Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/people/Angel.Lopez-Sanchez/posters/IAU212-mkn1087.pdf Äàòà èçìåíåíèÿ: Sun Feb 3 03:58:18 2008 Äàòà èíäåêñèðîâàíèÿ: Mon Apr 14 04:19:14 2008 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: interacting galaxies

MRK 1087: a puzzling suspected Wolf-Rayet galaxy
CÈsar Esteban1, àngel R. LÑpez-SÀnchez1, and MÑnica RodrÌguez2
2Ins 1Instituto de AstrofÌsica de Canarias, La Laguna, Tenerife, Spain tituto Nacional de AstronomÌa, ñptica y ElectrÑnica, Tonantzintla, Puebla, Mexico

-ABSTRACTWe present new observations of Mrk 1087 that indicate this suspected Wolf-Rayet galaxy formation event due to the interaction with -at least- two nearby galaxies, one of them is existence is reported by the first time in this poster. We discover that some of the non-stellar body of the galaxy are tidal dwarves which host a recent star formation event. These tidal dwar of material stripped from Mrk 1087. is experiencing a strong star a star-forming dwarf, which objects surrounding the main ves are connected by bridges

THE OBJECT
E S
Schaerer et al. (1999) classify Mrk 1087 (distance = 111 Mpc) as a suspected Wolf-Rayet (WR) galaxy due to the detectio n of He II 4686 å in spectra taken by Vaceli et al. (1997). The galaxy has an elliptical shape and is surrounded by a complex system of filaments. It has a companion galaxy (K72 103a, see Figure 1) at 81 kpc to the southw est and at the same distance. Keel (1988) proposes that the object is a galaxy in formation surrounded by filaments of infalling ionized gas. MÈndez & Esteban (2000) ­hereinafter ME00­ obtain deep CCD UBV images finding and characterizing several faint objects connected by bridges and tails with the main body of the galaxy. They interpret Mrk 1087 as a galaxy experiencing an interaction event w ith one or more of the neighboring objects.
Mrk 1087 - nucleus

companion

OUR OBSERVATIONS
We have obtained long-slit intermediate-resolution spectroscopy in five slit positions centered on the nucleus of the galaxy and covering most of the surrounding objects. We have used the ISIS spectrograph on the 4.2m William Herschel Telescope (WHT) and the IDS on the 2.5m Isaac New ton Telescope (INT) both at Roque de los Muchachos Observatory (La Palma, Canary Islands). An additional V-band CCD image was taken at the 2.2m telescope at Calar Alto Observatory (Figure 1).

50 arcsec

Figure 1: Deep CCD image in the V filter of Mrk 0187 and its surroundings

Figure 2: WHT spectra of the nucleus of Mrk 1087 and the northern companion.

A NEW COMPANION
In Figure 1, w e show the main non-stellar objects surrounding Mrk 1087 found by ME00. We designated the objects following those authors. Our new images show a new faint companion galaxy (MV ~ -18.2) just to the north of Mrk 1087 (labeled as "companion" in Figure 1). It has an elliptical shape and shows nebular emission, indicating that starformation is ongoing or has been very recent in th e object (see its spectrum in Figure 2). It show s a radial velocity of +117 km s-1 w ith respect to the main body of Mrk 1087. Therefore, both galaxies may be physically associated and at a relative distance of about 60 kpc.

ORIGIN OF THE SURROUNDING OBJECTS
We have detected faint nebular emission in #1, #3, and #7, but not in #12. Objects #2 and #11 w ere not observed. Objects #1 and #3 show radial velocities close to the systemic velocity of Mrk 1087 (see Table1). Keel (1988) interpreted #7 as a companion external object interacting w ith the main galaxy due to its large radial velocity how ever, w e do not confirm his result, in fact #7 seems to follow closely the general rotation pattern of the ionized gas of the main galaxy. We do not of the spectra and abundances from making use of the the [NII] 6584 å Table 1 w e show different objects. detect the [OIII] 4363 å line in any therefore w e have estimated the ionic the so-called empirical calibrations, R23 parameter (Pilyugin 2000) and/or / H ratio (DenicolÑ et al. 2002). In the O/H and N+/O+ ratios of the

Table 1: R elevant data of the objects
n u cl e u s M M
B

co m p a n i o n -18.7 2 + 11 7

#7 - 1 7 .3 - 1 7 .4 + 14 4 8.55
2

#1 -1 6 . 2 -1 6 . 5 -6 3 8.70
3

#3 - 16 . 9 - 17 . 3 + 10 8 .7 0
3

- 2 1. 7 - 2 1. 9 0 8 .7 0
2

V

v r2 (k m /s ) 1 2 +l o g O / H [N I I ]/ H N + /O
1

8.30

2

0 .3 3 - 0.99

0.07 -1.55

0 . 14 - 1. 1 5

0.28 -

0 .2 4 -

+

AGE OF THE OBJECTS
Table 2: Age estimations (in Myr)
A ge in d ica to r W (H )1 W ( [ O I I I ]) W (H ) U-B B-V
2 1 1

Radial velocity with respect to the nucleus of MRK 1087 2 From empirical calibrations using R23 and [NII]/H 3From empirical calibrations usi ng [NII] /H only

n u c le u s 7 7 5 -9 5 -6

c o m pa n ion 5 4-5 -

#7 4-6 5-7 4 6 -2 2

#1 8 7-59

#3 #7

10 >1 0 0
nucleus companion

2

The absolute magnitudes given in Table 1 indicate that all the objects except the main body of Mrk 1087 can be considered as dw arf galaxies. The O/H ratio estimated for objects #1, #3 from their [NII]/ H ratio is similar to that of the main galaxy and larger than the abundance expected for the metallicity-luminosity relation for "classical" dwarves (Richer & McCall 1995). This result indicates that objects #1 and #3 can be considered as tidal dwarves originated from material stripped from Mrk 1087. Object #7 show s O/H and N+/O+ ratios low er but not so different to that of the main galaxy. The O/H abundance is larger than that expected for a dwarf galaxy and therefore w e interpret #7 as an intense starforming zone off-center of Mrk 1087.

1 2

T his work using models by St asinska & L eitherer (1996) ME00 using models b y Leitherer & H eckman (199 5)

The O/H ratio obtained for the companion object at the north is clearly lower than that of the main galaxy and consistent w ith the metallicityluminosity relation for dw arves. Moreover, the N+/O+ is also low er than that of Mrk 1087 and of the order of the typical value for dwarf galaxies. Therefore, the north companion should be interpreted as an independent nearby dw arf galaxy which is probably interacting with Mrk 1087. This could explain the presence ­at least­ of the tidal dw arf #1 and its associated bridge, which is almost aligned w ith the companion galaxy.

In Table 2, we show the ages of the starburst contained in the different objects determined from our data and the previous ones by MEOO. We have used the models of Stasinska & Leitherer (1996) for determining the ages from W(H) ­see figure 3- and W([OIII]) -Figure 4, and the models of Leitherer & Heckman (1995) when using W(H). There is a very good agreement betw een the different indicators except w ith B-V, probably because the larger contamination of the emission from underlying older stellar populations in the V-filter.

Figure 3: W(H) vs. age of the bursts. Models by Stasinska & Leitherer (1996)

companion nucleus #7

EPILOGUE
The complex geometry of the filamentary structure Mrk1087 and the results obtained in this paper indicate that this galaxy is experiencing a strong star formation event due to the interaction w ith at least two external galaxies: the relatively bright K72 103a ­that could explain the bridges and the non-stellar objects (#11 and #13) located betw een both galaxies­ and the new dw arf companion at the north. We consider that Mrk 1087 and its surroundings to be considered as a compact group of galaxies.

References
· · · · · · · · · DenicolÑ et al. 2002, MNRAS 330, 69 Keel, W.C. 1988, A&A 202, 41 Leitherer, C., Heckman, T.M. 1995, ApJS 96, 9 MÈndez, D.I., Esteban, C. 2000, A&A 359, 493 (ME00) Pilyugin, 2000, A&A 362, 325 Richer, M.G., McCall, M.L. 1995, ApJ 445, 642 Schaerer et al. 1999, A&AS 335, 85 Stasinska, G., Leitherer, C. 1996 ApJS 107, 661 Vaceli et al. 1997, AJ 114, 1345

Figure 4: W([OIII]) vs. W(H) . Models by Stasinska & Leitherer (1996)