Tidal Tales of Minor Mergers: Star Formation in the Tidal Debris of Minor Mergers
aa r X i v : . [ a s t r o - ph . C O ] S e p **FULL TITLE**ASP Conference Series, Vol. **VOLUME**, c (cid:13) **YEAR OF PUBLICATION****NAMES OF EDITORS** Tidal Tales of Minor Mergers: Star Formation in the TidalDebris of Minor Mergers
Karen A. Knierman
School of Earth and Space Exploration, Arizona State University,Tempe, AZ 85287
Abstract.
How does the tidal debris of minor galaxy mergers contributeto structures in spiral galaxies or in the intergalactic medium? While majormergers are known to create structures such as tidal dwarf galaxies and starclusters within their tidal debris, less is known about minor mergers (mass ratiosbetween a dwarf galaxy and disk galaxy of less than one-third) and their tidaldebris. This work surveys 6 nearby minor mergers using optical broad-band and Hα narrow-band imaging to characterize star formation in their tidal debris.Young star clusters with ages less than the dynamical age of the tidal tails arefound in all 6 mergers, indicating that the star clusters formed in situ. Even ifminor mergers contribute less tidal debris per interaction than major mergers,they are more common and possibly contribute structure to all types of galaxiesand to the intergalactic medium throughout the history of the universe. Does the tidal debris of minor mergers contribute structures to halos of spiralgalaxies or IGM? Though major mergers are known to create structures from starclusters to tidal dwarf galaxies in their debris (Duc et al. 2000; Knierman et al.2003), less is known about minor mergers and their tidal debris. Yet, minormergers are more common and were presumably important in the early universe.Structures in minor mergers’ tidal debris may be captured by the more massivegalaxy and create structures in the galaxy’s halo like those found in the MilkyWay (Ibata et al. 1997), or contribute to the pollution of the IGM. Tidal debriscan also give insight into unusual environments of star formation and how gasbehaves in galaxy interactions. Studying the formation of structure in the tidaldebris of minor mergers can help us understand a common process in galaxyformation, particularly disk and halo formation.
Compared to major mergers, less is known about minor mergers (mass ratiosbetween a dwarf galaxy and a disk galaxy of less than 0.3) and their tidal debris.Previous work includes VLT observations of NGC 6872 (Bastian et al. 2005) andHST observations of UGC 10214 (Tran et al. 2003; de Grijs et al 2003). Thesestudies found the tidal debris to contain blue star clusters ( U − B ∼ − . B − V ∼ − .
2; de Grijs et al. 2003) with young agesindicating they were formed within the tidal tail(s).1
Knierman
Using the 1.8 m VATT, deep optical images of 15 minor mergers were taken in
U BV R and narrow-band Hα and reduced using standard IRAF tasks. Pointsources were found using DAOFIND. Final star cluster candidates were selectedusing the 3DEF code (Bik et al. 2003) and Bruzual & Charlot (2003) SSP modelswith a metallicity of 0 . Z ⊙ . These SED fits gave an age, mass, and extinctionfor each star cluster candidate. Results from the closest six out of the full sample of 15 minor mergers show thatstar clusters in the tidal debris have ages ranging from a few Myr to greaterthan a Gyr as well as masses ranging 10 . − M ⊙ (Figure 1). There maybe some contamination from foreground stars and background galaxies. Table1 shows properties of the minor mergers such as merger stage, as well as thedynamical age of the tail, number of star cluster candidates (SCC), mean ormedian values of M B , age, mass, and extinction, and the total mass of the starcluster candidates in the tidal debris area.In each merger, there are SCC with ages less than the tail age, which in-dicates that the star cluster formed in the tidal debris and was not pulled outfrom the central regions of the galaxies. Comparing the merger stages with age,early stage mergers have slightly younger median ages than late stage or mergedsystems, with the exception of UGC 260. The median age of clusters in thissystem are likely skewed by the presence of substantial Hα flux contaminatingthe clusters. In general, the median cluster age appears to be 10-20% of thedynamical age of the merger. The lowest median mass of the star clusters arefound in the early stage mergers of Arp 269 and Arp 279. These two galaxiesappear to be missing higher mass star clusters. This could indicate either thatthey have not yet formed more massive star clusters or that there is not enoughmaterial in their tails to form them. In the sample of 10 debris regions in 6 minor mergers, we find that 8 debrisregions host HII regions (see Table 2). The debris regions of NGC 3310 have thehighest median Hα luminosity, total Hα luminosity, and total star formationrate. The tidal tails of UGC 260 have the next highest values behind NGC 3310.In comparing the minor merger Hα luminosities, we find that they are of similarorders of magnitude to major mergers. So star formation occurs at similar levelsin both major and minor mergers. In comparing merger stages, we find thatearly stage mergers in the sample are forming smaller HII regions than those inlate or merged systems.There is a weak trend in Hα luminosity and thus star formation rate withmerger stage, merged systems having a higher rate. However, UGC 260 hasthe second highest rate in the sample. This indicates that individual galaxyparameters are more important than merger stage. idal Tales of Minor Mergers Table 1. Properties of Tidal Debris and Star Cluster CandidatesDebris Merger Tail Age
SCC M Bb Mass c Age c Total MassArea Stage a Myr mag log [ M ⊙ ] log ( yr ) log [ M ⊙ ]Arp269E 1 72.8 12 -7.89 4.99 6.82 7.07Arp269B 10 -8.47 5.13 6.44 6.3Arp279W 1 34.7 4 -10.37 5.68 6.71 6.8Arp279B 54.4 6 -10.08 5.57 6.51 6.4UGC260N 1 30.7 5 -12.20 6.26 8.06 7.1UGC260S 47.2 6 -12.02 6.37 8.41 7.3NGC2782E 2 200.0 32 -10.44 6.38 8.00 8.4NGC2782W 320.9 19 -9.86 5.75 7.32 7.6NGC3310 3 77.9 43 -9.96 5.47 7.18 8.1NGC7479 3 118.5 10 -12.13 6.42 7.91 8.0 a b Mean value for debris area c Median value for debris area
Table 2. Properties of HII regions in the Tidal Debris of Minor MergersDebris L Hα,lim N HII L Hα,med L Hα,tot
SF R ( Hα ) a ergs − ergs − ergs − M ⊙ yr − A269Br 0.562 4 3.16 13.6 0.00108A269Dw 0.562 11 6.30 318 0.0248A279Br 0.0942 10 11.5 141 0.0112U260N 0.117 8 58.5 648 0.0512U260S 0.117 5 92.7 713 0.0563N2782E 0.568 5 27.4 214 0.0169N2782W 2.67 1 137 137 0.0108N3310N 0.0894 31 60.5 5520 0.436N3310S 0.0894 18 69.1 1570 0.124N7479N c > > > . e e
170 0.013N7714/5Br e
180 0.014 a Kennicutt 1998 d Using HII regions from Jarrett et al. 2006, median, total, and SFR are taken from the moremassive HII region. e Major Merger tidal debris from Smith et al. 1999 and references therein.
Knierman
6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 l og ( M a ss ) log(Age) EarlyLateMergedCompleteness (U260)
Figure 1. Mass and Age for all star cluster candidates, sorted by mergerstage, fit to Bruzual & Charlot (2003) SSP models by the 3DEF code(Bik et al. 2003). The dotted line shows the completeness limit for UGC260.
These observations show that star clusters with ages less than the tail age arepresent in all sample tidal debris. This suggests that star formation in minormerger tidal debris is ubiquitous. Ages appear to correlate with dynamical ageof the merger. There is a weak trend in Hα luminosity and thus star formationrate with merger stage, merged systems having a higher rate. There may bedelays of >
10 Myr in star formation episodes as two debris regions in the earlystage systems did not harbor any HII regions.
Acknowledgments.
Thank you to the Galaxy Wars committee for provid-ing conference funding and to the Arizona/NASA Space Grant Consortium.
References
Bastian, N. et al. 2005, A&A, 435, 65.Bik et al. 2003, A&A, 397, 473Bruzual, G., & Charlot, S. 2003, MNRAS, 344, 1000.de Grijs, R., et al. 2003, New Astronomy, 8, 155.Duc, P.-A., et al. 2000, AJ, 120, 1238.Ibata, R. A., et al. 1997, AJ, 113, 634.Knierman, K. A., et al. 2003, AJ, 126, 1227.Kennicutt, R. 1998, ARA&A, 36, 189Smith, B., et al. 1999, AJ, 117, 1237. idal Tales of Minor Mergers5