Evolution of Dwarf Galaxies: a Dynamical Perspective
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A&A 563, A27 (2014) Astronomy DOI: 10.1051/0004-6361/201322658 & c ESO 2014 Astrophysics Evolution of dwarf galaxies: a dynamical perspective Federico Lelli1, Filippo Fraternali2,1, and Marc Verheijen1 1 Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands e-mail: [email protected] 2 Department of Physics and Astronomy, University of Bologna, via Berti Pichat 6/2, 40127 Bologna, Italy Received 12 September 2013 / Accepted 15 November 2013 ABSTRACT For a rotating galaxy, the inner circular-velocity gradient dRV(0) provides a direct estimate of the central dynamical mass density, including gas, stars, and dark matter. We consider 60 low-mass galaxies with high-quality H I and/or stellar rotation curves (including starbursting dwarfs, irregulars, and spheroidals), and estimate dRV(0) as VR /Rd,whereRd is the galaxy scale length. For gas-rich / dμ Σ dwarfs, we find that VRd Rd correlates with the central surface brightness 0, the mean atomic gas surface density gas,andthe star formation rate surface density ΣSFR. Starbursting galaxies, such as blue compact dwarfs (BCDs), generally have higher values of VR /Rd than dwarf irregulars, suggesting that the starburst is closely related to the inner shape of the potential well. There are, d / however, some “compact” irregulars with values of VRd Rd similar to BCDs. Unless a redistribution of mass takes place, BCDs must evolve into compact irregulars. Rotating spheroidals in the Virgo cluster follow the same correlation between VR /Rd and μ0 as gas- / d rich dwarfs. They have values of VRd Rd comparable to those of BCDs and compact irregulars, pointing to evolutionary links between these types of dwarfs. Finally, we find that, as for spiral galaxies and massive starbursts, the star-formation activity in dwarfs can be parametrized as ΣSFR = Σgas/τorb,whereτorb is the orbital time and 0.02. Key words. galaxies: dwarf – galaxies: starburst – galaxies: irregular – galaxies: evolution – galaxies: star formation – galaxies: kinematics and dynamics 1. Introduction interesting as the burst durations are typically of a few 100 Myr (McQuinn et al. 2010a), thus they must evolve into another type Low-luminosity, dwarf galaxies are the most common types of dwarf as the starburst fades. The possibility of morphological of galaxies in the Universe (e.g., Ferguson & Binggeli 1994). transformations between low-mass galaxies is also suggested by Despite numerous observational and theoretical studies, their the existence of “transition type” dwarfs, which have interme- formation and evolution is still not fully understood (e.g., diate properties between Sphs and Irrs/BCDs (e.g., Sandage & Tolstoy et al. 2009; Mayer 2011; Kormendy & Bender 2012). Hoffman 1991; Mateo 1998; Dellenbusch et al. 2007, 2008). Three main types of dwarfs exist in the nearby Universe: i) gas- Several photometric studies have shown that the underly- poor dwarfs that are not currently forming stars, which are ing, old stellar component of BCDs typically has a smaller usually called spheroidals (Sphs) or dwarf ellipticals (dEs), scale length and a higher central surface brightness than Irrs hereafter we refer to them as Sphs; ii) gas-rich dwarfs that are and Sphs of the same luminosity, suggesting that the evolu- forming stars at a relatively low rate, named irregulars (Irrs); tionary links between BCDs and Irrs/Sphs are not straightfor- and iii) starbursting dwarfs that are forming stars at an unusu- ward (e.g., Papaderos et al. 1996; Gil de Paz & Madore 2005; ally high rate. The last objects are often classified as amorphous Herrmann et al. 2013). However, it is generally difficult to ob- dwarfs (based on optical morphology, e.g., Gallagher & Hunter tain accurate structural parameters for starbursting dwarfs, since 1987; Marlowe et al. 1999), H II-galaxies (based on emission- the galaxy morphology is extremely irregular and young stars line spectroscopy, e.g., Terlevich et al. 1991), and/or blue com- may dominate the integrated light over much of the stellar body. pact dwarfs (BCDs, based on colors and surface brightness mea- Recently, Micheva et al. (2013) have obtained deep optical and surements, e.g., Gil de Paz et al. 2003). Hereafter, we refer to any near-infrared photometry and challenged the previous results, ar- starbursting dwarf as a BCD. As we show in Sect. 4.1,theterm guing that the structural parameters of the old stellar component “BCD” captures a fundamental observational fact: the starburst of BCDs are consistent with those of Irrs and Sphs. activity (the blue color) occurs mainly in galaxies with a steep ff gravitational potential (i.e., a compact mass distribution towards Adi erent approach is to consider dynamical information the galaxy center), providing that they also have a strong con- that directly traces the distribution of mass, such as H I rota- centration of gas. tion curves (e.g., Lelli et al. 2012a,b). Using qualitative esti- It is known that Sphs, Irrs, and BCDs follow the same cor- mates of the rotation velocities, van Zee et al. (2001) suggested that BCDs have steeper rotation curves than low surface bright- relations between the effective surface brightness μ ff,theef- e ness galaxies of similar luminosity (see also Meurer et al. 1998). fective radius Reff, and the total magnitude M, pointing at evo- lutionary links between them (e.g., Kormendy 1985; Binggeli In Lelli et al. (2012b), we considered a small sample of BCDs and Irrs with high-quality H I rotation curves and measured the 1994; Tolstoy et al. 2009). In this respect, BCDs are particularly / circular-velocity gradient VRd Rd,whereRd is the exponential Appendices are available in electronic form at scale length of the stellar body. We found that BCDs generally / http://www.aanda.org have higher values of VRd Rd than typical Irrs, implying that they Article published by EDP Sciences A27, page 1 of 17 A&A 563, A27 (2014) have a higher central dynamical mass density (including gas, may not be a reliable tracer of the gravitational potential, as stars, and dark matter). BCDs also have higher central H I sur- the galaxy strongly deviates from the baryonic TF relation (see face densities than Irrs (e.g., van Zee et al. 1998, 2001; Simpson Fig. 8 in Lelli et al. 2014), thus we exclude this object here. As & Gottesman 2000). This suggests that the starburst is closely we stressed in Sect. 1, we refer to any starbursting dwarf as a related to the inner shape of the gravitational potential and to the BCD. central concentration of gas. This connection must be the key We also added the well-studied BCD NGC 2915, which has to understanding the mechanisms that trigger and drive the star- been resolved into single stars by HST (Karachentsev et al. burst in BCDs. 2003), but its SFH has not yet been derived. NGC 2915 has In this paper, we confirm the results of Lelli et al. (2012b) a regularly-rotating H I disk (Elson et al. 2010), but the inner for a larger sample of BCDs and Irrs, and include star formation parts of the rotation curve are uncertain because of the presence rate (SFR) indicators in the analysis. We also consider a sample of strong non-circular motions (Elson et al. 2011), thus we as- of rotating Sphs. We use the dynamical information provided signed a conservative error of 15 km s−1 to the inner points of / by VRd Rd to constrain the possible evolutionary links between the rotation curve, which have rotation velocities between ∼30 dwarf galaxies. and ∼60 km s−1. The properties of our sample of 9 BCDs are given in Tables B.1 and B.2. For all these objects, the HST studies pro- 2. The sample vide accurate distances using the tip of the red giant branch We define a dwarf galaxy, in a dynamical sense, as an object (TRGB) method. For NGC 2366, we used the distance derived −1 by Tolstoy et al. (1995) from Cepheids observations, which with Vflat ≤ 100 km s ,whereVflat is the asymptotic velo- city along the flat part of the rotation curve. For a pressure- is consistent within the uncertainties with that obtained from √ the TRGB. supported system, Vflat can be estimated as 3σobs (McGaugh & Wolf 2010), where σobs is the observed velocity dispersion along the line of sight. According to the Tully-Fisher (TF) rela- 2.2. Irregulars −1 tion, Vflat 100 km s occurs at MB −16.5 mag (cf. Verheijen 2001), thus our definition of a dwarf galaxy qualitatively agrees We selected 37 Irrs from the sample of Swaters et al. (2009). with the standard one given by Tammann (1994), which is based We required that the galaxies have high-quality rotation curves on total luminosity and size. However, contrary to Tammann’s (quality-flag q ≤ 2, see Swaters et al. 2009 for details) and in- criteria, our definition is directly related to the potential well of clinations between 30◦ and 80◦, thus the rotation velocities and the galaxy and is less affected by the effects of recent star for- the central surface brightnesses can be measured with small un- mation, which can be serious for BCDs where the light is domi- certainties. The rotation curves of these galaxies have been de- nated by young stellar populations. The choice of 100 km s−1 is rived by Swaters et al. (2009) taking beam-smearing effects into −1 not arbitrary: in galaxies with Vflat ≤ 100 km s bulges tend to account. We also added another 6 objects that meet our quality- disappear (e.g., Kormendy & Bender 2012) and some cosmolog- criteria: UGC 6955 (DDO 105) and UGC 8320 (DDO 168) from ical models predict that mass loss from supernova feedback may Broeils (1992), UGC 6399 and UGC 6446 from Verheijen & start to affect the baryonic content (e.g., Dekel & Silk 1986).