II Zw 40 – 30 Doradus on Steroids

AbstractWe obtained HST COS G140L spectra of the enigmatic nearby blue compact dwarf galaxy II Zw 40. The galaxy hosts a nuclear super star cluster embedded in a radio-bright nebula, similar to those observed in the related blue compact dwarfs NGC 5253 and Henize 2-10. The ultraviolet spectrum of II Zw 40 is exceptional in terms of the strength of He II 1640, O III] 1666 and C III] 1909. We determined reddening, age, and the cluster mass from the ultraviolet data. The super nebula and the ionizing cluster exceed the ionizing luminosity and stellar mass of the local benchmark 30 Doradus by an order of magnitude. Comparison with stellar evolution models accounting for rotation reveals serious short-comings: these models do not account for the presence of Wolf-Rayet-like stars at young ages observed in II Zw 40. Photoionization modeling is used to probe the origin of the nebular lines and determine gas phase abundances. C/O is solar, in agreement with the result of the stellar-wind modeling.

Download Full-text

The Dark Blue Compact Dwarf Galaxy NGC 2915

Publications of the Astronomical Society of Australia ◽

10.1071/as97077 ◽

1997 ◽

Vol 14 (1) ◽

pp. 77-80 ◽

Cited By ~ 2

Author(s):

Gerhardt R. Meurer

Keyword(s):

Dark Matter ◽

Mass Distribution ◽

Dwarf Galaxy ◽

Dark Matter Halo ◽

High Mass ◽

Mass Star ◽

Normal Galaxies ◽

The Galaxy ◽

Dark Blue ◽

Blue Compact Dwarf Galaxy

AbstractRecent results on NGC 2915, the first blue compact dwarf galaxy to have its mass distribution modelled, are summarised. NGC 2915 is shown to have HI well beyond its detected optical extent. Its rotation curve is well determined and fit with maximum disk mass models. The dark matter halo dominates the mass distribution at nearly all radii, and has a very dense core compared to those of normal galaxies. High-mass star formation energises the HI in the centre of the galaxy, but appears to be maintained in viriai equilibrium with the dark matter halo. The implications of these results are briefly discussed.

Download Full-text

Haro15: Is it actually a low metallicity galaxy?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310000657 ◽

2009 ◽

Vol 5 (S265) ◽

pp. 243-244

Author(s):

Verónica Firpo ◽

Guillermo Bosch ◽

Guillermo Hägele ◽

Ángeles I. Díaz ◽

Nidia Morrell

Keyword(s):

Dwarf Galaxy ◽

Empirical Methods ◽

Chemical Abundances ◽

Physical Conditions ◽

Ionization Structure ◽

The Galaxy ◽

Low Metallicity ◽

Echelle Spectroscopy ◽

Abundance Determination ◽

Blue Compact Dwarf Galaxy

AbstractWe present a detailed study of the physical properties of the nebular material in multiple knots of the blue compact dwarf galaxy Haro 15. Using long slit and echelle spectroscopy, obtained at Las Campanas Observatory, we study the physical conditions (electron density and temperature), ionic and total chemical abundances of several atoms, reddening and ionization structure. The latter was derived by comparing the oxygen and sulphur ionic ratios to their corresponding observed emission line ratios (the η and η' plots) in different regions of the galaxy. Applying direct and empirical methods for abundance determination, we perform a comparative analysis between these regions.

Download Full-text

Nearby Blue Compact Dwarf Galaxy NGC 6789

Symposium - International Astronomical Union ◽

10.1017/s0074180900204191 ◽

1999 ◽

Vol 192 ◽

pp. 253-258

Author(s):

I. Drozdovsky ◽

N. Tikhonov

Keyword(s):

Surface Brightness ◽

Dwarf Galaxy ◽

High Surface ◽

Low Surface Brightness ◽

The Galaxy ◽

Metal Abundance ◽

The Mean ◽

Red Giant ◽

Halo Population ◽

Blue Compact Dwarf Galaxy

We present the results of a detailed BVRI and Hα study of the isolated nearby blue compact dwarf (BCD) galaxy NGC 6789. Judging from the literature the observed galaxy has not yet been resolved into stars up to now. On CCD frames obtained with 6m BTA telescope and 2.5m Nordic telescope the galaxy is well resolved. Its colour-magnitude diagram confirms the two component (core-halo) galaxy morphology, which consists of two stellar populations distinct in structure and colour: an inner high surface-brightness young population within 150 pc from the center of the galaxy, and a relatively low surface-brightness intermediate-age population extending out to at least 600 pc. The distance to the galaxy, estimated from the tip of the red giant branch (TRGB) is 2.1 Mpc which places NGC 6789 close to the Local Group. From the mean colour of the RGB, the mean metal abundance of the halo population is estimated as [Fe/H] ≃ −1 dex.

Download Full-text

VLT/FORS Surveys of Wolf-Rayet Stars in the Nearby Universe

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308020656 ◽

2007 ◽

Vol 3 (S250) ◽

pp. 327-332

Author(s):

Lucy J. Hadfield ◽

Paul A. Crowther

Keyword(s):

Local Group ◽

Narrow Band ◽

Dwarf Galaxy ◽

Narrow Band Imaging ◽

Interstellar Extinction ◽

Spectroscopic Studies ◽

Order Of Magnitude ◽

Low Metallicity ◽

Massive Clusters ◽

Blue Compact Dwarf Galaxy

AbstractWe present results from a series of VLT/FORS narrow-band imaging and spectroscopic surveys of Wolf-Rayet (WR) stars in nearby spiral galaxies and compare observed populations in high- and low metallicity environments. The metal-rich galaxy M 83 is seen to host an exceptional WR content, with over 1000 WR stars being detected. N(WC)/N(WN) ~ 1.2 and late-type WC subtypes dominate the WC population. At low metallicity, ~100 stars has been identified within NGC 1313, with N(WC)/N(WN) ~ 0.5. In contrast to M83, the WC population of NGC 1313 comprises solely early subtypes plus a WO star (the first WO star to be identified beyond the Local Group). Consequently, the dominant WC subtype may serve as a crude metallicity diagnostic for WR galaxies.In addition, the WR content of the blue compact dwarf galaxy NGC 3125 is examined. Previous UV and optical spectroscopic studies of knot A in NGC 3125 derive WR populations which differ by more than an order of magnitude. New VLT observations and archival HST spectroscopy reconcile this discrepancy via the use of LMC WR spectral templates and a reduced nebular-derived interstellar extinction. Empirical N(WR)/N(O) ratios for clusters within NGC 3125 are a factor of two higher than evolutionary synthesis predictions but are consistent with those observed for other young massive clusters.

Download Full-text

Globular clusters in the stellar stream surrounding the Milky Way analogue NGC 5907

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3382 ◽

2019 ◽

Vol 491 (4) ◽

pp. 5693-5701 ◽

Cited By ~ 4

Author(s):

Adebusola B Alabi ◽

Duncan A Forbes ◽

Aaron J Romanowsky ◽

Jean P Brodie

Keyword(s):

Spiral Galaxy ◽

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Young Star ◽

Star Cluster ◽

Wide Field ◽

Stellar Disc ◽

The Galaxy ◽

Specific Frequency

ABSTRACT We study the globular clusters (GCs) in the spiral galaxy NGC 5907 well-known for its spectacular stellar stream – to better understand its origin. Using wide-field Subaru/Suprime-Cam gri images and deep Keck/DEIMOS multi-object spectroscopy, we identify and obtain the kinematics of several GCs superimposed on the stellar stream and the galaxy disc. We estimate the total number of GCs in NGC 5907 to be 154 ± 44, with a specific frequency of 0.73 ± 0.21. Our analysis also reveals a significant, new population of young star cluster candidates found mostly along the outskirts of the stellar disc. Using the properties of the stream GCs, we estimate that the disrupted galaxy has a stellar mass similar to the Sagittarius dwarf galaxy accreted by the Milky Way, i.e. $\sim 10^8~\rm M_\odot$.

Download Full-text

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

Proceedings of the International Astronomical Union ◽

10.1017/s174392130802512x ◽

2008 ◽

Vol 4 (S255) ◽

pp. 387-391

Author(s):

A. Aloisi ◽

G. Clementini ◽

M. Tosi ◽

F. Annibali ◽

R. Contreras ◽

...

Keyword(s):

High Resolution ◽

Early Universe ◽

Dwarf Galaxy ◽

Hubble Space Telescope ◽

Local Universe ◽

Photometric Observations ◽

The Galaxy ◽

Cepheid Variables ◽

History Of ◽

Blue Compact Dwarf Galaxy

AbstractThe 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.

Download Full-text

Inferring the parallax of Westerlund 1 from Gaia DR2

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3628 ◽

2019 ◽

Vol 492 (2) ◽

pp. 2497-2509 ◽

Cited By ~ 3

Author(s):

Mojgan Aghakhanloo ◽

Jeremiah W Murphy ◽

Nathan Smith ◽

John Parejko ◽

Mariangelly Díaz-Rodríguez ◽

...

Keyword(s):

Stellar Evolution ◽

Bayesian Model ◽

Main Sequence ◽

Zero Point ◽

Star Cluster ◽

Initial Mass ◽

Data Release ◽

The Galaxy ◽

Late Stages ◽

Gaia Dr2

ABSTRACT Westerlund 1 (Wd1) is potentially the largest star cluster in the Galaxy. That designation critically depends upon the distance to the cluster, yet the cluster is highly obscured, making luminosity-based distance estimates difficult. Using Gaia Data Release 2 (DR2) parallaxes and Bayesian inference, we infer a parallax of $0.35^{+0.07}_{-0.06}$ mas corresponding to a distance of $2.6^{+0.6}_{-0.4}$ kpc. To leverage the combined statistics of all stars in the direction of Wd1, we derive the Bayesian model for a cluster of stars hidden among Galactic field stars; this model includes the parallax zero-point. Previous estimates for the distance to Wd1 ranged from 1.0 to 5.5 kpc, although values around 5 kpc have usually been adopted. The Gaia DR2 parallaxes reduce the uncertainty from a factor of 3 to 18 per cent and rules out the most often quoted value of 5 kpc with 99 per cent confidence. This new distance allows for more accurate mass and age determinations for the stars in Wd1. For example, the previously inferred initial mass at the main-sequence turn-off was around 40 M⊙; the new Gaia DR2 distance shifts this down to about 22 M⊙. This has important implications for our understanding of the late stages of stellar evolution, including the initial mass of the magnetar and the LBV in Wd1. Similarly, the new distance suggests that the total cluster mass is about four times lower than previously calculated.

Download Full-text

A multi-frequency study of the Wolf-Rayet blue compact dwarf galaxy He2-10

Symposium - International Astronomical Union ◽

10.1017/s0074180900206426 ◽

1999 ◽

Vol 193 ◽

pp. 610-611

Author(s):

David I. Méndez ◽

César Esteban ◽

Miroslav D. Filipović ◽

Matthias Ehle ◽

Prank Haberl ◽

...

Keyword(s):

High Resolution ◽

Supernova Remnants ◽

Dwarf Galaxy ◽

Mechanical Action ◽

Radio Continuum ◽

High Resolution Spectroscopy ◽

Continuum Emission ◽

Star Forming ◽

The Galaxy ◽

Blue Compact Dwarf Galaxy

We present preliminary results on multi-wavelength observations of the Wolf-Rayet Blue Compact Dwarf Galaxy He2–10. These observations include Hα and continuum imaging, high-resolution Hα spectroscopy, high-resolution radio-continuum mapping at 6.3 and 3.5 cm and X-ray mapping. The deep Hα image reveals that the galaxy consists of a complex system of different star-forming knots surrounded by kpc-scale bubble-like and filamentary structures. The most interesting structure is a bipolar superbubble centered on the most intense star-formation knot. High-resolution spectroscopy of this structure indicates that it is expanding with a velocity in the range 75–250 km s−1. This kind of outflows is likely to be produced by the mechanical action of stellar winds and supernovae explosions in the intense starbursts that the galaxy hosts. This scenario is consistent with the finding of a very steep radio spectral index in the extended radio continuum emission (α = −0.59) that confirms the presence of a large number of supernova remnants in the galaxy.

Download Full-text