Stellar populations in a Sample
of Tidal Dwarf Galaxies

Peter Weilbacher,
Uta Fritze-von Alvensleben,
Universtitäts-Sternwarte Göttingen

Pierre-Alain Duc,
CNRS and CEA Saclay

Summary of Talk given in Saclay, 26.Feb.01
 
 
 
 

Motivation
Study spectrophotometric evolution of Tidal Dwarfs:
How did they get visible and blue?
How will they look like later?

Sample
 

Name	Morphology	Velocity	Observations		MB	number
	from NED	km s-1	Broad Band	Spectroscopy	parents	TDG-Cs
AM 0529-565 A/D	Merging, dI	4420, 4645	B V R  H	MOS	-18.3, -15.4	4
AM 0537-292 A/B	Sb?, E	3700, 3880	B V R  H	2xMOS	-18.5, -16.9	5
AM 0545-434	S?, E, E	?	B R	-
AM 0547-244 A/B	S:pec, S?	13160, 13410	B V R  H K	MOS	-20.6, -19.1	3
AM 0547-474 A/B	Sa:pec, E	15130, ?	B V R  H	LS	-21.0, ?	2
AM 0607-444 A	Sc:pec	12750	B V R  H	MOS	-20.8	1
AM 0642-645	Merger	27300	B V R  H	LS	-21.4
AM 0748-665 A/B	S, E	?, ?	B V R  H	MOS	?, ?	3
AM 1054-325 A/B	Sm, S pec	3800, ~3800	B V R  H	MOS+LS	-19.0, -18.2	6
AM 1159-530	SAB(rs)a:pec	4530	B V R  J H K	MOS	< -18.4 mag	1
AM 1208-273 A	Sc	12500	B V R H	MOS	-20.5
AM 1237-364	S?	5700	B V R  H	MOS	-19.4	4
AM 1324-431	S?	10450	B V R H	MOS	-20.2	4
AM 1325-292 A/B	SB(s)b, E1:pec	4600, 4430	B V R  H	LS	-19.9, -20.4	2
AM 1353-272 A/B	Sc:pec, S:pec	11790, 12145	B V R  J H K	MOS	-20.3, -18.1	12

Model Inputs

Evolutionary synthesis using

• Input physics:
• Geneva stellar tracks with stellar metallicities

Z1 = 0.001 and Z3 = 0.008 ~ Z_sun/2,
possibly shortly also Z2 = 0.004.
• Gaseous continuum- and line emission included.
• Scalo IMF.
• Specific star formation history:
• Old component represented by constant SFR from the beginning to the epoch of interaction (up to 10...14 Gyrs).
• Young component with recent starburst
• Instantaneous, exponentially decreasing
• or Gaussian shaped
• parameters: max. SFR, timescale

Model Parameters

To produce observed colors vary parameters within these ranges

• max. SFR
  50...1000 M_sun yr^-1
• burst timescale
  5x10⁵...5x10⁶ yr
• use both metallicities
• each model gives a burst strength, defined here as the ratio of the stellar mass built during the burst to the total stellar mass ever built:
  b = S_burst / S_total = 0.01...0.18

In principle also variable, but not used as parameters are

• Interaction age,
• IMF.

Model Applications

Compare model with photometric data:

• Use two-color diagrams for optical data

Is observed color in the range of models?
• Begin "TDG analysis".
 
• Otherwise probably background object, reject as TDG candidate.
• From position get first estimates for burst age,
minimum burst strength b
• Is spectroscopy available?
• Use correct metallicity in model,
• then use H_alpha or H_beta to get better burst age,
• and continuum slope to get best fit burst parameters.
 
• Otherwise use nearest model, problems with internal extinction!
• Are NIR colors available?
• Create SED,
• restrict burst strength accurately,
• use scaled model to determine stellar mass.


Results

Young burst ages, up to 20 Myrs.

High burst strengths, from 2% to at least 20% for some TDG candidates.

=> Visible because of the recent star-formation activity.

=> Strong fading expected, after/if burst stops, 2.5 mag in B within 200 Myrs.

Peter Weilbacher, Thu Mar 8 13:27:17 CET 2001
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