Pohlen, M.

Trujillo, I.

English

Abstract: We have in recent years come to view the outer parts of galaxies as having vital clues about their formation and evolution. Here, we would like to briefly present our results from a complete sample of nearby, late-type, spiral galaxies, using data from the SDSS survey, especially focused on the stellar light distribution in the outer disk. Our study shows that only the minority of late-type galaxies show a classical, exponential Freeman Type I profile down to the noise limit, whereas the majority exhibit either downbending (stellar truncation as introduced 1979 by Piet van der Kruit) or upbending profiles

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   University of GroningenThe outer disks of galaxiesPohlen, M.; Trujillo, I.Published in:The Astrophysical JournalIMPORTANT 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:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Pohlen, M., & Trujillo, I. (2005). The outer disks of galaxies: "To be or not to be truncated?". TheAstrophysical Journal, 626(2).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: 13-02-2023arXiv:astro-ph/0509057 v1   2 Sep 2005THE OUTER DISKS OF GALAXIES:“TO BE OR NOT TO BE TRUNCATED?”Michael Pohlen1 and Ignacio Trujillo21Kapteyn Astronomical Institute, University of Groningen,The Netherlands2Max–Planck–Institut für Astronomie, Heidelberg, GermanyAbstract We have in recent years come to view the outer parts of galaxies s having vitalclues about their formation and evolution. Here, we would like to briefly presentour results from a complete sample of nearby, late-type, spiral galaxies, usingdata from the SDSS survey, especially focused on the stellarlight distribution inthe outer disk. Our study shows that only the minority of late-type galaxies showa classical, exponential Freeman Type I profile down to the noise limit, whereasthe majority exhibit either downbending (stellar truncation as introduced 1979by Piet van der Kruit) or upbending profiles.1. Historical introductionWhy study outer disks? The structure of galactic disks is of fundamentalimportance for observationally addressing the formation and evolution of spi-ral galaxies. Especially theouter edgesof disk galaxies are of interest, sincesubstructure, the so called fossil evidence, is expected tobe imprinted by thegalaxy formation process. Recent formation and evolution scenarios suggestfor example that galaxies continue to grow from the accretion and tidal disrup-tion of satellite companions.What is their shape? The general shape of the surface brightness profile ofgalactic disks is currently one of our favourite paradigms,namely a pure ex-ponential disk. Its success is based solely on empirical evidence (albeit datingback now nearly 50 years) and has in fact never been fully physically moti-vated.In view of the great variety of structures among spirals ... there is good evidence,however, that at least in ordinary spirals the smoothed radial luminosity distri-bution is approximately exponential in the outer parts (de Vaucouleurs 1959)Eleven years later the general nature of disks was finally settled by Ken Free-man in his 1970 paper.2Almost every disklike galaxy with measuredI(R) shows an exponential disk ...and its origin is certainly a significant cosmogonic problem. (Freeman 1970)However, only nine years later Piet van den Kruit indicated that he exponentialnature does not hold to infinity, but[..] at the edges of the disk the decrease in apparent surfacebrightness is exceed-ingly steep. This sharp drop implies that galaxies do not retain their exponentiallight distribution to such faint levels. (van der Kruit 1979)This marked the detection of truncations at a safe distance of ∼ 4.5 timesthe radial scalelength, so the paradigm of the exponential disk was not in realdanger at the time.Where are they truncated? After van der Kruit and Searle’s seminal pa-pers about the structure of galactic disks (e.g. van der Kruit & Searle 1981),the matter of imperfectly exponential disks was rather ignored for some moreten years, until Barteldrees & Dettmar (1994) confirmed their ex stence usingfor the first time modern CCD equipment, but placed the cut-off cl ser to thecenter, for some galaxies at a disturbing close< 3 times the scalelength.Finally starting from the year 2000 several groups (e.g. Pohlen et al. 2000, deGrijs et al. 2001, Florido et al. 2001, Kregel et al. 2002, Pohlen et al. 2002, orErwin et al. 2005a) followed up the question of where the disks are truncated,how the shape of the profile in the very outer parts looks like,and if all diskshave a truncation. For a recent review see Pohlen et al. (2004).To keep a long story short our answer is that we know now that not all galaxiesare truncated, but those which are, are now believed to be truncated ’early’ (at∼ 3h), often abruptly (but not completely) and the profiles are best d scribedas a broken exponential and so probably better calledbr aksthancut-offs. Theprototypical break is recently observed for M 33 by Ferguson(this volume)using the star-count method as an independent approach compared to the sofar purely surface photometric measurements. The currently favoured originfor these breaks are global star-formation thresholds as describ d by Martin &Kennicutt (2001) or Schaye (2004), although this does not explain the originof the material beyond the break (see Pohlen et al. 2004).What next? What we still need is a complete census of the outer disk struc-ture in the local universe extending the work done by Courtea(1996) and deJong (1996) including a detailed discussion of the light profile in the outer re-gion to answer our famous question: “To be or not to be (truncated)?”. Thisis now done and will be presented briefly here and later in detail in Pohlen &Trujillo (2005, in prep.) for late-type galaxies and in Erwin et al. (2005b, inprep.) for early-type galaxies.The outer disks of galaxies 32. SampleWe used the LEDA online catalogue (the richest, most complete and up-to-date catalogue with homogeneous parameters of galaxies forthe largest avail-able sample) to select our initial galaxy sample using the following selectioncriteria: 2.99 < T < 8.49 (Sb-Sdm),log r25 < 0.301, vvir < 3250 km/s,|b2| > 20◦, andMabs < −18.4 B-mag. This leaves us with an unbiased,volume limited sample of late-type, face-on (i <∼ 60 deg), nearby (local) diskgalaxies (avoiding the Milky Way). Our sample complements the CCD imag-ing sample of∼ 65 early-type SB0-SBb galaxies by Erwin et al. (2005b, inprep.). The actual data we use for our present study come fromthe Sloan Digi-tal Sky Survey (SDSS, data release 2) providing images of∼ 15% (98/655) ofthe galaxies in our original LEDA sample.3. ResultsSDSS profiles: To convince the reader, and first of all ourselves, that SDSSimages with a rather short exposure time of only∼ 60s are indeed deep enoughto trace the outer disk we compared for three galaxies SDSS images with deepsurface photometry. The deep data is presented in Pohlen et al. (2002) andwas obtained at the CAHA 2.2m telescope using CAFOS. The total exposuretime of these images is about 3 hours reaching reliably down tµlim = 27.2R-mag/sqarcsec. As shown in Fig.1 (for two of the galaxies) the agreement isastonishingly good, allowing us to safely use SDSS images tostudy the profileclearly beyond the break radius.Figure 1. Comparison of the azimuthally averaged, radial surface brightness profiles fromSDSS images with much deeper imaging by Pohlen et al. (2002).The small, green squares areproduced by using the transformation to convert SDSS g’ and r’ into standard Johnson R bandfollowing Smith et al. (2002).4Classification: We classify each galaxy profile by eye according to the fol-lowing nomenclature. If there is no indication for any obvious break in theprofile the galaxy is classified as Type I following Freeman (1970). In the samesense, galaxies showing a profile better described as a broken exponential witha clear break and adownbending, steeper outer region are defined here as TypeII:Type II hasI(R) < I0 exp(−αR) in an intervalR1 < R < R2 not far fromthe center. (Freeman 1970)This definition includes the class of truncated galaxies shown by Pohlen etal. (2002). Although “not far from the center” may suggest toexclude trun-cated galaxies (where the break is at several radial scalelengths) from this class,thus following quote allows us to generalise Ken’s definitio:It is worth pointing out that for the[...] galaxies in which the Type II characteris-tic is most prominent[e.g. NGC 7793], the exponential disk begins outside themain region of the spiral-arm activity. (Freeman 1970)Finally, following Erwin et al. (2005a), galaxies showing abroken exponentialprofile with a break, but followed by anupbending, shallower outer region, arecalled Type III. Three proto-typical cases are shown in Fig.2.Frequencies and parameter distribution: The Type II class is split intoseveral sub classes (discussed in detail by Pohlen & Trujillo, 2005, in prep.)but the majority, about32.9% ± 5.1% of the total sample, are what we callnow classical truncations(Type CT), associated with those having a star for-mation threshold origin. An equal amount of galaxies (also32.9%) are infact anti-truncatedand fall into the Type III class. Finally, only the minority(15.3% ± 3.9%) of galaxies is barely consistent with being pure exponentialdisks of Type I.We find indications for a trend with Hubble Type: Early-type galaxies (Sb-Sc)are more commonly Type III while Type CTs seem to be significantly more fre-quent in later types. Counting the neighbouring galaxies around each galaxyreveals the fact that Type III galaxies statistically prefea high density, Type CTgalaxies a low density environment. However, this relationis far from beingclear-cut.The break radius for galaxies with classical truncations, the ones discovered byPiet van der Kruit in 1979, happens in the r’ band atRbreak = 2.5 ± 0.6 hinnerranging between 1.4 and 4.2. Since we can follow the profile ofthe Type Igalaxies down to∼ 6 − 8 times their scalelength, they seem to be genuinelyuntruncated galaxies.While the scalelength varies with filter (being larger in g’ compared to r’) asknown before, we do not find a systematic difference for the break radius. Wedo, however, find that the distribution of the surface brightness at the breakThe outer disks of galaxies 5Figure 2. The three main disk types: Type I, Type CT, and Type III(from top to bottom). Leftcolumn: Azimuthally averaged, radial (in units of[′′]) SDSS surface brightness profiles in theg’ and r’ band overlayed by r’ band exponential fits to the individual regions: single disk; innerand outer disk. Right column: r’ band images with the break radius marked as a red ellipse. Thewhite ellipse for the first Type I galaxy corresponds to roughly t e noise limit at∼ 140′′.radiusµbreak for galaxies with a classical truncation is peaked around a me nvalue. Together with the absence of a relation betweenRbreak/hinner and mass(rotational velocity), this favours a star formation threshold scenario for itsorigin.64. Stellar Disks truncations at high-zPérez (2004, and this volume) showed that it is possible to detect trunca-tions even out to high redshift (z ∼ 1). So we carefully defined a completesample of high redshift galaxies (Trujillo & Pohlen, 2005) using the ACS dataof the Hubble Ultra Deep Field. From the final sample of 36 galaxies, 21 showtruncations. Now, using the position of the truncation as a direct estimator ofthe size of the stellar disk it becomes possible to outright observe inside-outgrowth of galactic disks comparing the ACS to our local SDSS sample. Theresults suggest that the radial position of the truncation has increased with cos-mic time by∼ 1 − 3 kpc in the last∼ 8 Gyr indicating a small to moderate(∼ 25%) inside-out growth of the disk galaxies sincez ∼ 1 (see Trujillo &Pohlen, 2005).AcknowledgmentsM.P. would like to thank Peter Erwin and John Beckman for their stimulat-ing discussions and useful suggestions during this work. Part of this work wassupported by a Marie Curie Intra-European Fellowship within t e 6th Euro-pean Community Framework Programme.ReferencesBarteldrees, A., & Dettmar, R.-J. 1994, A&AS, 103, 475Courteau, S. 1996, ApJS, 103, 363de Grijs, R., Kregel, M., & Wesson, K. H. 2001, MNRAS, 324, 1074de Jong, R. S. 1996, A&A 313, 45de Vaucouleurs, G. 1959, Handbuch der Physik, 53, 311Erwin, P., Beckman, J. E., & Pohlen, M. 2005a, ApJ, 626, L81Florido, E., Battaner, E., Guijarro, A., Garzón, F., & Jiménez-Vicente, J. 2001, A&A, 378, 82Freeman K. C. 1970, ApJ, 160, 811Kregel, M., van der Kruit, P. C., & de Grijs, R. 2002, MNRAS, 334, 646Martin, C. L., & Kennicutt, R. C., Jr. 2001, ApJ, 555, 301Pérez, I. 2004, A&A, 427, L17Pohlen, M., Dettmar, R.-J., & Lütticke, R. 2000, A&A, 357, L1Pohlen, M., Dettmar, R.-J., Lütticke, R., & Aronica, G. 2002, A&A, 392, 807Pohlen, M., Beckman, J. E., Hüttemeister, S. H., Knapen, J. H., Erwin, P., & Dettmar, R.-J.2004, In Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning Fork Strikesa New Note, ed. D. L. Block, I. Puerari, K. C. Freeman, R. Groess, & E. K. Block (Dordrecht:Springer), 713Schaye, J. 2004, ApJ, 609, 667Smith, J.A., Tucker, D.L., Kent, S.M., et al. 2002, AJ, 123, 2121Trujillo, I., & Pohlen, M. 2005, ApJ, 630, L17van der Kruit, P. C. 1979, A&AS, 38, 15van der Kruit, P. C., Searle, L., 1981, A&A 95, 105

The outer disks of galaxies:"To be or not to be truncated?"

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The outer disks of galaxies:"To be or not to be truncated?"

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