Blok, W.J.G. de

Blyth, S.-L.

Bouchard, A.

Heyden, K. van der

Holwerda, B.W.

Prizkal, N.

English

The upcoming new perspective of the high redshift Universe in the 21 cm line of atomic hydrogen opens possibilities to explore topics of spiral disk evolution, hitherto reserved for the optical regime. The growth of spiral gas disks over Cosmic time can be explored with the new generation of radio telescopes, notably the SKA, and its precursors, as accurately as with the Hubble Space Telescope for stellar disks. Since the atomic hydrogen gas is the building block of these disks, it should trace their formation accurately. Morphology of HI disks can now equally be quantified over Cosmic time. In studies of HST deep fields, the optical or UV morphology of high-redshift galaxy disks have been characterized using a few quantities: concentration (C), asymmetry (A), smoothness (S), second-order-moment (M20), the GINI coefficient (G), and Ellipticity (E). We have applied these parameters across wavelengths and compared them to the HI morphology over the THINGS sample. NGC 3184, an unperturbed disk, and NGC 5194, the canonical 3:1 interaction, serve as examples for quantified morphology. We find that morphology parameters determined in HI are as good or better a tracer of interaction compared to those in any other wavelength, notably in Asymmetry, Gini and M20. This opens the possibility of using them in the parameterization pipeline for SKA precursor catalogues to select interacting or harassed galaxies from their HI morphology. Asymme try, Gini and M20 may be redefined for use on data-cubes rather than HI column density image

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   University of GroningenQuantified Morphology of HI Disks in the UniverseBlok, W.J.G. de; Holwerda, B.W.; Bouchard, A.; Blyth, S.-L.; Heyden, K. van der; Prizkal, N.Published in:Panoramic Radio Astronomy: Wide-field 1-2 GHz research on galaxy evolutionIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2009Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Blok, W. J. G. D., Holwerda, B. W., Bouchard, A., Blyth, S-L., Heyden, K. V. D., & Prizkal, N. (2009).Quantified Morphology of HI Disks in the Universe. In Panoramic Radio Astronomy: Wide-field 1-2 GHzresearch on galaxy evolution University of Groningen.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 14-02-2023arXiv:0908.0693v2  [astro-ph.CO]  6 Aug 2009Quantified Morphology of HI Disks in the UniverseB. W. Holwerda ∗, W. J. G. de Blok, A. Bouchard, S-L. Blyth, K. van der Heyden,Department of Astronomy, University of Cape Town, Private Bag X3, 7700 Rondebosch,Republic of South AfricaE-mail: holwerda@ast.uct.ac.zaN. PrizkalSpace Telescope Science Institute, Baltimore, MD 21218, USAThe upcoming new perspective of the high redshift Universe in the 21 cm line of atomic hydrogenopens possibilities to explore topics of spiral disk evoluti n, hitherto reserved for the opticalregime. The growth of spiral gas disks over Cosmic time can beexplored with the new generationof radio telescopes, notably the SKA, and its precursors, asaccurately as with the Hubble SpaceTelescope for stellar disks. Since the atomic hydrogen gas is the building block of these disks, itshould trace their formation accurately.Morphology of HI disks can now equally be quantified over Cosmic time. In studies of HSTdeep fields, the optical or UV morphology of high-redshift galaxy disks have been characterisedusing a few quantities: concentration (C), asymmetry (A), smoothness (S), second-order-moment(M20), the GINI coefficient (G), and Ellipticity (E). We have applied these parameters acrosswavelengths and compared them to the HI morphology over the THINGS sample. NGC 3184, anunperturbed disk, and NGC 5194, the canonical 3:1 interaction, serve as examples for quantifiedmorphology.We find that morphology parameters determined in HI are as good or better a tracer of interactioncompared to those in any other wavelength, notably in Asymmetry, Gini andM20. This opens thepossibility of using them in the parameterization pipelinefor SKA precursor catalogues to selectinteracting or harassed galaxies from their HI morphology. Asymmetry, Gini andM20 may beredefined for use on data-cubes rather than HI column density image.Panoramic Radio Astronomy: Wide-field 1-2 GHz research on galaxy evolution - PRA2009June 02 - 05 2009Groningen, the Netherlands∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licen. http://pos.sissa.it/Quantified HI Disk Morphology B. W. Holwerda1. IntroductionIn the coming years, the SKA precursors, MeerKAT, ASKAP, WSRT/APERTIF and EVLAwill take 21 cm line emission data of a large number of nearby and distant galaxies. The resultingdata-cubes and zero-order maps will need automated schemesfor source detection and characteri-sation. The morphology of HI disks is greatly informed by recent interaction (Hibbard etal., 2001)and harassment by passing dark matter haloes (Kazantzidis et al., 2008). Therefore an automatedmorphology classification of HI data is desirable, to comb the future catalogues of HI surveys forinteracting or perturbed galaxies. We propose to adapt morphological parameters currently in useto characterise UV/optical data of nearby and distant galaxies for use on HI data in order identifydifferent types of galaxy disks.2. Quantified MorphologyThe morphology of spiral galaxies has been quantified in Hubble Space Telescope images ofdistant galaxies and reference nearby galaxy samples in therestframe UV and optical using twoparameter schemes or a combination thereof. After the initial work by Abraham et al. (1994, 1996),the Concentration-Asymmetry-Smoothness (CAS) space was est blished by Conselice (2003) andthe Gini and Asymmetry parameters by Lotz et al. (2004). Scarlata et al. (2007) added ellipticityfor Hubble type classification. The benefits of quantified morph logy parameters are that there isno need for an observer, they can be applied to rest-frame UV images, which trace the triggeredstar-formation in interacting galaxies and the galaxies have an intrinsically high surface brightness,aiding their detection.The parameters used are:Concentration:C = 5 log( r80r20),wherer f is the radius of the circular aperture containingf percent of the light.Asymmetry:A = (I−I180)I ,whereI is the image,I180 is the image rotated by 180◦ around the galaxy’s centerSmoothness:S= (I−IS)I ,where I is the image,IS is the image smoothed with a certain kernel. The 0.2 Petrosian radiusboxcar is in use, but we used a 6" Gaussian. This is effectively a parameterized version of unsharpmasking (Takamiya, 1999; Malin & Hadley, 1997).Gini: G = 1Īn(n−1)Σi(2i −n−1)Ii ,whereIi is the value of pixel i in an ordered list,n is the number of pixels in the galaxy image, andĪ is the mean pixel value in the image. The Gini parameter is an index of equality in economics,with G=1 is perfectly unequal (all the flux in one pixel) and G=0 is perfectly equal (all pixel valuesthe same).2Quantified HI Disk Morphology B. W. HolwerdaFigure 1: THINGS H I maps (NA) for NGC 3184 and NGC 5194. The edge of the optical or stellar disk isat 3×1021 atoms cm−2, indicated by the red contour, and the extent of the HI disk (1019 atoms cm−2, theTHINGS detection limit) is marked with the blue contour.M20: the relative contribution of the 20% brightest pixels to thesecond order moment of the light inthe image. The second order moment of pixel i is defined as:Mi = Ii [(xi −xc)2+(yi −yc)2], whereIiis the value of pixel i in the image, andxi andyi are the x and y coordinates of that pixel andxc andycare the position of the galaxy’s centre. The total second order is:Mtot = ΣIi [(xi −xc)2+(yi −yc)2],andM20 is defined as:M20 = log(Σmi MiMtot), wherem is the 20% brightest pixels.Ellipticity : E = 1−B/A where A and B are the major and minor axes of the image.We note that many of these parameters need a predefined central position of the galaxy and allneed a definition of which pixels in an image belong to the object (a segmentation map). We usethe areas on the sky defined by HI contours to determine which pixels belong to our target galaxyand the dynamical centres reported in Walter et al. (2008).3. A case of two galaxies: NGC 3184 and NGC 5194The above parameters were computed within two contours, onec rr sponding to approxi-mately the extent of the stellar disk (3×1021 atoms/cm2) and one to the extent of the HI disk thatcould be observed with THINGS (Walter et al., 2008), (Figure1). We applied these contours onimages of two galaxies, NGC 3184 and NGC 5194, spanning UV, optical, near and far-infraredfrom GALEX, SDSS and Spitzer (see also Holwerda et al., 2010a, in preparation). The resultingmorphology parameter values are plotted in Figure 2, as computed in the stellar disk (red) and overthe extent of the HI disk (blue).3Quantified HI Disk Morphology B. W. HolwerdaWe note that the area over which these parameters are computed does not matter much formany of the parameters but there is extra information at eachw velength. The exception is the Giniparameter, which increases significantly from the stellar disk to the HI disk at all wavelengths. Theparameters dependence on wavelength is more significant; Asymmetry, Smoothness and Gini arehigh in star-formation and 21 cm line emission and Concentration peaks in the optical.4. HI morphology as a interaction tracerIn Figure 2 we can also compare the isolated, unperturbed galaxy NGC 3184 to the interactingNGC 5194 (M51). In the morphological parameters we can see that Concentration increases in theoptical and near-infrared from isolated to interacting. Asymmetry decreases a little from isolatedto interaction in UV and 24µm emission (the star-formation tracers) but increases for HI 21 cmemission. Both Gini andM20 increase, notably in HI. From Figure 2 we can conclude that thereis as much signal of interaction in HI as in the star-formation tracers (24µm and UV) but any HIclassification scheme will have to be fine-tuned for this signal. Fortunately, we can use existinguniform H I surveys such WHISP (van der Hulst et al., 2001) and THINGS (Walter et al., 2008) totest these morphology parameters (see Holwerda et al., 2010b,c,d, all in preparation)5. Use in Future SurveysThe coming HI surveys with the SKA precursors will produce a great number of resolved HIdatacubes, using a data-pipeline, producing data-cubes, moment order maps and a catalogue withdynamical (W20, W50, Vsys, Vrot etc.) and flux information. Morphological parameters such as theabove, CAS, Gini andM20, of the zero-order moment map (the HI surface density map) could beadded to the catalogue, with the intention to expand the catalogue’s parameter space such that firstcuts can easily be made for isolated, harassed, and interacting galaxies.6. Applicability to data-cubesThe application of morphological parameters to HI is thusfar limited to the zero-order map,predominantly for comparison to other wavelengths but one strength of HI data is its third dimen-sion, frequency. Like the spatial dimensions of the data, this i has a finite sampling and range. Inthe discussions at the Panoramic Radio Astronomy conferenc, P. Serra mentioned the possibilityof redefining these parameters for use on data-cubes. Following his suggestion, three of the aboveparameters could, in principle, easily be redefined to 3D morph logy parameters: Asymmetry, GiniandM20. In the case of asymmetry, the datacube would be mirrored around the central position (xc,yc, fc) in space and frequency, the Gini parameter would simply have more input-pixels ranked byvalue (unfortunately most of those would be essentially noise), and the second order moment of apixel i in the datacube would be redefined as:Mi = Ii [(xi −xc)2+(yi −yc)2+( fi − fc)2] with M20 tofollow (see Holwerda et al., 2010d). These three 3D morphology parameters, probably combinedwith zero-order map morphological parameters and dynamical indicators, could span a parameterspace in which it could be possible to determine which datacube has unique HI phenomena likewarps, “beards” (anomalous velocity gas on one side of the disk) and of course interaction andharassment.4Quantified HI Disk Morphology B. W. Holwerda5101520 00.511.5-6-4-2000.20.40.60.800.20.40.60.81 -1.5-1-0.50-6-4-2000.20.40.60.815101520 00.511.5-6-4-2000.20.40.60.800.20.40.60.81 -1.5-1-0.50-6-4-2000.20.40.60.81Figure 2: The six structural parameters for the isolated NGC 3184 and interacting NGC 5194 as a functionof wavelength. The values for the optical disk (red) and those f r the HI disk (blue). The dashed lines arefor the contours with equal pixel weights (set to 1) to serve as a reference (for Gini this value is 0, prefectequality). The optical values from Conselice (2003) for NGC3184 are indicated as black dots and the valuesfor the 3.6 and 24µm. from Bendo et al. (2007) are indicated with open squares.5Quantified HI Disk Morphology B. W. HolwerdaReferencesAbraham R. G., Valdes F., Yee H. K. C., van den Bergh S., 1994, ApJ 432, 75Abraham R. G., van den Bergh S., Glazebrook K., Ellis R. S., Santiago B. X., Surma P., GriffithsR. E., 1996, ApJS, 107, 1Bendo G. J., Calzetti D., Engelbracht C. W., Kennicutt R. 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Quantified Morphology of HI Disks in the Universe

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Quantified Morphology of HI Disks in the Universe

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