Dwarf galaxies in the Local Group

Cold dark matter based cosmology theories predict that the smallest dark halos are the first to collapse, leading to the formation of dwarf galaxy sized objects very early on. However, the formation epoch of today's dwarf galaxies and their subsequent evolution cannot yet be directly predicted from structure formation models. The only way to measure the significance of the mechanisms proposed to delay baryonic infall and/or star formation is to use the fossil record of star formation in nearby dwarf galaxies to derive their age distributions in detail. The approximately 60 dwarf galaxies of the Local Group display such a wide range of star formation histories and chemical enrichment that we still lack a comprehensive scenario explaining what triggers and regulates star formation in them. Dwarf spheroidal galaxies are usually gas-poor systems with little, if any, recent star formation. On the other hand, dwarf irregular galaxies are gas-rich systems that have experienced a continuous star formation history and are currently forming stars. Some "transition" systems show signs of discrete and multiple star formation episodes that are separated by quiescent periods. The variety of star formation histories, metallicities, and distances from the Milky Way and M31, makes the Local Group dwarf galaxies a unique laboratory where one can address the properties of their more distant counterparts at high redshift.

STELLAR POPULATIONS AND STAR FORMATION HISTORY

p0431ba rid

At the Padova Observatory the origin and evolution of dwarf galaxies in the Local Group and in the nearest groups has been studied for several years, using different techniques to pinpoint their star formation history and chemical enrichment.  Stellar populations in dwarf spheroidal and irregular galaxies have been addressed using HST and ground-based instrumentation. Among dwarf irregulars, the Sagittarius dwarf (SagDIG, see figure) was observed with the Advanced Camera for Surveys on board HST (cycles 12 and14 proposal "SagDIG: a benchmark for understanding star formation in extreme low-metallicity galaxies", P.I. Momany). Located at the border of the Local Group, this dwarf irregular galaxy is very metal-poor and exceptionally rich in atomic gas. On-going star formation in such an extremely metal-poor environment makes it an ideal target to locally test the hypothesis that really young galaxies may exist. Our ACS study of the stellar populations of SagDIG (Momany et al. 2005) shows that, despite the fact that young stars are the most noticeable feature, the galaxy began forming stars a long time ago, possibly more than 10 Gyr ago. A preliminary reconstruction of its star formation history (Held et al. 2007) suggests a broad episode of star formation between 3 and 8 Gyr ago, with a low-intensity tail of star formation at older ages, and a SF enhancement in the last Gyr.
Among dwarf spheroidals, we have recently studied the Fornax dSph, one of the most luminous dwarf spheroidal satellites of the Milky Way. This dSph is remarkable in many ways: it hosts 5 globular clusters, color-magnitude diagrams shows some relatively young stars (200 Myr old: Saviane et al. 2000, Rizzi et al. 2007), and has faint sub-structures that have been interpreted as signs of recent interactions. In order to learn the complete, spatially-resolved star formation history of Fornax and establish a link between its evolutionary path and the predictions from numerical simulations, we have set up a large international collaboration to conduct an extensive HST study (Cycle 16 proposal "The Star Formation History of the Fornax Dwarf Spheroidal Galaxy", P.I. Held).

EVOLVED STELLAR POPULATIONS IN THE NEAR-INFRARED

fornaxjk

Intermediate-mass stars on the Asymptotic Giant Branch (AGB), and especially carbon-rich (C) stars, have been known for a long time to be important tracers of intermediate-age populations in galaxies.  Because of their high luminosities and distinctive spectral features, AGB stars can significantly contribute to the integrated light of galaxies. The population of carbon stars is a function of the star-formation history and the chemical enrichment of a galaxy. Our team has obtained wide-area near-infrared (JHKs) photometry of several dwarf galaxies in the Local Group as a mean to provide a reliable census of their evolved stellar populations. We especially want to characterize their Asymptotic Giant Branch and Red Giant and Supergiant stellar populations to understand star formation histories in dwarf galaxies between 0.2 and 10 Gyr ago. Our observations in the near infrared are optimal to study the cool Asymptotic Giant Branch stars (see the color-magnitude of Fornax dSph in the figure), which trace the intermediate-age stellar populations, as well as Red Giant Branch stars, from which the metallicity distribution of the stars can be derived (Gullieuszik et al. 2007).
During the bright AGB evolutionary phase, the delicate interplay of several physical processes is very difficult to model, and stellar evolutionary tracks are particularly uncertain. At the same time, being a short lived stage, the observational data sets are usually depleted of this kind of stars, hampering robust empirical calibrations. For these reasons, the use of bright AGB stars as proxies for the star formation history at intermediate ages is far from straightforward. Our team includes researchers with long standing experience on the stellar evolution theory, and on the application of the synthetic CMD method to derive the star formation history of galaxies. We will follow two approaches for interpreting our data: one based on the currently most complete and updated data set of AGB tracks (Marigo & Girardi 2007), another based on a new method (Greggio 2002), that exploits basic principles of the stellar evolution theory to derive star formation rates averaged over relatively wide age ranges. By applying both methods to the same data set we will derive a better estimate of the SFH at intermediate ages, and, perhaps more importantly, we will improve our ability to use these bright stars as tracers of the Star Formation History at intermediate ages. This is especially important for interpreting the CMDs of more distant galaxies, for which only the intrinsically bright stars are accessible.

VARIABLE STARS AS STELLAR POPULATION PROBES

 
fornaxVars

Variable stars are very effective diagnostic tools of different epochs of star formation, including the oldest generation of stars (traced by RR Lyrae's) and the intermediate-age stellar populations (Anomalous Cepheids). The presence of significant populations of RR Lyrae stars in dwarf galaxies provides direct evidence for a first epoch of star formation common to the majority of Local Group dwarf galaxies, both star forming and presently quiescent. Our group has pursued a long-term project in collaboration with researchers in the Bologna and Brera/Merate Observatories aimed at detecting RR Lyrae, Anomalous Cepheids, and Dwarf Cepheids in Local Group Galaxies (Held et al. 2001; Baldacci et al. 2005, Poretti et al. 2008) and their globular clusters (Greco et al. 2007, 2008). We are conducting a systematic search of all types of short-period pulsating stars in the Fornax dwarf (see the figure, which refers to partial data). In addition, follow-up spectroscopy of RR Lyrae has been used to investigate the metal abundances of the oldest stars in dwarf galaxies (as in Sculptor dSph: Clementini et al. 2005).

SPECTROSCOPY

The photometric studies are complemented with spectroscopy of individual stars and interstellar gas, in order to explore the chemical enrichment history of the galaxies and fully disentangle the "age-metallicity" degeneracy which affects the color-magnitude diagrams. Low- and high-resolution spectroscopy obtained with FORS2 and FLAMES at the ESO VLT is currently analysed to investigate the metallicity distribution as well as the internal kinematics and dark matter content of Local Group dwarf galaxies. Our long-range plan is to provide a global view of the dwarfs by studying individual stars across the galaxies.

People: E.V. Held, L. Girardi, L. Greggio, M. Gullieuszik, Y. Momany, S. Zaggia

Collaboration: L. Rizzi (JACH Hawaii), I. Saviane (ESO), G. Clementini (INAF OA Bologna), E. Poretti (INAF OA Brera-Merate), A. Cole (Tasmania Univ.), G. Da Costa (ANU), P. Marigo (Padova Univ.), M. Mateo (Michigan Univ.), M. Rejkuba (ESO), M. Rich (UCLA), E. Tolstoy (Groningen Univ.)

 Publications:  Gullieuszik et al. (2007), A&A 467,1025; Rizzi et al. (2007), MNRAS 380,1255; Greco et al. (2007), ApJ 670,332; Held et al. (2007), IAU Symp. 241,339; Poretti et al. (2008), ApJ 685,947; Saviane et al. (2008), A&A 487,901; Gullieuszik et al. (2008), MNRAS 388,1185; Gullieuszik et al. (2008), A&A 483,5; Held et al. (2009), in Science with the VLT in the ELT Era (Springer),27


 

Dwarf Galaxies outside the Local Group

1705 pressrelease
Figure 1:

Late-type dwarf galaxies (dwarf Irregulars (dIs) and Blue Compact dwarfs (BCD)), with their low metallicity and high gas content, can be regarded as most similar to primeval galaxies. Thus, they are ideal bench marks to understand how galaxies evolved in the early Universe. These systems are also actively forming stars, which makes them unique laboratories for testing our ideas about the occurrence of starbursts, the evolution of massive stars, and the occurrence of galactic winds. Understanding how late-type dwarfs evolve and what were their conditions at the early epochs is then crucial for cosmological purposes. To this aim, the quantitative determination of their star formation history (SFH) is of fundamental importance. Thanks to the high sensitivity and high spatial resolution of HST, in the last decade it has been possible to resolve single stars and derive deep color magnitude diagrams (CMD) for late-type dwarfs up to distances of ∼ 20 Mpc.

NGC4449 pressrelease
Figure 2:

At OAPd we are carrying on a long-term project, in collaboration with researchers at the Bologna Observatory and at STScI, to study the stellar populations in some late-type dwarfs of particular interest: NGC 1705 (the best evidence of a galactic wind), IZw18 (the most metal-poor BCD), NGC 4449 (one of the most active dwarfs). Our goal is to derive the SFHs of these galaxies through the method of the synthetic CMD, applied to deep HST photometry. The results of these monographic studies, complemented with those from similar investigations on other nearby dwarfs (e.g. NGC 1569), will allow us to derive some general conclusions on the formation and evolution of star forming dwarfs.

The BCD NGC 1705 (Figure 1), located at ∼ 5 Mpc distance, hosts a young central super star cluster which accounts for ∼ 50 % of the total integrated UV light, and shows the best observational evidence of an ongoing galactic wind. From the analysis of multiband (U,B,V,I,J,H) HST CMDs we derived a detailed SFH, probing ages up to a Hubble time ( Annibali et al. 2003).
Adding more recent data, we confirmed the recent SF activity (Annibali et al. 2009, AJ, submitted), in particular the burst occurred ∼ 15 Myr ago, which has probably powered the observed galactic wind, as well as the current star formation episode. The latter is characterized by a SF rate much stronger than what usually observed in late-type dwarfs, and only comparable to the exceptional SF activity ongoing in NGC 1569.

NGC 4449 (Figure 2) is another starbursting dwarf in the category of the Magellanic Irregulars, which may be experiencing interaction or merger with a companion. We are currently analyzing our ACS data in the B,V and I bands of the ∼400,000 resolved stars ( Annibali et al. 2008).

4449 cmd
Figure 3:

The deep and rich CMD (Figure 3) includes all key stellar evolution branches, which will allow us to recover the SFH over the whole Hubble time. The BCD IZw18 (Figure 4) is one of the most intriguing nearby objects and has fascinated generations of astronomers since its discovery in 1966 by Zwicky. With a metallicity of ∼ 1/50 solar, it holds the record of the lowest abundance ever measured in a star-forming galaxy in the Local Universe.

IZw18 mb
Figure 4:

IZw18 is also very blue and full of gas. Because of all these properties, it has been indicated as the best candidate for a truly "primordial" galaxy in the Local Universe. While all the late-type dwarf galaxies studied so far are known to host old stellar populations, the epoch at which SF started in IZw18 is still subject of debate.

The major limitation in this respect concerns the distance of this galaxy. Based on recent HST/ACS V and I time series observations of Cepheids, we have shown that IZw18 is located at a distance of ∼ 18 Mpc ( Aloisi et al. 2007), substantially father away than previously thought. These proprietary data, combined with archival ones, yield an accurate CMD which we are currently analyzing to recover the SFH.

 

People: F. Annibali, L. Greggio, E. Held

Collaboration: M. Tosi, G. Clementini, R. Contreras, P. Montegriffo, L. Angeretti (OABo), A. Aloisi, R. van der Marel, A. Grocholski, M. Sirianni, J. Mack, C. Leitherer (STScI), G. Fiorentino (KAY), M. Marconi, I Musella (OADC), A. Saha (NOAO).

Publications: Grocholski et al. (2008), ApJ 686,79; Annibali et al. (2008), AJ 135,1900; Aloisi et al. (2007),ApJ 667,151

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