1. Mergers in the Local Universe
9/6/2018
Brad Whitmore
Galaxy Mergers: From the Local Universe to the Red Sequence STScI, October 4, 2006
Top “Ten’’ Merger Papers - (1972 to 1996)
- What did we learn?
- Do we still believe it?
I am here this morning to talk about and enhancement to the Hubble archives called the Hubble Legacy Archive.

2. Ground Rules
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Merger paradigm took off ~ 35 years, so focus on this time period.
Earlier papers mainly just pointed out something interesting was going on without leading to much follow up.
- Duncan (1923) – Antennae photo and description.
- Holmberg (1941) – First “N-body” simulation using light
bulbs ! (since light follows same inverse square law as gravity).
- Zwicky – Systematic study of peculiar galaxies.
- Vorontsov-Velyaminov (1959), Arp (1966), Arp & Madore (1987) - Catalogs of peculiar galaxies -

3. CTIO B,V,R Image courtesy of
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ApJ 57, 137 (1923)
NGC 4038/39
CTIO B,V,R Image courtesy of
John Hibbard

4. Eight Famous Merger Papers
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Eight Famous Merger Papers
1. Toomre & Toomre (1972) – Recognizing mergers; N-body simulations
2. Ostriker & Tremaine (1975) – Dynamical friction; cD’s from galactic cannibalism
3. Toomre (1977) – Mergers between spirals make ellipticals?
4. Larsen & Tinsley (1978) – Mergers promote star formation. -
5. Tremaine (1981), van den Bergh (1982) – Dissent.
6. Sanders et al. (1988) – Mergers make quasars?
7. Holtzman et al. (1992) – Mergers make globular clusters?
8. Abrahams (1996) – Lots of mergers in early universe.

5. 9/6/2018
# 1. Toomre + Toomre
(1972) -

6. # 1 – Toomre & Toomre (1972) – “Galactic Bridges & Tails”
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# 1 – Toomre & Toomre (1972) – “Galactic Bridges & Tails”
Showed that many “peculiar galaxies” were mergers.
Opened the era of N-body simulations.
Still believe ? - Yes
- May be in danger of overdoing it. Anything asymmetric is often consider a merger now. -

7. Mergers can also be recognized by their HI signatures.
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Mergers can also be recognized by their HI signatures.
e.g., Hibbard & van Gorkom (1996) -

8. Dozens of papers with N-body simulations of mergers have been written.
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Dozens of papers with N-body simulations of mergers have been written.
e.g., Barnes (1988) – Antennae “look-alike” -

9. # 2. Ostriker & Tremaine (1975)
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# 2. Ostriker & Tremaine (1975) -

10. - Harassment may affect non-central galaxies as well.
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# 2 – Ostriker & Tremaine (1975) – “Another Evolutionary Correction to the Luminosity of Giant Galaxies”
The central giant galaxies in clusters (i.e., cD galaxies) are made by “galactic cannibalism” through the process of dynamical friction.
Still believe ? - Yes
- Harassment may affect non-central galaxies as well. -
Dressler (1979)

11. # 3. Toomre (1977) - “Mergers and Some Consequences”
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# 3. Toomre (1977) - “Mergers and Some Consequences”
Toomre’s calculation of the total number of merger remnants based on the fact that there are ~10 NGC galaxies in the Toomre Sequence, each lasting ~0.5 Gyr.
Beautiful example of how a single graphic can make an argument crystal clear. -

12. # 3 - Toomre (1977) – “Mergers and Some Consequences”
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# 3 - Toomre (1977) – “Mergers and Some Consequences”
Predicts 10 % of galaxies should be merger remnants (like Toomre sequence galaxies)
~10 % of NGC galaxies are ellipticals.
Could it be that mergers form ellipticals ?
Still believe ? – Yes, more or less
- Hubble sequence might be transition.
- Not so simple. Some issues fitting dynamics. (Rothberg talk) -

13. NGC 7252 “Atoms-for-Peace” galaxy as an example.
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Schweizer (1982) helped convince people that mergers could produce galaxies with single elliptical-looking nuclei, using
NGC 7252 “Atoms-for-Peace” galaxy as an example. -

14. 9/6/2018
HST increased the sample of protoglobular cluster candidates from a half dozen to several hundred.
Also showed that the galaxy was still forming stars (and clusters) in the inner region. -

15. “Could it be like Leonardo da Vinci before him,
Edwin Hubble put his most secret thoughts in
mirror writing?” – Schweizer (1990)

16. 9/6/2018
# 4 – Larson & Tinsley
(1978)
Galaxies in the Arp Atlas have more scatter, and are found blueward (younger) compared to a sample of “normal galaxies from the Hubble Atlas. -

17. Mergers promote star formation
# 4 – Larson & Tinsley (1977) – “Star Formation in Normal and Peculiar Galaxies”
Mergers promote star formation
U-B vs. B-V diagrams are very useful diagnostics
Still believe ? - Yes
- Most of the star formation turns out to be in clusters. (Fall talk)
9/6/2018 -

18. 9/6/2018
Schweizer & Seitzer (1992)
Showed that a “Fine Structure index” could be used to identify intermediate-age mergers.
The index included terms for ripples, jets of luminous material, x-structure, boxiness -

19. #5 – Tremaine (1981) #5 – van den Bergh (1982)
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#5 – Tremaine (1981)
#5 – van den Bergh (1982) -
“After a collision, a car is a wreck, not a new type of car.” - deVaucouleurs (1974)
"I do not think you can make rocks by merging clouds…“ Gunn (1987)

20. # 5 – Tremaine (1981), van den Bergh (1982) - Dissent
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# 5 – Tremaine (1981), van den Bergh (1982) - Dissent
Merger hypothesis can’t explain:
High % of E’s in galaxy clusters.
Low angular momentum in observed ellipticals. …
Globular cluster S_n.
Still believe ? – No
- Most mergers in outer subgroups
- Ellipticals have had more complex histories
- New globular clusters can form in mergers
van den Bergh (1982) suggested that mergers of spirals might form spirals.
Probably right in many cases -

21. 9/6/2018
# 6. Sanders et al. (1988) -

22. # 6 – Sanders (1988) – “ULIRGS and the Origins of Qusars”
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# 6 – Sanders (1988) – “ULIRGS and the Origins of Qusars”
Nearly all ULIRGS are mergers.
The central energy source is a mixture of starburt and AGN.
This phenomena might be what makes quasars.
Still believe ? – ULIRGS are
mergers: yes
- Mergers make quasars: maybe -

23. 9/6/2018
# 7 – Holtzman et al.
(1992) -
Note: Ashman & Zepf (1982) predicted mergers could make globular clusters before the Holtzman observations !

24. Dozens of similar studies since.
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# 7 – Holtzman et al (1992)
Used HST to discover ~60 luminous, young compact clusters in NGC Suggested they were protoglobular clusters.
Dozens of similar studies since.
(e.g., Antennae - Whitmore et al 1995, 1999; starbursts – Meurer 1993; normal spirals – Larsen & Richtler 2002) -
Still believe ? – Yes,
- Though only ~1 in 1000 survive (Fall et al. 2005).
- May not explain all bimodality in colors of globular clusters.

25. 9/6/2018
Top Ten Countdown
The -

26. 9/6/2018
HST increased the sample of protoglobular cluster candidates from a half dozen to several hundred.
Also showed that the galaxy was still forming stars (and clusters) in the inner region. -

27. The colors and luminosities of the clusters found in merging galaxies follow the predictions from the Bruzual and Charlot (1996) spectral evolution models.
Whitmore et al. 1997
Metal poor
Metal rich

28. Whitmore et al., 1995 – M87
The distribution of colors for the globular clusters in M87, and most giant elliptical galaxies , are bi-modal, as expected if there was a second epoch of cluster formation.
Metal poor
Metal rich
This was a prediction by Zepf & Ashman (1993) before the observations were definitive.
Zepf & Ashman (1993)

29. 9/6/2018
# 8 – Abrahams et al. (1996) -

31. There is a large % of peculiar/irr/mergers at high z
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# 8 – Abrahams et al. (1996)
An asymmetry-concentration index can be used to classify galaxies at high redshift.
There is a large % of peculiar/irr/mergers at high z
Still believe ? – Yes
- But be careful comparing different wavelengths.
- And some large spirals are being found at high redshift. -

32. Eight Famous Merger Papers
9/6/2018
Eight Famous Merger Papers
1. Toomre & Toomre (1972) – Recognizing mergers; N-body simulations
2. Ostriker & Tremaine (1975) – Dynamical friction; cD galaxies from galactic canabalism
3. Toomre (1977) – Mergers make ellipticals?
4. Larsen & Tinsley (1978) – Mergers promote star formation. -
5. Tremaine (1981), van den Bergh (1982) – Dissent.
6. Sanders et al. (1988) – Mergers make quasars?
7. Holtzman et al. (1992) – Mergers make globular clusters?
8. Abrahams (1996) – Lots of mergers in early universe.

33. And the winner is: 1. Toomre & Toomre (1972) - 1272 citations
9/6/2018
1. Toomre & Toomre (1972) citations
2. Sanders et al. (1988) citations
3. Larson & Tinsley (1978) citations
4. Arp (1966) citations Atlas
5. Toomre (1977) citations.
6. Barnes & Hernquist (1992) citations ARAA review
7. Barnes & Hernquist (1991) citations Fueling starbursts
8. Ashman & Zepf (1992) citations Formation of GCs in merger
9. Meurer et al. (1995) citations Starbursts and star clusters
10. Barnes (1988) citations N-body of Antennae look-alike
11. Whitmore et al. (1995) citations Antennae -

34. 12. Holtzman et al. (1992) - 282 citations
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12. Holtzman et al. (1992) citations
13. Schweizer (1982) citations NGC 7252
14. Abrahams et al. (1996) citations
15. Schweizer & Seitzer (1992) citations Fine Structure
16. Ostriker & Tremaine (1975) citations
17. Vorontsov-Velyaminov (1959) citations Catalog
18. Zepf & Ashman (1993) citations Bimodality for GCs in ellipticals
19. Hibbard & van Gorkom (1996) citations HI in mergers
20. Arp & Madore (1987) citations Catalog
21. Tremaine (1981) citations
22. van den Bergh ( citations
23. Homberg (1941) citations Light bulb model -

35. A Basic Reference Schweizer 1996 – Lecture Notes for Saas-Fee Course
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A Basic Reference
Schweizer 1996 – Lecture Notes for Saas-Fee Course -