1. New Clusters in the Halo of M31
The presentation will include:
goals of the Research Programme
work carried out to date
future work, i.e. the programme to completion
PURPOSE:
The aim of the presentation is to report to the members of the C.A.R., the goals of my Research Programme (MSc by Research), the work carried to date and the programme to completion.
CONTENT:
background theory
Post-AGB stars
dust in evolved stars
O-rich
C-rich
binarity and disks
where is the O-rich dust?
project description
goal of the project
use developed realistic models of the dust emission from post-AGB stars using a radiation transport code to, comparing with observational constraints, determine where is the located the O-rich material.
thesis: the O-rich material is located in stable circumbinary disc
schedule of work
radiative transfer code
ray tracing method
monte-carlo method
interpret and analyze the result
constraint models results with observational SEDs and images
Results
fits SEDs vs observations
observed images vs models images
case studies
a detailed understanding of exceptional objects may be useful for uncovering and evaluating the key physical processes involved in a class of objects
hd 44179 …
programme to completation
Structure:
Introduction
Purpose and context
Overview of main points
tell them what you are going to say
Main poits
Point1
development and explanation of ideas
examples
restatement of the point
Point 2 (…)
Summary and Conclusion
reinstatement and review of main points
conclusions and implications
New Clusters in the Halo of M31
Avon Huxor
Nial Tanvir, Mike Irwin, Rodrigo Ibata, Annette Ferguson, Geraint Lewis and Terry Bridges
‘The M31 Team’
March 2006
Centre for Astrophysics Research : Science & Technology Research Institute : University of Hertfordshire

2. Overview
Motivation
Search for and discovery of new outer halo clusters
The extended clusters
Properties of outer halo GC system
Spatial distribution
Luminosity distribution
Radial profile of cluster system
Discussion
Future work
Centre for Astrophysics Research University of Hertfordshire
March 2006

3. Motivation: Merger History of M31
Overall goal to study the assembly history of M31
Others on M31 Team focussing on stellar component. Found significant sub-structure, including Giant Southern Stream – see figure right. (Ferguson et al. 2005)
Keck data (Ibata et al.2005) show an extended disk, to 40 kpc (and components to 70 kpc.)
My primary aim to discover new outer halo globular clusters in M31 as chemical & dynamical probes. What can the halo GCs tell us about the history of M31?
northern spur
NGC205 loop NE
Shelf
G1 clump
stream
Centre for Astrophysics Research University of Hertfordshire
March 2006

4. Published Surveys
Revised Bologna Catalogue (of catalogues): based on original (3º x 3º), inner square region (green)
Inner regions and axes have subsequent follow-up, e.g. Perrett et al. spectroscopy (red), and small areas from HST archive,
Outer regions of Catalogue still contaminated and/or uncertain
Far Halo not in catalogue at all.
Inner ellipse - semi-major axis of 2 at i = 77.5
Outer ellipse – 4o ~55 kpc radius projected
Centre for Astrophysics Research University of Hertfordshire
March 2006

5. Search & Survey areas
Used semi-automated technique to identify GC candidates. Search on cuts in parameter space in magnitude, colour, size and ellipticity. Subsequent visual inspection gave final list
Mainly Isaac Newton Telescope – Wide Field Camera Survey. Over 50 square degrees. Now moving to CFHT Megacam.
Total of ~70 square degrees.
INT (red and blue)
- Plate scale 0.33”/pixel
CFHT 12K (green)
Plate scale 0.206”/pixel
CFHT MegaCam (grey: stipple and hatched)
Plate scale 0.187“/pixel
Centre for Astrophysics Research University of Hertfordshire
March 2006

6. New clusters
Discovered 39 new GCs, which is > 10% of the Revised Bologna Catalogue (RBC) list of 336 confirmed. But > 75% more than the RBC list of 50, for r > 1 deg
Of these 39, twelve have extended morphology (right)
In addition, ‘revised’ the RBC where possible. For r > 1 deg:
5 RBC confirms not GCs
1 RBC ‘star’ a GC
1 RBC unconfirmed a GC
Centre for Astrophysics Research University of Hertfordshire
March 2006

7. Spatial distribution New extended clusters (red) New GCs (green)
‘Consolidated’ RBC (blue stars) with r > 1 deg
Perrett spectroscopic survey (blue points)
New clusters extend far into the halo, > 100 kpc projected, and found in all survey areas to date, suggesting further clusters to be discovered
Centre for Astrophysics Research University of Hertfordshire
March 2006

8. Location in Mv – Rh of extended clusters (a)
Originally, it appeared that these three first clusters stood out in a plot of absolute magnitude vs half-light radius
Hence we took then as a separate population (Huxor et al. 2005)
Plot of Mv against log(Rh) for range of objects:
M31 clusters - filled circles
MW GCs – asterisks
MW dSphs – crosses
M31 dSphs – squares
UCO – triangles
NGC5128 massive GCs – points
Willman object – crossed circle
Also shown are the area for faint fuzzies, the line proposed by van den Bergh and above which stripped dwarfs are found, and a line of constant SB, indicating possible selection effect
Centre for Astrophysics Research University of Hertfordshire
March 2006

9. Location in Mv – Rh of extended clusters (b)
Gomez et al show that clusters in NGC 5128 move into this part of the parameter space
Centre for Astrophysics Research University of Hertfordshire
March 2006

10. Location in Mv – Rh of extended clusters (c)
The additional extended clusters in M31 show more of a continuum with MW GCs
But all 12 still in unoccupied region for MW GCs
McConnachie & Irwin (2006) note that the
M31 dSphs (squares) are also systematically larger than those of the MW (crosses) - an environmental effect of M31 system c.f. the MW?
Centre for Astrophysics Research University of Hertfordshire
March 2006

11. What do the extended clusters indicate?
Now a total of 12 extended (many of which are Pal-type) clusters with Rh > 10 pc
Mackey & van den Bergh (2005) list 12 MW GCs with Rh > 10 pc, of which 9 ‘young halo’ (YH), 2 ‘old halo’ (OH) , 1 ‘Sag Dwarf’ (SG)
Their YH is defined by second parameter effect, and believed to be of external origin (and note that Zhao 1998 attributes the 2 OH clusters: NGC 2419 & Pal 15, to accretion)
Very recently Hwang (2005) discovered very extended clusters in the LG dIrr NGC6822 very similar to my largest examples
Suggests that the M31 extended clusters too may be part of a YH analogue - an accreted system.
Centre for Astrophysics Research University of Hertfordshire
March 2006

12. Luminosity distribution
Mackey & van den Bergh also find that the MW YH population has an excess of less luminous clusters
GCLF for M31 halo is suggestive of this, distribution ‘less peaky’ as galactocentric distance increases, but K-S test does not show that they share parent distribution with YH, or any, MW subsystem.
May be due to low numbers, and M31 sample has faint magnitude limit. New MegaCam data will improve depth
Centre for Astrophysics Research University of Hertfordshire
March 2006

13. Radial profile of M31 GC system
Not unexpectedly, most MW YH clusters in far halo, with 33% have Rgc > 40 kpc. Compared to 0% for BD and 0.01 % for OH. What is the situation for all GCs in M31 far halo?
Plot of R1/4 vs log (GCs/arcmin2), with likely young BLCCs (Fusi Pecci et al. 2005) removed from sample, to obtain more natal sample.
Radial profile indicates an excess of clusters at large galactocentric radii
Unless we have all clusters in our outer survey areas
Errors can be driven down by further surveys
R1/4 arcmin
Log (GCs/arcmin2) 2o
Centre for Astrophysics Research University of Hertfordshire
March 2006

14. Stellar radial profile of M31
Irwin et al. (2005)
The V-band profile is illustrated in green and blue, and the i-band profile in black and red
The green and black circles: surface photometry, whereas the blue and red points are derived from star counts
The green dashed lines show a de Vaucouleurs R1/4 law
Break at ~ 2 deg, to power law profile
Guhathakurta et al. (forthcoming) find a similar result, from 20 out to 150 kpc
Centre for Astrophysics Research University of Hertfordshire
March 2006

15. Simulated radial profiles
Abadi et al. (2006), using numerical simulations. ‘In situ’ stars form in most massive progenitor, ‘accreted’ stars in other galaxies accreted later
Within certain radius, SB profile fit by an R1/4 + disk
Beyond this radius, profile dominated by accreted stars
Is this what we see in Irwin et al. ?
Centre for Astrophysics Research University of Hertfordshire
March 2006

16. M31: an accreted GC population in the far halo?
Extended clusters indicative of MW analogue YH population – possibly accreted
Radial profile matches recent stellar profiles with new, shallower, component
Although not statistically significant, LF suggestive of YH
NGC 6822 type dwarf irregular as source of very extended clusters?
Not unexpected given prevailing model of hierarchical assembly
However, results still tentative - additional data required
Centre for Astrophysics Research University of Hertfordshire
March 2006

17. Future Work
Work in progress
HST/ACS: Colour-magnitude diagrams for 4 extended and 9 other clusters. To determine metallicity, and any second parameter effect, indicative of ‘YH’ status
Keck/DEIMOS and Gemini/GMOS: velocity dispersion of extended clusters that is most distant (projected) from M31 (dark matter component?)
WHT/ISIS: radial velocities for many of the new clusters (most luminous)
New work
Extend surveys further. New MegaCam already taken, and more applied for.
Can we locate additional clusters and confirm the radial profile?
Can we identify any stellar substructure in far outer halo to match that found in the inner halo?
Centre for Astrophysics Research University of Hertfordshire
March 2006

18. Summary 39 new GCs found in the outer halo of M31
12 have an extended morphology
Extended clusters ‘connect’ to MW GC in parameter space, but still stand apart
Extended clusters suggestive of an accreted YH component, tentatively supported by radial profile
New data will resolve many of the outstanding issues:
Second parameter clusters?
Additional GCs to confirm radial profile and determine LF
Stellar substructure in far halo
Centre for Astrophysics Research University of Hertfordshire
March 2006

19. Clusters C1- 4
Two colour composite from HST/ACS F606W and F814W data (images: Nial Tanvir)
Centre for Astrophysics Research University of Hertfordshire
March 2006

20. Discovery of extended cluster
While investigating globular cluster candidate (in red circle)
Serendipitously discovered an extended cluster in field of view (yellow circle)
Unlike typical GC as seen in INT-WFC survey. So sought all surveys areas by eye
All cluster images 1’x1
Centre for Astrophysics Research University of Hertfordshire
March 2006

21. Photometry of C1-C3 GC-like luminosities (if at M31), and colours
But large half-light radii, all 12 have Rh.>10 pc
C1, C2 and C3 all have good fit to King profile
Similar to ‘faint fuzzies’, but more extended, bluer and not associated with visible disk ID V
(V-I)0
Rh (pc) MV C1
17.6
0.88 34
-7.1 C2
17.1
0.93 26
-7.7 C3
617
1.02
Centre for Astrophysics Research University of Hertfordshire
March 2006

22. Examples of new M31 halo globulars
INT i-band 0.5 arcmin x 0.5 arcmin images of 15 of the newly discovered GCs
Centre for Astrophysics Research University of Hertfordshire
March 2006

23. Isochrones below at [Fe/H] =-2.17 (M15), -1.58 (M2), -0.71 (47 Tuc)
Colour
CMDs consistent with metal-poor population at M31 distance - although C3 may be nearer
Isochrones below at [Fe/H] =-2.17 (M15), (M2), (47 Tuc)
Centre for Astrophysics Research University of Hertfordshire
March 2006