F. Annibali

I Zw 18 Revisited with HST ACS and Cepheids: New Distance and Age*

We present new V- and I-band HST ACS photometry of I Zw 18, the most metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been argued in the past that I Zw 18 is a very young system that started forming stars only 500 Myr ago, but other work has hinted that older (1 Gyr) red giant branch (RGB) stars may also exist. Our new data, once combined with archival HST ACS data, provide a deep and uncontaminated optical color-magnitude diagram (CMD) that now strongly indicates an RGB. The RGB tip (TRGB) magnitude yields a distance modulus (m - M)0 = 31.30 ± 0.17, i.e., D = 18.2 ± 1.5 Mpc. The time-series nature of our observations allows us to also detect and characterize for the first time three classical Cepheids in I Zw 18. The time-averaged Cepheid V and I magnitudes are compared to the VI reddening-free Wesenheit relation predicted from new nonlinear pulsation models specifically calculated at the metallicity of I Zw 18. For the one bona fide classical Cepheid with a period of 8.63 days this implies a distance modulus (m - M)0 = 31.42 ± 0.26. The other two Cepheids have unusually long periods (125.0 and 129.8 days) but are consistent with this distance. The coherent picture that emerges is that I Zw 18 is farther away than previously assumed and older than suggested by some previous works. The presence of an RGB population rules out the possibility that I Zw 18 is a truly primordial galaxy formed recently (z 0.1) in the local universe.

Starbursts in the Local Universe: New Hubble Space Telescope Advanced Camera for Surveys Observations of the Irregular Galaxy NGC 4449

We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V), and F814W (I) broadband filters, and in the F658N (Hα) narrowband filter. Our photometry includes ≈300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V 28, and ≈400,000 objects in the (V, I) CMD, down to I 27. A subsample of ≈200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (0.2 dex). Also, we detect an interesting resolved star cluster on the west side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Hα emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D = 3.82 ± 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I) broad-band filters, and in the F658N (Halpha) narrow-band filter. Our photometry includes ~300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V < 28, and ~400,000 objects in the (V, I) CMD, down to I < 27 . A subsample of ~200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (< 0.2 dex). Also, we detect an interesting resolved star cluster on the West side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Halpha emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D=3.82 \pm 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.

Starbursts in the Local Universe: new HST/ACS observations of the irregular galaxy NGC 4449

We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V), and F814W (I) broadband filters, and in the F658N (Hα) narrowband filter. Our photometry includes ≈300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V 28, and ≈400,000 objects in the (V, I) CMD, down to I 27. A subsample of ≈200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (0.2 dex). Also, we detect an interesting resolved star cluster on the west side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Hα emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D = 3.82 ± 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I) broad-band filters, and in the F658N (Halpha) narrow-band filter. Our photometry includes ~300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V < 28, and ~400,000 objects in the (V, I) CMD, down to I < 27 . A subsample of ~200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (< 0.2 dex). Also, we detect an interesting resolved star cluster on the West side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Halpha emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D=3.82 \pm 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.

A New Hubble Space Telescope Distance to NGC 1569: Starburst Properties and IC 342 Group Membership

We present deep HST ACS/WFC photometry of the dwarf irregular galaxy NGC 1569, one of the closest and strongest nearby starburst galaxies. These data allow us, for the first time, to unequivocally detect the tip of the red giant branch and thereby determine the distance to NGC 1569. We find that this galaxy is 3.36 ± 0.20 Mpc away, considerably farther away than the typically assumed distance of 2.2 ± 0.6 Mpc. Previously thought to be an isolated galaxy due to its shorter distance, our new distance firmly establishes NGC 1569 as a member of the IC 342 group of galaxies. The higher density environment may help explain the starburst nature of NGC 1569, since starbursts are often triggered by galaxy interactions. On the other hand, the longer distance implies that NGC 1569 is an even more extreme starburst galaxy than previously believed. Previous estimates of the rate of star formation for stars younger than 1 Gyr become stronger by more than a factor of 2. Stars older than this were not constrained by previous studies. The dynamical masses of NGC 1569s three super star clusters, which are already known as some of the most massive ever discovered, increase by ~53% to (6-7) × 105 M☉.

A New Deep HST/ACS CMD of I Zw 18: Evidence for Red Giant Branch Stars

We present results from new deep HST/ACS photometry of I Zw 18, the most metal-poor blue compact dwarf galaxy in the nearby universe. It has been previously argued that this is a very young system that started forming stars only 1 Gyr) red giant branch (RGB) stars may exist in this galaxy. Our deeper data indeed reveal evidence for an RGB. Underlying old (> 1 Gyr) populations are therefore present in even the most metal-poor systems, implying that star formation started at z > 0.1. The RGB tip (TRGB) magnitude and the properties of Cepheid variables identified from our program indicate that I Zw 18 is farther away (D = 19.0 +/- 1.8 Mpc) than previously believed.

Multi-Epoch HST Observations of IZw18: Characterization of Variable Stars at Ultra-Low Metallicities

Variable stars have been identified for the first time in the very metal-poor Blue Compact dwarf galaxy IZw18, using deep multi-band (F606W, F814W) time-series photometry obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We detected 34 candidate variable stars in the galaxy. We classify three of them as Classical Cepheids, with periods of 8.71, 125.0 and 130.3 days, respectively, and other two as long period variables with periodicities longer than a hundred days. These are the lowest metallicity Classical Cepheids known so far, thus providing the opportunity to explore and fit models of stellar pulsation for Classical Cepheids at previously inaccessible metallicities. The period distribution of the confirmed Cepheids is markedly different from what is seen in other nearby galaxies, which is likely related to the star bursting nature of IZw18. The long period Cepheids we have detected in IZw18 seem to indicate that massive stars at the metallicity of IZw18 (Z=0.0004) may cross the instability strip long enough to be observed. By applying to the 8.71 days Cepheid theoretical Wesenheit (V, I) relations based on new pulsation models of Classical Cepheids specifically computed for the extremely low metallicity of this galaxy (Z=0.0004, Y=0.24), we estimate the distance modulus of IZw18 to be μ0= 31.4 ± 0.3 (D=19.0 +2.8 −2.5 Mpc) for canonical models of Classical Cepheids, and of 31.2 ± 0.3 mag (D=17.4 −2.2 Mpc) using over luminous models. The theoretical modeling of the star’s light curves provides μ0 = 31.4± 0.2 mag, D=19.0 −1.7 Mpc, in good agreement with the results from the theoretical Wesenheit relations. These pulsation distances bracket the distance of 18.2 ±1.5 Mpc inferred by Aloisi et al. (2007) using the galaxy’s Red Giant Branch Tip. Subject headings: — galaxies: individual (IZw18) — stars: variables: other Classical Cepheids — distance scale — galaxies: Blue Dwarf Galaxy — techniques: photometry

Multi-Epoch Hubble Space Telescope Observations of IZw18

Variable stars have been identified for the first time in the very metal-poor Blue Compact dwarf galaxy IZw18, using deep multi-band (F606W, F814W) time-series photometry obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We detected 34 candidate variable stars in the galaxy. We classify three of them as Classical Cepheids, with periods of 8.71, 125.0 and 130.3 days, respectively, and other two as long period variables with periodicities longer than a hundred days. These are the lowest metallicity Classical Cepheids known so far, thus providing the opportunity to explore and fit models of stellar pulsation for Classical Cepheids at previously inaccessible metallicities. The period distribution of the confirmed Cepheids is markedly different from what is seen in other nearby galaxies, which is likely related to the star bursting nature of IZw18. The long period Cepheids we have detected in IZw18 seem to indicate that massive stars at the metallicity of IZw18 (Z=0.0004) may cross the instability strip long enough to be observed. By applying to the 8.71 days Cepheid theoretical Wesenheit (V, I) relations based on new pulsation models of Classical Cepheids specifically computed for the extremely low metallicity of this galaxy (Z=0.0004, Y=0.24), we estimate the distance modulus of IZw18 to be μ0= 31.4 ± 0.3 (D=19.0 +2.8 −2.5 Mpc) for canonical models of Classical Cepheids, and of 31.2 ± 0.3 mag (D=17.4 −2.2 Mpc) using over luminous models. The theoretical modeling of the star’s light curves provides μ0 = 31.4± 0.2 mag, D=19.0 −1.7 Mpc, in good agreement with the results from the theoretical Wesenheit relations. These pulsation distances bracket the distance of 18.2 ±1.5 Mpc inferred by Aloisi et al. (2007) using the galaxy’s Red Giant Branch Tip. Subject headings: — galaxies: individual (IZw18) — stars: variables: other Classical Cepheids — distance scale — galaxies: Blue Dwarf Galaxy — techniques: photometry

A New Age and Distance for I Zw 18, the Most Metal-Poor Galaxy in the Nearby Universe

The blue compact dwarf galaxy I Zw 18 holds the record of the lowest metallicity ever observed in the local universe. As such, it represents the closest analog to primordial galaxies in the early universe. More interestingly, it has recurrently been regarded as a genuinely young galaxy caught in the process of forming in the nearby universe. However, stars of increasingly older ages are found within I Zw 18 every time deeper high-resolution photometric observations are performed with the Hubble Space Telescope (HST): from the original few tens of Myrs to, possibly, several Gyrs. Here we summarize the history of I Zw 18 age and present an ongoing HST/ACS project which allowed us to precisely derive the galaxy distance by studying its Cepheid variables, and to firmly establish the age of its faintest resolved populations.

New HST/ACS Data of the Starburst Irregular Galaxy NGC 4449

We present B, V and I photometry with HST/ACS of the resolved stellar populations in the Magellanic starburst galaxy NGC 4449. Our photometry provides ≈ 300,000 objects in the (B, V) color-magnitude diagrams (CMDs) and ≈ 400,000 objects in the (V, I) CMDs. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. Our data clearly indicate the presence of an age gradient, with young and intermediate-age stars concentrated toward the galactic center, and old stars present everywhere. We derive the distance of NGC 4449 with an accuracy of 5% from the tip of the red giant branch: D = 3.82 ± 0.18 Mpc.

New HST/ACS Observations of Dwarf Star-Forming Galaxies with Extreme Properties of Cosmological Relevance

We present deep Color Magnitude Diagrams (CMDs), based on HST/ACS data, of two star-forming dwarf galaxies, NGC4449 and IZw18.