On MOND and dark matter in ultra compact dwarf galaxies

Transcription

1 On MOND and dark matter in ultra compact dwarf galaxies Jörg Dabringhausen Argelander-Institut für Astronomie, University of Bonn Pavel Kroupa Michael Hilker, Michael Fellhauer, Steffen Mieske, Michael Marks,...

2 Properties of ultra compact dwarf galaxies (UCDs) luminosities between 10 7 and 10 solar luminosities 6

3 Properties of ultra compact dwarf galaxies (UCDs) luminosities between 10 7 and 10 solar luminosities Half-light radii of less than 50 pc 6

4 Properties of ultra compact dwarf galaxies (UCDs) luminosities between 10 7 and 10 solar luminosities Half-light radii of less than 50 pc 6 high densities

5 Properties of ultra compact dwarf galaxies (UCDs) luminosities between 10 7 UCD and 10 solar luminosities 6 Half-light radii of less than 50 pc de high densities Image by M. Hilker

6 Properties of ultra compact dwarf galaxies (UCDs) luminosities between 10 7 UCD and 10 solar luminosities 6 Half-light radii of less than 50 pc de high densities intermediate to high ages Image by M. Hilker

7 Properties of ultra compact dwarf galaxies (UCDs) luminosities between 10 7 UCD and 10 solar luminosities 6 Half-light radii of less than 50 pc de high densities intermediate to high ages most of them are found in galaxy clusters Image by M. Hilker

8 From close distance, a UCD probably looks similar to this: Image from ESO

9 From close distance, a UCD probably looks similar to this: ω Cen Image from ESO

10 The M/L ratios of UCDs The M/L ratios of UCDs are of particular interest: GCs UCDs (Data from Mieske et al.2008)

11 The M/L ratios of UCDs The M/L ratios of UCDs are of particular interest: GCs UCDs A part of the difference between ʻclassicalʼ globular clusters and UCDs can be explained by the evolution of ʻclassicalʼ globular clusters......but not all of it. (Data from Mieske et al.2008)

12 The M/L ratios of UCDs Compare the M/L ratio of UCDs with predictions from models for stellar populations.

13 The M/L ratios of UCDs Compare the M/L ratio of UCDs with predictions from models for stellar populations. For this, an IMF has to be assumed:

14 The M/L ratios of UCDs Compare the M/L ratio of UCDs with predictions from models for stellar populations. For this, an IMF has to be assumed: The canonical IMF

15 The canonical IMF: α=1.3 α=2.3

16 The canonical IMF: α=1.3 α =2.3 This IMF is apparently invariant in open clusters (Kroupa 2001), which makes it a good initial assumption for UCDs as well.

17 The M/L ratios of UCDs predicted M/L ratio

18 The M/L ratios of UCDs 32 UCDs predicted M/L ratio 14 UCDs

19 The most likely M/L ratios of UCDs exceed the prediction of even very old simple stellar population models (with the canonical IMF)! (Mieske & Kroupa 2008) (Dabringhausen, Hilker & Kroupa 2008)

20 MOND in UCDs?

21 MOND in UCDs? GCs UCDs average radii of UCDs increase with their mass! (Data from Mieske et al.2008)

22 MOND in UCDs? half-mass radius: 33pc Image from Haghi et al. (2009)

23 MOND in UCDs? half-mass radius: 33pc UCDs Image from Haghi et al. (2009)

24 MOND in UCDs? half-mass radius: 33pc UCDs MONDian and Newtonian mass estimaes are almost the same for UCDs Image from Haghi et al. (2009)

25 MOND in UCDs? half-mass radius: 33pc UCDs MONDian and Newtonian mass estimaes are almost the same for UCDs MOND does not solve the problems of the elevated M/L ratios in UCDs Image from Haghi et al. (2009)

26 Dark matter in UCDs?

27 Dark matter in UCDs? This cannot be non-baryonic cold dark matter - UCDs are to compact to contain an appreciable amount of it (Murray 2009).

28 Dark matter in UCDs? This cannot be non-baryonic cold dark matter - UCDs are to compact to contain an appreciable amount of it (Murray 2009). So, could the IMF vary after all? - This would allow for more baryonic (almost) dark matter in some systems.

29 Dark matter in UCDs? This cannot be non-baryonic cold dark matter - UCDs are to compact to contain an appreciable amount of it (Murray 2009). So, could the IMF vary after all? - This would allow for more baryonic (almost) dark matter in some systems. Motivation: UCDs may be associated with the most extreme star bursts (not observed locally). Thus, we may observe an invariant IMF in resolved systems, but it needs not be invariant everywhere.

30 A possible explanation: A top-heavy IMF

31 A possible explanation: A top-heavy IMF top-heavy IMF: for old populations, high M/L ratio through over-abundance of stellar remnants

32 Top-heavy IMF in UCDs Find an IMF so that stellar remnants can explain the M/L ratio of UCDs (Dabringhausen, Kroupa & Baumgardt 2009)

33 Top-heavy IMF in UCDs Find an IMF so that stellar remnants can explain the M/L ratio of UCDs (Dabringhausen, Kroupa & Baumgardt 2009)

34 Top-heavy IMF in UCDs Find an IMF so that stellar remnants can explain the M/L ratio of UCDs (Dabringhausen, Kroupa & Baumgardt 2009)

35 Would UCDs with a top-heavy IMF survive their early evolution?

36 Would UCDs with a top-heavy IMF survive their early evolution? Perform N-Body simulations of UCDs with mass-loss through gas explusion and stellar evolution

37 Would UCDs with a top-heavy IMF survive their early evolution? Perform N-Body simulations of UCDs with mass-loss through gas explusion and stellar evolution UCDs can also form with top-heavy IMFs, but this implies extreme initial conditions for them. (Dabringhausen, Fellhauer & Kroupa 2010)

38 Initial parameters thereby implied for UCDs small present-dayucd massive present-day UCD Mass [10 Solar masses ] (Dabringhausen, Fellhauer & Kroupa 2010) 1000

39 Consider a UCD with some 10 solar masses today 7

40 Consider a UCD with some 10 solar masses today 7 Initially, it may have had: A mass of some 10 solar masses... 8

41 Consider a UCD with some 10 solar masses today 7 Initially, it may have had: A mass of some 10 solar masses but a half-mass radius of only a few pc! (expansion through mass-loss!)

42 Consider a UCD with some 10 solar masses today 7 Initially, it may have had: A mass of some 10 solar masses but a half-mass radius of only a few pc! (expansion through mass-loss!) A population of 10 O-stars... 6

43 Consider a UCD with some 10 solar masses today 7 Initially, it may have had: A mass of some 10 solar masses but a half-mass radius of only a few pc! (expansion through mass-loss!) A population of 10 O-stars......with a total luminosity of 10 solar luminosities. 6 11

44 Another clue to top-heavy IMFs: Abundance of neutron stars Compared to one with the canonical IMF, a stellar system with a top-heavy IMF should have many neutron stars.

45 Another clue to top-heavy IMFs: Abundance of neutron stars Compared to one with the canonical IMF, a stellar system with a top-heavy IMF should have many neutron stars. Thus, it can have many binary systems where a neutron star accretes matter from a close companion star, so called low-mass X-ray binaries (LMXBs).

46 Low-mass X-ray binaries LMXBs make neutron stars visible as bright X-ray sources.

47 Low-mass X-ray binaries LMXBs make neutron stars visible as bright X-ray sources. The creation of LMXBs is driven by encouters involving stars and neutron stars - such encounters can make binaries close enough for accretion from the star to the neutron star.

48 Low-mass X-ray binaries LMXBs make neutron stars visible as bright X-ray sources. The creation of LMXBs is driven by encouters involving stars and neutron stars - such encounters can make binaries close enough for accretion from the star to the neutron star. The frequency of such encounters is measured by the encounter rate: Γ n s n ns r 3 c σ (Verbunt 2003)

49 LMXBs in globular clusters and UCDs in Virgo The encounter rate is given as Γ n s n ns r 3 c σ. GCs constant radii UCDs radii vary with mass Γ for non-changing stellar mass function

50 LMXBs in globular clusters and UCDs in Virgo The encounter rate is given as Γ n s n ns r 3 c σ.

51 LMXBs in globular clusters and UCDs in Virgo The encounter rate is given as Γ n s n ns r 3 c σ.?

52 LMXBs in globular clusters and UCDs in Virgo The encounter rate is given as Γ n s n ns r 3 c σ.? Is there an IMF, such that the probability for an LMXB in a UCD can be proportional to Γ?

53 LMXBs in globular clusters and UCDs in Virgo The encounter rate is given as Γ n s n ns r 3 c σ.? Is there an IMF, such that the probability for an LMXB in a UCD can be proportional to Γ? A changing IMF implies n s n ns changes in, and σ.

54 There is such an IMF:

55 There is such an IMF: (Dabringhausen, Marks & Kroupa, in preparation)

56 There is such an IMF: result similar to the one that came from the M/L ratios (Dabringhausen, Marks & Kroupa, in preparation)

57 LMXBs in globular clusters and UCDs in Virgo?

58 LMXBs in globular clusters and UCDs in Virgo top-heavy IMF

59 Conclusions

60 Conclusions UCDs generally have a higher mass-to-light ratios than expected, if they are a pure stellar population that formed with the canonical IMF.

61 Conclusions UCDs generally have a higher mass-to-light ratios than expected, if they are a pure stellar population that formed with the canonical IMF. A top-heavy IMF is a possible explanation for this finding.

62 Conclusions UCDs generally have a higher mass-to-light ratios than expected, if they are a pure stellar population that formed with the canonical IMF. A top-heavy IMF is a possible explanation for this finding. A top-heavy IMF does not contradict the survival of UCDs until today...but their initial conditions must have been very extreme.

63 Conclusions UCDs generally have a higher mass-to-light ratios than expected, if they are a pure stellar population that formed with the canonical IMF. A top-heavy IMF is a possible explanation for this finding. A top-heavy IMF does not contradict the survival of UCDs until today...but their initial conditions must have been very extreme. The frequency of LMXBs (i.e. neutron stars) in Virgo- UCDs also suggests a top-heavy IMF.