The Globular Cluster System of NGC 1399 IV. Some noteworthy objects
a r X i v :a s t r o -p h /0505320v 1 16 M a y 2005
Astronomy &Astrophysics manuscript no.objekterev2astroph February 2,2008
(DOI:will be inserted by hand later)
The Globular Cluster System of NGC 1399IV.Some noteworthy
objects
T.Richtler 1,B.Dirsch 1,https://www.sodocs.net/doc/df9687613.html,rsen 2,M.Hilker 3,and L.Infante 4
?
1
Grupo de Astronom′?a,Departamento de F′?sica,Universidad de Concepci′o n,Casilla 160-C,Concepci′o n,Chile e-mail:tom@coma.cfm.udec.cl
2
European Southern Observatory,Karl-Schwarzschild-Str.2,D-85748Garching,Germany 3
Sternwarte der Universit¨a t Bonn,Auf dem H¨u gel 71,D-53121
4
Departamento de Astronom′?a y Astrof′?sica,P.Universidad Cat′o lica,Vicu?n a Mackenna 4860,Santiago 22,Chile
Received /Accepted
Abstract.We present 8bright globular clusters and /or objects of less familiar nature which we found in the course of scruti-
nizing the globular cluster system of NGC 1399.These objects are morphologically striking,either by their sizes or by other structural properties.Some of them may be candidates for stripped dwarf galaxy nuclei,emphasizing the possible role of ac-cretion in the NGC 1399cluster system.They are all highly interesting targets for further deep spectroscopy or HST-imaging.Since these objects have been found within an area of only 42′2,we expect many more still to be detected in a full census of the NGC 1399cluster system.
Key words.galaxies:elliptical and lenticular,cD –galaxies:individual:NGC 1399–galaxies:star clusters
1.Introduction
NGC 1399is the central galaxy in the Fornax cluster.Due to its proximity (19Mpc)and richness,its globular cluster system is one of the most attractive cluster systems to study (for a compi-lation of literature on NGC 1399see the introduction of Dirsch et al.2003).
How such rich GCSs of giant elliptical galaxies have formed,is intimately linked to the formation history of the host galaxy itself.Several possibilities have been discussed:Tidal stripping of globular clusters (GCs)by encounters of neigh-bouring galaxies (Kissler-Patig et al.1999),accretion of GCs through the accretion of dwarf galaxies (Hilker et al.1999,C?o t′e et al.1998),formation of GCs in merger events (Ashman &Zepf 1992),in-situ formation during collapse (Forbes et al.1997).
In particular the brightest objects attract closer scrutiny.In NGC 1399,GC-like objects as bright as M V ≈?13mag have been identi?ed (Hilker et al.1999,Drinkwater et al.2000)which Phillips et al.(2001)labelled ”Ultracompact Dwarfs”(UCDs).Do these objects simply constitute the bright wing
of
2T.Richtler et al.:The Globular Cluster System of NGC1399IV.Some noteworthy objects
lar clusters and revealing structures which went unnoticed until now.
In this paper,we present a list of some of our brightest GCs showing under various aspects some sort of peculiarity regard-ing their sizes or other structural properties.Our aim is to draw the community’s attention to these interesting objects in order to investigate their properties further by better spatial or spec-troscopic resolution.
2.Observations
The spectroscopic observations,which resulted in radial veloc-ities of about470objects,have been obtained with FORS2 and the Mask Exchange Unit(MXU)at ESO’s Very Large Telescope in the period11/29/2000-12/1/2000.The grism was 600B,resulting in a resolution of about4?.These observa-tions are described in Dirsch et
al.2004.During this campaign, two FORS2?elds(area6.5×6.5′2)have been imaged in V and I in excellent seeing(0.6arcsec),resulting in marginal reso-lution of many GCs.These images are already documented in Dirsch et al.2003.The images have been exposed for300 sec in both V(Bessell V)and I(Bessell I).A few objects are well resolved and in some cases show faint large extensions or irregular shapes.These are the objects,which we list here. The quoted Washington photometric values are taken from Dirsch et al.2003,who used the4m CTIO telescope equipped with the MOSAIC camera to obtain wide-?eld photometry in Washington C1and Kron-Cousins R.
3.Morphological description
The objects which we present have been found by visual in-spection of the PSF-subtracted images.Most faint structure are better or only visible on the V-frame due to the much lower sky background with respect to the I frame.The PSF subtraction works excellent,removing also bright stars and leaving only faint residuals.Many of the bright globular clusters,which can be spectroscopically identi?ed,leave much brighter residuals than stars of comparable or even distinctly higher brightness. Of course,the resolution is not good enough to measure except for the largest ones reliable structural parameters,but it demon-strates that large GCs can be identi?ed by ground-based data out to a distance of20Mpc.In NGC1399,Larsen et al.(2001) measured e?ective radii with HST,but the present FORS?elds do not overlap with the HST-?elds.Here we select only objects which are particularly striking.
For estimating Washington metallicities,we used the cali-bration formula given by Harris&Harris(2002):
[Fe/H]=?6.04·(1?0.82(C?T1)+0.16(C?T1)2).
3.1.ISHAPE
ISHAPE is a code designed to measure the best?tting shape parameters of an object by iteratively convolving an analytic model with the point spread function(PSF)(Larsen1999). Here we choose a King model with a concentration param-eter of30,which has been applied to HST-observations of Fig.1.The?gure shows the ISHAPE-residuals for90:12and 91:93(upper panels).Displayed in each panel are the model convolved by the PSF(upper left),the object(upper right),the residual(lower left),and the weighting array(lower right).The residuals show that90:12is somewhat asymmetric and that a King30pro?le might be not the best representation of91:93. For comparison,we also show the residuals for an unresolved globular cluster(92:39,lower left)and a star of approximately R=19mag.
NGC1399clusters before(Larsen et al.2001)(named King30-pro?le in the following).The model reads
μ(r)～ 11+(r r c)2 2,
μ(r)being the surface brightness,r c the core radius,and r t the tidal radius.
The code gives the full-width-at-half-maximum(FWHM) and chi-square values of the model?t.Moreover,it returns the chi-square values of a convolution of a delta-function with the PSF.This is useful for an assessment,how well a given object is resolved.In our case the FWHM is always much smaller than the PSF(except for one object),so the resulting values cannot be accurate,but rather?rst guesses of their magnitudes.For all objects,we used a?tting radius of10pixels.Table2lists the values for our objects.The e?ective radii are of course model dependent and have been calculated by adopting r t as the ra-dius containing the total brightness.The corresponding central surface brightnesses are very uncertain and only can indicate a trend.More comments are given in the individual descrip-tions.For comparison,we give the?t values also for an unre-solved globular cluster(92:39)and a19th mag(R-band)star. In both cases ISHAPE returns a stellar appearance.Here the chi-square values of the King30models are meaningless.The delta-function chi-square values are very small,indicating an excellent?t.Figure1illustrates for90:12and91:93(UCD2), how the quality of the?ts can be assessed.The upper left panel for each object displays the model,the upper right panel the image,the lower left panel the residual,and the lower right panel the weighting scheme(black means low weight).More
T.Richtler et al.:The Globular Cluster System of NGC 1399IV .Some noteworthy objects 3
Table 1.The identi?cation of objects uses the numbering of Dirsch et al.(2004).The label FCOS is adapted from Mieske et al.(2002,2004),the label UCD from Drinkwater et al.(2003).We list coordinates and Washington colours as well from Dirsch et al..They are not available for two objects.V and I photometry may not be more accurate than 0.1mag,since no nightly calibration has been done.We used the standard zeropoints and neglected colour terms.Three objects are found only on the preimages and have no V ,I photometry assigned.Radial velocities are from Dirsch et al..Remarks refer to crossidenti?cations or peculiarities.
object
RA[2000]
Dec[2000]
T1
C-T1
V
V-I
rad.vel.
Remarks
Fig.3.Four spectra of the brightest objects are plotted in the wavelength range 4000?-5500?.The spectra are not ?ux calibrated and are shifted (except for 90:12)to permit a con-venient display.Individual shifts are indicated.Except perhaps for 91:93,they are too noisy to measure line indices.However,the high metallicity of 91:93and the strong Balmer lines of 78:12are striking.The ”emission”in 80:115at 5050?is an artefact.
comments are given in the individual descriptions.In addition,we give the residuals for the two unresolved objects.Only tiny residuals are left.
3.2.Description of Objects
Our spectra were designed to give radial velocities,not line in-dices.Therefore,most of them are too noisy to say anything about abundances or ages.In Fig.34spectra of the brighter
are not ?ux-calibrated (the rela-the spectrograph’s ?at ?eld)and are the individual shifts.In refer a few times to Balmer line marginally resolved by ISHAPE,
superposition on a faint,elongated N-E.The PSF-subtraction does not colours indicate a metallicity of gives for the appendage a V-I than the object itself,still con-colour.However,the appendage is might be not reliable.that the object is superimposed it is a candidate for a bright glob-tail.
stunning.The PSF-subtraction un-
covers a large halo,extending on our image to about 3′′to the North,corresponding to 270pc,which clearly is a lower limit.The e ?ective radius of 11pc only refers to the core.For com-parison,ω-Centauri has an e ?ective radius of 6.4pc,but with an absolute magnitude of M V =?10.5is only 0.4mag fainter.The central surface brightness consequently is low,but still nor-mal for globular clusters.The V-image shows two faint struc-tures,one going in the N-W direction,the other,like a tiny ”spiral arm”to the S-W.The N-W structure is also visible on our MOSAIC R-image.It is hardly visible on the VLT I-image probably due to the much higher sky background.The metal-licity from Washington photometry is -1.3dex.The spectrum shows strong Balmer lines.H γis much stronger than the neigh-bouring G-band,which one rather ?nds for very metal-poor clusters.However,higher spectroscopic S /N is needed for any attempt to determine its age.In total,78:12does not resemble neither a globular cluster nor a dwarf galaxy and apparently is a candidate for an initially larger but strongly distorted /stripped nucleated dwarf galaxy.
4T.Richtler et al.:The Globular Cluster System of NGC1399IV.Some noteworthy objects
Table2.This table lists the results of ISHAPE and the corresponding e?ective radii and central surface brighnesses in the V-band. The latter values are rough estimates only,using the relations quoted by Larsen(2001).A King30model has been applied.The di?erence between theχ2-values for a King pro?le and a Delta-function indicates the degree of resolution.
objectχ2(King-model/Delta-function)FWHM[′′]r e f f[pc]μ0(V)[mag/arcsec2]
Object80:115This object only appears on our V-preimage, which has been exposed for30s(seeing0.7′′).Deeper imag-ing is not available,thus the S/N is not su?cient for ISHAPE to work reliably.The PSF subtraction however shows a remnant with an approximate diameter of2′′.The real extent is presum-ably much larger and may even be larger than that of78:12 given the short exposure time.The Washington metallicity is -1.2dex and the Balmer lines are less strong than in the case of 78:12.
Object89:22Washington photometry of this resolved object is not available.The V-I colour indicates a metal-poor object, while the Balmer lines are only moderately strong.It looks compact,without a visible halo.The diameter is roughly2′′, corresponding to180pc.The e?ective radius according to ISHAPE is as large as that of78:12.It could be a fainter version of an UCD(Drinkwater et al.2003).
Object89:3389:33is only marginally resolved,as are many objects on our frames.It is interesting by showing a faint ap-pendage,perhaps a tidal tail?
Object90:12This is the object for which ISHAPE found the largest FWHM.It appears slightly elliptical.We estimate its total extension along the major axis to be at least350pc ac-cording to the remnant of the PSF subtraction.The e?ective radius is27pc,larger than those of the majority of the UCDs (Drinkwater et al.2003).Mieske et al.(2004)quote1331±149 km/s for its radial velocity,deviating by about2-sigma from our value.
It can be seen in Fig.1that the spherically symmetric model leaves a residual.Apparently,the object is asymmetric.No Washington photometry exists,but among our objects it has the bluest V-I colour,pointing to a strong metal de?ciency. The spectrum is noisy,but the Balmer lines appear not much weaker as those of78:12.The spatial resolution is good enough for an estimation of the central surface brightness.A value of μ0(V)≈19mag/arcsec2is quite low for a globular cluster.We estimate the central mass density using formula(7)of Larsen (2001)and getρ0≈170M⊙pc?3for an adopted(M/L)V of 4.There are just few galactic globular clusters having lower central mass densities.This combination of size and surface brightness,however,does not exist for any known cluster(see Fig.7of Huxor et al.2004).
Object91:93This is one of the UCDs(UCD2),which has been imaged by HST(Drinkwater et al.2003,de Propris et al. 2005).The Washington metallicity is-0.6,which excellently ?ts to the metallicity given by Mieske et al.(2002)derived from line indices.We show the spectrum for illustration,which is typical for a metal-rich globular cluster.This object gives us the possibility to compare the e?ective radius of ISHAPE with the more reliable one of HST.de Propris et al.(2005)quote an e?ective radius of20.3pc so our value of16pc(which is model dependent)shows that a King pro?le with c=30measures its extension only approximately.Fig.1shows a residual.It is sym-metric but our King30model underestimates the brightness of the central parts and the outer regions,while overestimating the brightness in an intermediate region.However,our central sur-face brightness ofμ0(V)≈16.3mag/arcsec2?ts?ts well to the pro?le of de Propris et al.
The remnant of the PSF has a diameter of about3′′,cor-responding to280pc.No halo is visible but there is a faint structure at the N-E,too faint to measure its colour reliably. Deeper images could provide colour measurements in order to see whether this is a faint background galaxy.Mieske et al. (2004)quote1280±58km/s for its radial velocity which agrees with our value within the uncertainty.
Object92:74The Washington metallicity of this object is ?0.9.ISHAPE resolves it marginally.It is superimposed on a faint elongated structure pointing towards N-E,the length be-ing approximately3arsec.This structure is marginally visible on our R-frame and not visible on the I-frame.Like75:85,it may be a candidate for showing a bright tidal tail.Mieske et al.(2004)quote997±152km/s for its radial velocity and thus agrees with our more accurate value.
FCOS1-063Mieske et al.(2002)list this object as a globu-lar cluster.Our image immediately reveals its elongation.The
T.Richtler et al.:The Globular Cluster System of NGC1399IV.Some noteworthy objects
5
Fig.2.Centered on the objects,these images show the residuals of9objects after PSF subtraction.The size of each image is 35×35′′.North is up,East to the left.See the text for individual descriptions.
subtraction of the PSF further shows that it actually consists of two sources,superimposed on a larger structure with major and minor axes of approximately2.5and1.5arcsec of length, corresponding to140kpc and230kpc,respectively.The sec-ondary object is2mag fainter than the primary.No di?erence in V-I color can be detected.It still could be a superimposed star but in conjunction with its other peculiarities it would be a quite strange coincidence.
The Washington color indicates a metallicity of-1.1.The absolute magnitude is M V=?11.The radial velocity relative to NGC1399is almost800km/s.If we assume(very proba-ble incorrectly)that the radial component represented the full orbital velocity around NGC1399the object was at its peri-galactic distance.This is the miminal space velocity which is possible relative to NGC1399.Already then it must have an extremely elongated orbit(Richtler et al.2004).The radial sep-aration from NGC1399is6arcmin,corresponding to a peri-galactic distance of33kpc.The apogalactic distance is then about200kpc(see Fig.20of Richtler et al.2004).The true space velocity relative to NGC1399probably is higher and
6T.Richtler et al.:The Globular Cluster System of NGC1399IV.Some noteworthy objects
the apogalactic distance larger than200kpc.Then the question arises whether such object can be bound to NGC1399or rather should be considered as an interlooper bound only to the entire Fornax cluster.
4.Discussion
The main aim of our presentation is to call the community’s attention to the present objects.Given that almost all have been found on two FORS?elds only after a modest exposure time, the cluster system of NGC1399must host a multitud of similar objects.Extended speculations on the nature of these sources are inappropriate but a few remarks may be given.
What is the nature of the secondary point source in the case of FCO1-063?Its association with the main source is un-clear,although likely.It can be a candidate for a nucleus of a stripped dwarf galaxy with one globular cluster,which sur-vived tidal stripping.One is tempted to think of a cluster merg-ing with the nucleus,as predicted by Oh&Lin(2000)and Lotz et al.(2001).Long-slit spectroscopy in excellent seeing would prove/disprove its association with FCO1-063.
The interpretation of UCD’s discovered by Drinkwater et al.(2000)and Hilker et al.(1999)as remnants of dwarf galax-ies may also hold for fainter objects.Depending on the ini-tial con?guration and the details of the interaction process,”galaxy threshing”(Karick et al.2003,Bekki et al.2001,2003) may produce a large variety of morphological appearance,the UCDs only being the brightest ones.However,de Propris et al.2005compared surface brightness pro?les of UCDs and dwarf galaxy nuclei and found the latter to have systemati-cally lower central surface brightnesses,weakening the case for galaxy threshing.Regarding their central surface brightness, our objects,except90:12,resemble fainter versions of UCDs rather than dwarf galaxy nuclei.90:12and perhaps80:115 (where deeper observations are required)seem to be quite dif-ferent from UCDs.
The role of accretion of dwarf galaxy nuclei might thus be important in giant ellipticals,particularly for a central giant el-liptical like NGC1399.This is also interesting in the following context:Bimodal colour distributions are common in globu-lar cluster systems.However,although the bimodality in NGC 1399is very pronounced it disappears for the bright end of the luminosity function(Dirsch et al,2003)indicating that this population has a di?erent history than the bulk of the fainter clusters.The sample of bright compact objects of Mieske et al. (2004)emphasizes this point again.
Another possibility is that Blue Compact Dwarf galaxies (BCDs)may be progenitors.An interesting object in that re-spect is POX186(e.g.Kunth1981,Corbin&Vacca2002, Guseva et al.2004).Such extremely compact star-forming dwarf galaxies may well develop into objects like78:12or 80:115after star formation has ceased.The formation of”super star clusters”and perhaps their subsequent merging(Fellhauer &Kroupa2002)does not need a merger of large spiral galax-ies,but also occurs in BCDs(e.g.Vanzi2003).Dwarf galaxies may therefore contribute in various ways to the population of bright and compact objects.Also the”faint fuzzies”in lentic-ular galaxies(Brodie&Larsen2003)and the recently discov-ered globular clusters with large(30pc)e?ective radii in M31 (Huxor et al.2004)show that the morphology of globular clus-ters is much broader than previously thought.The brightnesses of our objects are compatible with the bright tail of the lumi-nosity function of GCs in NGC1399(e.g.Dirsch et al.2003). Once a statistically signi?cant sample of these objects has been assembled,one can compare their luminosity function with that of GCs(for a recent review see Richtler2003)and search for di?erences.
It is therefore of high interest to perform a complete census around NGC1399.HST with the Advanced Survey Camera would of course be preferable,but ground-based imaging with a seeing of0.5”is also feasible and probably would uncover many more interesting and surprising objects. Acknowledgements.We thank an anonymous referee for constructive remarks.T.R.,B.D.,and L.I.acknowledge support from the FONDAP center for astrophysics,Conicyt15010003.
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