the ones which show the most promise as tracers of interaction in 21 cm line observations.

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arXiv2011

Quantified HI Morphology III: Merger Visibility Times from HI in Galaxy Simulations

Major mergers of disk galaxies are thought to be a substantial driver in galaxy evolution. To trace the fraction and the rate galaxies are in mergers over cosmic times, several observational techniques, including morphological selection criteria, have been developed over the last decade. We apply this morphological selection of mergers to 21 cm radio emission line (HI) column density images of spiral galaxies in nearby surveys. In this paper, we investigate how long a 1:1 merger is visible in HI from N-body simulations. We evaluate the merger visibility times for selection criteria based on four parameters: Concentration, Asymmetry, M20, and the Gini parameter of second order moment of the flux distribution (GM). Of three selection criteria used in the literature, one based on Concentration and M20 works well for the HI perspective with a merger time scale of 0.4 Gyr. Of the three selection criteria defined in our previous paper, the GM performs well and cleanly selects mergers for 0.69 Gyr. The other two criteria (A-M20 and C-M20), select isolated disks as well, but perform best for face-on, gas-rich disks (T(merger) ~ 1 Gyr). The different visibility scales can be combined with the selected fractions of galaxies in any large HI survey to obtain merger rates in the nearby Universe. All-sky surveys such as WALLABY with ASKAP and the Medium Deep Survey with the APETIF instrument on Westerbork are set to revolutionize our perspective on neutral hydrogen and will provide an accurate measure of the merger fraction and rate of the present epoch.

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arXiv2011

Quantified HI Morphology IV: The Merger Fraction and Rate in WHISP

The morphology of the atomic hydrogen (HI) disk of a spiral galaxy is the first component to be disturbed by a gravitational interaction such as a merger between two galaxies. We use a simple parametrisation of the morphology of HI column density maps of Westerbork HI Spiral Project (WHISP) to select those galaxies that are likely undergoing a significant interaction. Merging galaxies occupy a particular part of parameter space defined by Asymmetry (A), the relative contribution of the 20% brightest pixels to the second order moment of the column density map (M20) and the distribution of the second order moment over all the pixels (GM). Based on their HI morphology, we find that 13% of the WHISP galaxies are in an interaction (Concentration-M20) and only 7% based on close companions in the data-cube. This apparent discrepancy can be attributed to the difference in visibility time scales: mergers are identifiable as close pairs for 0.5 Gyr but ~1 Gyr by their disturbed HI morphology. Expressed as volume merger rates, the two estimates agree very well: 7 and 6.8 x 10^-3 mergers Gyr^-1 Mpc^-3 for paired and morphologically disturbed HI disks respectively. The consistency of our merger fractions to those published for bigger surveys such as the Sloan Digital Sky Survey, shows that HI morphology can be a very viable way to identify mergers in a large HI survey. The relatively high value for the volume merger rate may be a bias in the selection or WHISP volume. The expected boon in high-resolution HI data by the planned MeerKAT, ASKAP and WSRT/APERTIF radio observatories will reveal the importance of mergers in the local Universe and, with the advent of SKA, over cosmic times.

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