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Star-forming Dwarf Galaxies: Ariadne's Thread in the Cosmic Labyrinth

A multiwavelength analysis of Blue Compact Dwarf Galaxies: HI results
´ ´ ´ ¨ ´ Angel R. Lopez-Sanchez1 , Barbel Koribalski1 , Cesar Esteban2 , 1,3 , Janine van Eymeren4 & John Hibbard5 Attila Popping
1 2 3 4 5

CSIRO / Australia Telescope National Facility, Sydney, Australia Instituto de Astrof´ isica de Canarias, Tenerife, Spain Kapteyn Astronomical Institute, University of Groningen, the Netherlands Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, University of Manchester, UK National Radio Astronomy Observatory, USA

E-mail: Angel.Lopez-Sanchez@csiro.au Abstract. We obtained deep multi-wavelength data of a sample of nearby blue compact dwarf galaxies (BCDGs) combining broad-band optical/NIR and H photometry, optical spectroscopy and 21-cm radio continuum and line observations. The selected BCDGs are found in different environments, from apparently isolated to compact groups. Our aims are to analyze the chemical and physical properties of their gas, their star formation activity, the kinematics, the stellar populations and environment. Here we present new exciting H I results obtained with the Australia Telescope Compact Array for some BCDGs, all showing interaction features. This analysis strongly suggests that interactions of BCDGs with lowluminosity dwarf galaxies are the main trigger mechanism of the observed star-forming bursts.

AMS classification scheme numbers: 95.85.Bh, 98.52.Wz, 98.54.Ep, 98.58.Ge, 98.58.Nk, 98.62.Ai, 98.65.At, 98.65.Bv

1. Introduction Blue compact dwarf galaxies (BCDGs) are a subset of low-luminosity galaxies undergoing strong and short-lived episodes of star formation at the present time. They usually exhibit a compact, irregular morphology and display intense, narrow emission lines superposed on a blue continuum. Due to the low metallicity (10 % solar) and the considerable gas consumption involved in the violent starformation, it is mostly believed that unlike spirals the star formation in BCDGs takes place in transient, sporadic bursts. However, the origin and nature of their starburts is still poorly understood. There is increasing observational evidence that interaction of BCDGs with dwarf galaxies trigger the star formation activity in these systems [24];[18];[20];[12]. Nevertheless, optical images of BCDGs usually do rarely show interaction features. Furthermore, hierarchical formation models of galaxies [7];[21] predict that interactions between dwarf galaxies are more common at high redshifts. Detailed analysis of local BCDGs would have an important impact on our knowledge of the evolution of the galaxies. Although there has been an increasing amount of BCDG data in the last years, they mainly consist of optical/NIR imagery [2];[13], H images [3];[13], optical spectroscopy [6];[12] and single-dish H I surveys [22];[8];[4];[4]. However, the few interferometric H I studies of BCDGs available so far have resulted in surprises. For example, the H I extent of NGC 2915 is over 5 times the optical Holmberg radius and 22 times the exponential scale length in the B band, resulting in a mass-to-light ratio of MDyn /LB =76 [19]. Extended H I envelopes, neighboring H I clouds without stars, H I outflows, disturbed H I morphologies and intriguing H I kinematics have been also detected in BCDGs.


H I analysis of Blue Compact Dwarf Galaxies

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Table 1. General properties and HIPASS data [1];[8] of the sample of BCDGs. All H I

measurements were extracted from the BGC [8], except for Tol 9 and ESO 108-G017 for which we use the HICAT [1], and POX 4 for which we use previous interferometric results.
Galaxy NGC 1510 He 2-10 Tol 9 POX 4g Tol 30 NGC 5253 IC 4662 TOL 1924-416 IC 4870 ESO 108-G017
a b c d e f g

R.A. (2000) [h m s ] 04 08 10 11 13 13 17 19 19 22 03 36 34 51 05 39 47 27 37 10 32.6 15.2 38.7 11.6 46.7 55.9 08.8 58.2 37.6 47.2

a

Dec. (2000) [ - - - - - - - - - -


m

B

HIPASS name

b

H I Flux (259±17) 12.9±3.5 10.9 1.0 17.8±3.7 44.4±4.7 130±12 14.2±2.8 24.7±3.2 14.6

v

HI

W

50

M

HI

c

M

Dyn

d

Env.e P I G I G G I P I I

"] 24 24 35 36 25 38 38 34 48 52 01 34 00 02 20 24 30 32 43 09

[mag] 13.47 12.45 14.40 14.56 15.50 10.87 11.74 13.30 13.89 14.44

[Jy km s-1 ] [km s-1 ] [km s-1 ] [log M ] [log M ]
f

43 26 28 20 28 31 64 41 65 66

J0403-43 J0836-26 J1034-28 ... J1305-28 J1339-31A J1747-64 J1927-41 J1937-65 J2210-66

(898±3) (234±6) 868±12 117±24 3456.4 248.5 3580 65 2282±9 105±18 407±3 67±6 302±3 86±6 2832±8 66±16 879±3 76±6 2133 159.7

... 8.28 9.66 8.7 9.51 7.97 8.11 9.67 8.76 9.39

... 10.4 10.5 9.8 10.6 8.87 8.79 10.1 9.96 10.3

Data for columns 2-4 were extracted from the NED. Data for columns 5-8 were extracted from HIPASS. The H I mass was extracted from [8] or computed from the H I flux provided by [1] assuming vLG . The dynamical mass, MDyn , was derived using W50 and considering the optical size of the galaxy. Environment in which the BCDG resides: G (galaxy group), P (galaxy pair) or I (apparently isolated). The HIPASS data actually are the data of the galaxy pair NGC 1512/1510, see [11]. Data provided by J. Ott (priv. comm.) from preliminary analysis of the ATCA H I data.

H I observations of BCDGs are therefore important, for not only do they provide an estimate of the total dynamical mass (consisting of stars, gas, dust and dark matter), but also reveal their neutral gas content which, together with the molecular gas component, is the raw material for the star formation. H I observations have been established as a very powerful means of tracing low surface brightness gas far away from the centres of galaxies. The H I kinematics can also help to answer other key questions regarding the rotational or turbulent behaviour of the neutral gas. The measurement of the H I rotation curve leds to an estimate of the amount of dark matter in these galaxies. Finally, neutral hydrogen gas is the best tracer for galaxy-galaxy interactions. The H I distribution is usually several times larger than the optical extent of a galaxy and therefore more easily disrupted by tidal forces from neighboring galaxies than the stellar disk. All these data combined with parameters such as the absolute luminosity, star formation rate, stellar and dust content, oxygen abundance, etc., can furnish powerful clues about the nature and evolution of BCDGs. 2. ATCA observations Our group obtained deep multi-wavelength data of a sample of nearby BCDGs combining broadband optical/NIR and H photometry and optical spectroscopy with deep 21-cm radio continuum and line observations. The latter were obtained with the Australia Telescope Compact Array (ATCA), interferometer consisting of the six antennae provides two different frequency bands. We selected 10 bright BCDGs that were detected in the H I Parkes All-Sky Survey (HIPASS; [1];[8]). Some are apparently isolated, while others belong to a galaxy pair (TOL 1924-416, see Figure 1) or a galaxy group (Tol 9 and Tol 30, see Figure 2; and NGC 5253). The radio data of the galaxies NGC 1510, NGC 5253 and IC 4662 are part of The Local Volume H I Survey (LVHIS).
LVHIS is a large project [9];[10] that aims to provide detailed H I distributions, velocity fields and star formation rates for


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Figure 1. ATCA H I distribution of the BCDG Tol 1924-416 and surroundings using the 6A (baselines up to 6 km length, left) and 750A (baselines up to 750 m, right) overlaid onto an R-band image from the DSS. Important objects are labeled. Notice that the elliptical shape of the beam size in the 750A map is consequence of the short integration time (5 hours).

The basic properties of our sample of BCDGs are compiled in Table 1. We are using full synthesis observations (12 hours) in four different array configurations of the ATCA interferometer to get deep H I data. The reason for choosing several array configurations is because we need a large range of baselines to detect structures on all scales (see Figure 1). While the short baselines recover the extended H I distribution, the larger baselines are needed to provide more details such as we can compare with UV/optical/IR data [11]. The final radio maps will be obtained by combining all available arrays, having both high sensitivity (the H I column is around 5 â 1019 cm-2 for a 40" beam) and good angular ( 20 - 30") and velocity (10 - 20 km s-1 ) resolution. We can achieve a range of angular resolutions by weighting the data differently, thereby affecting the image sensitivity. The main aim of our H I observations is to estimate the distribution of the atomic gas in each BCDG and its surroundings, measuring the amount of fuel available for driving the current and future star formation processes. We will also look for H I clouds and morphological features that reveal interaction processes (merger remnants, arcs, or tails) and compare with the optical/NIR morphology and the H distribution of H II regions. The kinematics of the neutral gas may also reveal interaction features such as tidal tails or peculiar motions such as outflows that can not be explained by rotation. From the measurement of the total H I flux we can derive the neutral hydrogen mass ( MH I ) in each BCDG. In those galaxies in which we can measure the rotation curve we will also derive their total dynamical mass ( MDyn ), their mass-to-light ratios and their MH I / MDyn ratio. Using the 20-cm (1.4 GHz) radio continuum maps, we derive an extinction-free star formation rate and compare, when available, with UV-, H- and FIR-based SFR estimates, both globally and locally. We will also explore the Schmidt-Kennicutt star formation law comparing the SFR per unit area with the mean surface density of cold gas.
a complete sample of nearby, gas-rich galaxies belonging to the Local Volume (LV), the sphere of radius 10 Mpc centred on the Local Group. With the ATCA we observed all LV galaxies that were detected in HIPASS and reside south of approx. ­30 declination. See http://www.atnf.csiro.au/research/LVHIS for details.


H I analysis of Blue Compact Dwarf Galaxies

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Figure 2. ATCA H I distribution of the galaxy groups associated with Tol 9 (left, [16]) and Tol 30 (right, [17]) overlaid onto an R-band image from the DSS. Note that the angular resolution in both images is very similar but we show a different area (the primary beam, the size of the full field of view, is 33').

3. BCDGs in galaxy groups One of the key points in our analysis is to compare the properties derived for BCDGs belonging to galaxy groups with those that are in pairs or apparently isolated. So far, our study has revealed a number of surprises. The analysis of the H I kinematics in NGC 5253 reveals a velocity gradient along the optical minor axis of the galaxy; it does not show any sign of regular rotation [15]. Some authors suggested that this feature is an outflow. However, we think that its origin may be the disruption/accretion of a dwarf gasrich companion or the interaction with another galaxy in the M 83 subgroup. Our finding of a distorted H I morphology in the external parts of the galaxy supports this hypothesis. Another interesting BCDG is Tol 9, located in the center of the Klemola 13 group. The optical properties were published in [12];[13];[14]. The H I map (Figure 2, left) shows that the neutral gas is found in two regions: one associated with the spiral galaxy ESO 436-46 and the other embedding Tol 9 and two nearby objects, revealing a long H I tail in direction to ESO 436-44 and ESO 436-45, galaxies mainly composed by old stellar populations. The H I kinematics are also very intriguing, because the H I cloud in which Tol 9 an its surrounding dwarf galaxies are embedded seems to rotate as a single entity [16]. The kinematics of the tail suggest that it is of tidal origin. Our analysis of the H I gas in Tol 30 has revealed two faint tails starting at opposite sides of the galaxy, where the brightest H II regions are located (Figure 2, right). The northern tail hosts around 15% of the total H I mass of the system. We have detected a detached H I cloud at the NE of Tol 30 that, besides its small size, seems to show rotation. Our deep optical images confirm the detection of an object within this H I cloud. Combining the optical and radio data, we suggest that it is not a tidal dwarf galaxy but an independent nearby low-luminosity dwarf galaxy with an important old stellar population that is in interaction with Tol 30. An analysis of both the ionized and the neutral gas within this system, as well as its stellar populations, will be finished soon [17].


H I analysis of Blue Compact Dwarf Galaxies

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Figure 3. Preliminary ATCA H I maps of the galaxies IC 4870 (left) and ESO 108-G107 (right) overlaid onto optical DSS images. Both H I distributions show interaction features: IC 4870 seems to be a merger, and ESO 108-G107 possesses a prominent H I tail with a peculiar kinematics.

4. BCDGs in galaxy pairs Two of the BCDGs in our sample actually reside in a galaxy pair. H I maps of TOL 1924-416 are shown in Figure 1. Our preliminary results reveal a huge amount of neutral gas in this BCDG and its surroundings and an H I bridge between Tol 1924-416 and the companion galaxy ESO 338-004B. The H I bridge has 33% of all the neutral gas of the galaxy pair. The mass-to-luminosity ratios computed for the galaxies indicate that the H I detected in the bridge has probably been expelled from TOL 1924416. Their kinematics also support this scenario: the BCDG shows no rotation but a kind of tidal stream towards the bridge, while the neutral gas in ESO 338-004B seems to rotate about its optical minor axis. Another beautiful example is the NGC 1512/1510 system. It is composed of the barred spiral galaxy NGC 1512 and the BCDG NGC 1510 separated by only 5 (13.8 kpc). A detailed analysis of the neutral gas within this system and the study of the star-formation activity in hundreds of U V rich regions surrounding the system is presented in [11]. Our data strongly support the interactioninduced scenario to explain both the H I features and the star-formation activity in the system. Indeed, NGC 1512/1510 may be experiencing the first stages of a minor merger. 5. Apparently isolated BCDGs The analysis of the H I properties of the BCDG IC 4662 is presented in [23]. The neutral gas distribution consists of two parts: the inner high column density system (that coincides with its optical extent) is perpendicular to the outer low column density system ending in a kind of tail towards the east. The kinematics are very disturbed: the overall velocity gradient runs from the north-east with velocities of 220 km s-1 to the south-west with velocities of 380 km s-1 , changing direction by about 90 in the centre of IC 4662. The chemical properties of some of the H II regions are also intriguing and may suggest the presence of two objects that have experienced different chemical evolution.


H I analysis of Blue Compact Dwarf Galaxies

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The optical appearance of IC 4870 shows a compact star-forming core (35") embedded in an elliptical low-luminosity component with a size of 1.4 â 0.4 . However, the H I distribution (Figure 3, left) reveals a lot of neutral gas and two long tails with sizes of 3.7 (northern tail) and 4.2 (southern tail) arising in opposite directions. We detect two maxima of the H I emission located at the beginning of the tails. The northern tail is quite straight and does not show important variations in its kinematics, but the southern tail is slightly curved towards the W. This tail possesses a knot at the south that hosts 14% of all the H I mass of the system (it may seem detached from the tail if the H I image is not deep enough) and shows a velocity gradient. All these facts suggest that IC 4870 is experiencing a merging of two independent H I clouds, being the origin of its strong star-formation activity. The H I distribution found in the BCDG ESO 108-G107 (Figure 3, right) is more than 5 times the optical size. We detected a long tail towards the NW that has peculiar kinematics and is not aligned with a faint optical tail found at the W of the BCDG. The H I distribution and kinematics of the eastern neutral gas also suggest the presence of a tail in this area. 6. Conclusions We performed a multi-wavelength analysis of some nearby BCDGs combining broad-band optical/NIR and H photometry, optical spectroscopy and 21-cm radio observations. We show that the H I data are fundamental to understand both the nature and the dynamical evolution of the galaxies. All analyzed BCDGs show interaction features despite if they are located in a galaxy group, a galaxy pair or are apparently isolated. Our study strongly confirms the main result found in the analysis of Wolf-Rayet galaxies performed by [12] that interactions with or between low-luminosity dwarf galaxies are the main trigger mechanism of starbursts, specially on BCDGs. However these dwarf objects are only detected when deep optical images and complementary H I observations are obtained. References
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] Barnes, D.G. et al. 2001, MNRAS, 322, 486 ´ ´ ´ ´ Cairos, L.M., V´ ilchez, J.M., Gonzalez Perez, J.N., Iglesias-Paramo, J. & Caon, N. 2001, APJS, 133, 321 Gil de Paz, A. & Madore, B.F. 2005, ApJS, 156, 345 Huchtmeier, W.K., Krisna, G., & Petrosian, A., 2005, A&A, 434, 887 Huchtmeier, W.K., Petrosian, A., Gopal-Krishna, & Kunth, D., 2007, A&A, 462, 919 Izotov, Y.I. & Thuan, T.X. 1999, ApJ, 511, 639 Kauffman, G. & White, S.D.M. 1993, MNRAS, 261, 921 Koribalski, B.S. et al. 2004, AJ, 128, 16 Koribalski, B.S. 2008, proceedings of Galaxies in the Local Volume conference, Springer, p.41 Koribalski, B.S. et al. 2009, in prep. ´ ´ ´ Koribalski, B.S. & Lopez-Sanchez, A.R. 2009, MNRAS, in revision ´ ´ ´ Lopez-Sanchez, A.R. 2006, Massive star formation in dwarf Wolf-Rayet galaxies, PhD Thesis, Univ. La Laguna, Spain ´ ´ ´ Lopez-Sanchez, A.R. & Esteban, C. 2008, A&A, 491, 131 ´ ´ ´ Lopez-Sanchez, A.R. & Esteban, C. 2009, A&A, submitted ´ ´ ´ Lopez-Sanchez, A.R., Koribalski, B. & Esteban, C. 2008a, proc. of Galaxies in the Local Volume, Springer, p.53 ´ ´ ´ Lopez-Sanchez, A.R., Koribalski, B., Esteban, C. & Hibbard, J. 2008b, Galaxies in the Local Volume, Springer, p.301 ´ ´ ´ Lopez-Sanchez, A.R., Koribalski, B., Esteban, C. & Hibbard, J. 2009, in prep. ´ Mendez, D.I. & Esteban, C., 2000, A&A, 359, 493 Meurer, G.R., Carignan, C., Beaulieu, S.F. & Freeman, K.C., 1996, AJ, 111, 1551 ´ Noeske, K.G., Iglesias-Paramo, J., V´ ilchez, J.M., Papaderos, P. & Fricke, K.J. 2001, A&A, 371, 806 Springel, V., White, S. et al. 2005, Nature, 435, 629 Thuan, T.X., Lipovetski, V.A., Martin, J.-M. & Pustilnik, S.A. 1999 A&ASS, 139, 1 van Eymeren, J. 2008, PhD Thesis, Ruhr Universitaet Bochum, Germany Wilcots, E. M., Lehman, C., & Miller, B. 1996, AJ, 111, 1575