genoveva micheva- ultra-compact dwarf galaxies

8/3/2019 Genoveva Micheva- Ultra-Compact Dwarf Galaxies

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Galaxies course essey

Ultra-Compact Dwarf Galaxies

Genoveva Micheva

[email protected]


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CONTENTS UCD Galaxies

Contents

1 History 2

1.1 The Fornax Cluster Spectroscopic Survey . . . . . . . . . . . 21.2 Follow-up with the VLT/Keck . . . . . . . . . . . . . . . . . . 31.3 UCDs in Abell1689 with the HST ACS . . . . . . . . . . . . . 41.4 The Fornax Compact Object Survey(FCOS) . . . . . . . . . . 51.5 Observations of the Virgo Cluster with the 2dF AAT . . . . . 6

2 Discussion 7

3 References 12

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UCD Galaxies

1 History

1.1 The Fornax Cluster Spectroscopic Survey

Ultra-Compact Dwarf galaxies(UCDs) were first observed in the Two-degree Field (2dF) Fornax Cluster Spectroscopic Survey (FCSS) at the 3.9mAnglo-Australian Telescope(AAT) (Drinkwater et al. 2001, Phillips et al.2001) [1, 2]. The survey provided a complete spectroscopic sample of allobjects in the magnitude range 16.5 < bJ < 19.7, regardless of morphology,i.e. star or galaxy, with the aim to detect all dwarf galaxies in an areacentered on the Fornax Cluster1. The FCSS gave four unresolved objectsand a fifth marginally resolved2.

Figure 1: HST UCDs in Fornax Figure taken from Drinkwater et al(2004) [8].

Drinkwater et al. found a new population of Fornax Cluster dwarf galaxiesthat are unlike any known type of stellar system, being so compact that theywere previously mistaken for stars in the Galaxy, see figure 1. Their redshiftindicates, however, that they are members of the Fornax Cluster. These5 objects were named Ultra-Compact Dwarfs(UCDs) due to their appear-ance. They have intrinsic sizes of 100pc and absolute MB in the range14.0 < MB < 11.5. The spectra obtained with the FCSS show them tobe old stellar systems - no Balmer lines are detected. The UCDs are smaller

1 Fornax cluster distance 20Mpc. Central massive galaxy - NGC1399.2Resolved means that the object would have to be larger than the PSF of the telescope

and does not appear as a point source.

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1.2 Follow-up with the VLT/Keck UCD Galaxies

and more concentrated than any known dwarf galaxy, but are 2-3 magni-

tudes more luminous than the largest globular clusters in the Galaxy. Theonly known objects they resemble both in luminosity and morphology arethe nuclei of nucleated dwarf ellipticals(dE,N) but without the surroundinglow surface brightness envelope.

The Fornax UCDs have luminosities of13.4 < MV < 11.9 mag, which isintermediate between typical globular clusters of MV 8 mag

3 and nor-mal4 dwarf galaxies of16 MV 11 mag. Since the UCDs thus suggestto occupy the empty space in the Kormendy diagram between globular clus-ters and dwarf ellipticals, this teams interpretation is that the UCDs arethe stripped nuclei of dwarf elliptical galaxies and as such are a new tracer

of galaxy disruption processes in clusters, see figure 3.

Figure 2: Properties of the Fornax UCDs as measured with the 2dF AAT. Table takenfrom Drinkwater et al(2004) [8]

1.2 Follow-up with the VLT/Keck

Drinkwater et al.(2004) [8] further observed the UCDs with VLT UV EchelleSpectrograph(UVES) and the Keck Telescope Echelle Spectrograph to studythe internal dynamics, and estimate the masses. Their results indicate thatthe internal velocity dispersion for the Fornax UCDs ranges from 24 to 37kms1, which is considerably higher than those of Galactic globular clus-

ters, see figure 3. Assuming the UCDs are dynamically relaxed systems onecan use their sizes and velocity dispersions to estimate their masses. Us-ing a Kings model mass estimator they obtain masses of 1-5 107M forthe UCDs, compared to 1.4 107M for a dE,N galaxy nucleus and around4106M for the most massive globular clusters. With all five Fornax UCDsspatially resolved in these observations their effective radii could be measured

3 -Cen, the brightest cluster in the Milky Way has an absolute magnitude MV 10.2

mag4 normal here means dE or dSph.

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1.3 UCDs in Abell1689 with the HST ACS UCD Galaxies

Figure 3: Kormendy diagram. Plot taken from Drinkwater et al(2004) [8].

to lie between 10 and 30 pc, see figure 2. Additionally, a lower surface bright-ness compact halo was detected around one of the UCDs. Using complemen-tary HST data to measure the total V-band luminosities of the UCDs, the de-rived UCD mass-to-light ratios were (M/L) = 2-4 (M/L), which is greaterthan the mass-to-light for typical globular clusters, (M/L) (M/L).

It is worth pointing out that star clusters are expected to have low M/L val-ues, whereas compact galaxies would have higher M/L values, due to darkmatter. Even though the mass-to-light ratio for UCDs is higher than that ofglobulars, oppinions differ on the existance of dark matter halos in galaxiesat such low masses.

1.3 UCDs in Abell1689 with the HST ACSMieske and Infante(2004) [10] searched for UCDs in Abell1689 with the HSTAdvanced Camera for Surverys(ACS). They found at least 10 possible UCDswith 12.7 < MV < 11.5 mag. The UCDs in Abell1689 were brighter andlarger than the Fornax UCDs.

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1.4 The Fornax Compact Object Survey(FCOS) UCD Galaxies

1.4 The Fornax Compact Object Survey(FCOS)

Mieske et al. 2004 [7] attempted to close the magnitude gap between theUCDs and bright GCs. To this end, they have carried out the Fornax Com-pact Object Survery (FCOS). In order to separate Fornax members fromMilky Way stars and background galaxies, a velocity slice of550 kms1 < vrad < 2400 kms

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was considered, where vrad is the radial velocity. The mean radial veloc-ity for Fornax members is 1425 45 kms1, with a velocity dispersion of = 326+42

32kms1. This refers to the entire sample of detected Fornax

compact objects, not just the UCDs. This vrad is consistent with previousresults concerning globular clusters.

Since the sample of compact objects belonging to Fornax was too small, theauthors were unable to make any quantitative statements regarding any dif-ference between radial velocity, magnitude, color and distribution in spaceof the UCDs compared to the other objects in the sample. Nevertheless,plotting the apparent V magnitude vs. the radial velocity in figure 4, showsthat the bright5 FCOS Fornax members have a higher mean velocity thanthe faint ones. The fainter objects agree with the GC System better thanthe brighter ones.

Figure 4: Apparent V magnitude plotted vs radial velocity. Crosses: All FCOS Fornaxmembers. The UCDs are those brighter than V = 19.5 mag(shaded area). Plot taken from Mieskeet al(2004) [7].

5Bright here means mV < 20 mag, faint is mV > 20 mag.

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1.5 Observations of the Virgo Cluster with the 2dF AAT UCD Galaxies

1.5 Observations of the Virgo Cluster with the 2dF AAT

The discovery of UCDs in the Fornax Cluster raises the question of howcommon these type of galaxies are in clusters and whether the local envi-ronment in a cluster affects their number densities or properties. It is thusnecessary to check the existance of UCDs in other galaxy clusters such asVirgo. A detection of UCDs around the Virgo Cluster would be informativeas it would indicate that their membership in galaxy clusters is paramountto their existance.

Figure 5: UCDs in the Virgo Cluster. Optical images. Each frame is 2 wide, north is up.Figure taken from Jones et al. 2005

Figure 6: Properties of the VUCDs. Table taken from Jones et al. 2005

Since the Virgo cluster is several times less dense than Fornax, Jones etal.(2005) [9] expected to find fewer UCDs per volume. However, they found 9Virgo UCDs(VUCDs) within a distance of14 to 150 kpc ofM87, see figure 5and 6. The absolute magnitudes of these objects range from MB = 12.9

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to 10.7, which is similar to the ones found in the Fornax Cluster, with

MB = 13.8 to 11.6. From figure 7 is it clear that these are old stellarpopulations.

Figure 7: Spectra of the 9 VUCDs. Figure taken from Jones et al. 2005 [9]

2 Discussion

To recap, the UCDs have luminosities, sizes and velocity dispersions simi-lar to the cores of nucleated dwarf galaxies both in the Fornax and Virgoclusters but are larger and have brighter mass-to-light ratios than even thelargest globular clusters. In a Kormendy diagram, figure 3, the UCDs liewell off the globular cluster L 1.7 relation, filling a previously unoccupiedpart of the diagram, but on an extrapolation of the elliptical galaxy L 4

Faber-Jackson law.

Many theories have been suggested in an attempt to explain their formation.Perhaps the simplest one is that these compact objects are simply super mas-sive globular clusters. Mieske et al. (2001) [5] suggest that the four (at thattime) unresolved objects detected in the Fornax Cluster are in fact globularsat the high luminosity end. With the later VLT observations the UCDs werespacially resolved, however, and their sizes, while very compact and smallindeed, outsize typical globular cluster scales of 3 pc. Nevertheless, Mieskeet al.(2004) [7] claim that one cannot readily discard the possibility of UCDs

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being GCs even though Drinkwater et al.(2004) [8] find that in the veloc-

ity dispersion-absolute magnitude (-M) diagram, aka. Kormendy diagram,the UCDs lie off the extrapolation of the relation defined by the GCs andrather appear to follow the Faber-Jackson relation for ellipticals. Mieske etal. argue that as this disagreement of the UCD values is only with respectto the extrapolation of the GC relation defined at fainter magnitudes and isthus an insufficient condition to separate UCDs from GCs. In other wordswe do not know where GCs at the brighter end would lie in a Kormendydiagram.

Fellhauer and Kroupa [4, 5] examine the possibility of the UCDs being

Figure 8: Magnitude vs surface brightness for dE,N and UCDs. 2.5 upper limitsused for the UCDs, shown as arrows. The two UCDs with formal detections are shown as squares.Virgo(large circles) and Fornax(small circles) dE,N galaxies are shown for comparison. Figuretaken from Jones et al. 2005

formed through the merging of stellar super-clusters in a tidal field. TheUCDS could have evolved through a phase of violent star-cluster interac-

tions to form a young stellar super cluster, which itself will turn into anobject with similar properties as the UCDs after having aged several Gyrs.Even for highly eccentric orbits these merger objects are stable when sub-

ject to strong tides and maintain most of their mass after 10Gyrs. As anexample one could consider the interactions of gas-rich disk galaxies like theAntennae, who produce knots of intense star-formation, which result in clus-ters of massive young star clusters6 that have masses ranging from 107 to

6 These clusters of star clusters are not to be confused with super stellar clusters. The

latter are individual massive star clusters.

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Figure 9: Plot Plot taken from Drinkwater et al. 2004 [8]

108 M and dimensions of a few hundred pc. N-body simulations of super-clusters demonstrate that they are very likely to merge, resulting in compact

merger objects with sizes spanning from massive globular clusters like OmegaCentaurus (-Cen) to Ultra-Compact Dwarfs up to very small dwarf ellip-ticals. Thus, merging of gas-rich galaxies folowed by interaction-triggeredstar-formation are possible origins for second-generation dwarf galaxies. Fell-hauer and Kroupa [6] further speculate if the most massive GC -Cen couldbe a UCD, since it shows signs of rotation and has unusual properties for aGC, such as star populations of different ages and metallicity. However, theN-body simulations for -Cen give a merger object that is too large and notheavy enough compared to -Cen.

Could the UCDs be the nuclei of extremely low surface brightness nucleated

dE,N galaxies? This possibility was considered both by Drinkwater et al.(2001,2004 [1, 8]) and Jones et al.(2005) [9] but quickly discarded - figures 8and 9 show that the UCDs are displaced from the dE,N galaxies by 5-6magnitudes in surface brightness. Careful analysis of UCD images showsno sign of low-luminosity envelopes around the UCDs in the Fornax surveys(Drinkwater et al. 2001,2004 [1, 8], Mieske et al. 2004 [7]). There are twoUCDs in the Virgo survey with detected low surface brightness envelopes,VUCD #6 and VUCD #7 (Jones et al. 2005 [9]). However, VUCD #6 hasthree discreet sources within 6 of its center, while VUCD #7 has severalsources within both 10 and 20. Even though the sources were masked

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UCD Galaxies

out, residual signal may still be contaminating the signal from the VUCDs.

Therefore it is not possible to claim with any confidence that there are ex-tended halos around the VUCDs.

Perhaps the most popular theory on UCD formation is they are the rem-nant nuclei of stripped dwarf galaxies which have lost their outer parts inthe course of tidal interaction in a process called galaxy threshing. Bekki,Couch and Drinkwater(2001) [3] carry out computer simulations involving atemplate dwarf galaxy interacting with a massive central galaxy(NGC1399)in the Fornax clusters potential to test the galaxy threshing scenario. Thesimulations show how nucleated dSph galaxies would evolve dynamicallyunder the strong tidal field of NGC1399. After about 4 passages the dwarf

loses its envelope almost entirely. The central nucleus also loses mass, buta small amount ( 18%). Since it is compact enough it survives. Varyingthe parameters of the simulation shows that not all nucleated dwarfs can betransformed into UCDs by tidal stripping. The two major implications fromthis simulation are that the stellar population in the UCDs is unlikely tobe young, because tidal stripping takes a long time and that the luminosityfunction of the UCDs is not necessarily similar to that of the nuclei of nu-cleated dwarfs because tidal stripping is a selective process: the extractionof the nucleus depends strongly on the orbits and masses of the dwarfs.

Since the repeated simulations of Bekki et al.(2003) [11] result in a galaxy

threshing that leaves a low surface brightness envelope around the nuclei ofthe dwarf, a detection of such envelopes around UCDs would be necessaryto formally support the galaxy threshing formation scenario. As previouslymentioned, only two UCDs in the Virgo cluster have such detections andthose are highly uncertain. Further, necessary conditions for the progenitordwarf would have to be a very eccentric orbit and a sufficiently high mass topartially survive the tidal disruption, as well as a shallow dark matter core inorder to allow substantial tidal disruption. These conditions are qualitativelyconsistent with the radial distribution of the UCDs found by Drinkwater etal. (2001,2004 [1, 8]) and Phillipps et al. (2001 [2]).

Mieske et al. 2004 [7] find supporting evidence for the galaxy threshing sce-nario. They argue that in a Color-Magnitude diagram (CMD) during thecourse of the transformation from dE,N to UCD, a galaxy would move in themagnitude-axis by the amount corresponding to its luminosity-loss. Its CMrelation is thus shifted redwards producing a magnitude difference betweenthe original dE,N and the final UCD of 4.1 mag. This would bring the CMrelation very close to the one found in Mieske et al. (2004 [7]). However,one needs to assume that the nucleus has the same color as the entire hostgalaxy and that the stellar population does not change during the threshing,so that only the mass and thereby the luminosity decrease during the pro-

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Figure 10: Fields 1,2,3 of the FCSS. Cluster members are solid points, possible membersare open circles. Size scales by apparent magnitude. The 6 UCDS are shown as crosses, UCD 6is labeled. Figure taken from Jones et al. 2005

cess. On average this is not true. The nuclei are found to vary, being about0.07 mag bluer than the underlying host galaxy. Mieske et al. [10] furtherargue that the UCDs found in Abell1689 are brighter and larger than theFornax and Virgo UCDs because they are in an intermediate stage of thestripping process.

Jones et al. (2005 [9]) find additional circumstancial evidence supportingboth the galaxy threshing and the star cluster merging scenario. They ex-amine two additional fields in the Fornax cluster, see figure 10. There are noUCDs detected in fields 2 and 3, despite similar selection criteria as in field1. This suggests that the existance of UCDs is strongly related to the pres-ence of a massive central galaxy in a dense environment. If their birth wasnot through merging of star clusters or galaxy threshing one would expectto detect them also at large distances from the cluster center. Instead, theyare all found inside a radius of 170 kpc of the cluster center.

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3 References

References

[1] M. Drinkwater, K. Bekki, W. Couch, S. Phillipps, B. jones, M. GreggUltra-Compact dwarf galaxies: a new class of compact stellar system

discovered in the Fornax Cluster, astro-ph, 20 June 2001

[2] S. Phillipps, M.J. Drinkwater, M.D. Gregg, J.B. Jones Ultra-CompactDwarf galaxies in the Fornax Cluster, astro-ph, 21 June 2001

[3] K. Bekki, W. Couch, M. Drinkwater Galaxy threshing and the forma-tion of ultra-compact dwarf galaxies, astro-ph, 22 June 2001

[4] M. Fellhauer, P. Kroupa The Formation of Ultra-Compact DwarfGalaxies, astro-ph, 29 Oct 2001

[5] M. Fellhauer, P. Kroupa Merging Massive Star Clusters as BuildingBlocks of Dwarf Galaxies?, astro-ph, 5 Dec 2001

[6] M. Fellhauer, P. Kroupa Cen - an Ultra Compact Dwarf Galaxy?,astro-ph, 20 Sep 2002

[7] S. Mieske, M. Hilker, L. Infante Fornax compact object survey FCOS:On the nature of Ultra Compact Dwarf galaxies, astro-ph, 29 Jan 2004

[8] M. Drinkwater, M.D. Gregg, M. Hilker, K. Bekki, W.J. Couch, H.C.Ferguson, J.B. Jones & S. Phillipps A class of compact dwarf galaxies from disruptive processes in galaxy clusters., astro-ph, 1 Dec 2004

[9] J.B. Jones et al. Discovery of Ultra-Compact Dwarf Galaxies in theVirgo Cluster, astro-ph, 6 Sep 2005

[10] S. Mieske, L. Infante Ultra Compact Dwarf galaxies in Abell1689: aphotometric study with ACS, astro-ph, 28 Jun 2004

[11] K. Bekki, W.J. Couch, M. Drinkwater and Y. Shioya Galaxy threshing

and the origin of ultra-compact dwarf galaxies in the Fornax cluster,astro-ph, 14 Aug 2003

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