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Register a new userThe Mid-Infrared Properties of Blue Compact Dwarf Galaxies
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The unprecedented sensitivity of the Spitzer Space Telescope has enabled us for the first time to detect a large sample of Blue Compact Dwarf galaxies (BCDs), which are intrinsically faint in the infrared. In the present paper we present a summary of our findings which providing essential information on the presence/absence of the Polycyclic Aromatic Hydrocarbon features in metal-poor environments. In addition, using Spitzer/IRS high-resolution spectroscopy, we study the elemental abundances of neon and sulfur in BCDs and compare with the results from optical studies. Finally, we present an analysis of the mid- and far-infrared to radio correlation in low luminosity low metallicity galaxies.
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We study the correlation between the radio, mid-infrared and far-infrared properties for a sample of 28 blue compact dwarf (BCD) and low metallicity star-forming galaxies observed by Spitzer. We find that these sources extend the same far-infrared to radio correlation typical of star forming late type alaxies to lower luminosities. In BCDs, the 24um (or 22um) mid-infrared to radio correlation is similar to starburst galaxies, though there is somewhat larger dispersion in their q_24 parameter compared to their q_FIR. No strong correlations between the q parameter and galaxy metallicity or effective dust temperature have been detected, though there is a hint of decreasing q_24 at low metallicities. The two lowest metallicity dwarfs in our sample, IZw18 and SBS0335-052E, despite their similar chemical abundance, deviate from the average q$_{24}$ ratio in opposite manners, displaying an apparent radio excess and dust excess respectively.
We report basic far-infrared (FIR) properties of eight blue compact dwarf galaxies (BCDs) observed by AKARI. We measure the fluxes at the four FIS bands (wavelengths of 65 um, 90 um, 140 um, and 160 um). Based on these fluxes, we estimate basic quantities about dust: dust temperature, dust mass, and total FIR luminosity. We find that the typical dust temperature of the BCD sample is systematically higher than that of normal spiral galaxies, although there is a large variety. The interstellar radiation field estimated from the dust temperature ranges up to 100 times of the Galactic value. This confirms the concentrated star-forming activity in BCDs. The star formation rate can be evaluated from the FIR luminosity as 0.01--0.5 $M_odot$ yr$^{-1}$. Combining this quantity with gas mass taken from the literature, we estimate the gas consumption timescales (gas mass divided by the star formation rate), which prove to span a wide range from 1 Gyr to 100 Gyr. A natural interpretation of this large variety can be provided by intermittent star formation activity. We finally show the relation between dust-to-gas ratio and metallicity (we utilize our estimate of dust mass, and take other necessary quantities from the literature). There is a positive correlation between dust-to-gas ratio and metallicity as expected from chemical evolution models.
SBS0335-052 is a blue compact dwarf galaxy (BCD) with one of the lowest known metallicities, Z$sim$Z$_{sun}$/41, making it a local example of how primordial starburst galaxies and their precursors might appear. A spectrum obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope clearly shows silicate absorption features, emission lines of [SIV] and [NeIII], and puts strong upper limits on the PAH emission features. The observed low resolution spectrum (R~90) extends from 5.3 to 35microns and peaks at ~28microns. The spectrum is compared to IRS observations of the prototypical starburst nucleus NGC7714. SBS0335-052 is quite unlike normal starburst galaxies, which show strong PAH bands, low ionization emission lines, and a continuum peak near 80microns. The continuum difference for $lambda >30 mu$m implies a substantial reduction in the mass of cold dust. If the spectrum of this very low metallicity galaxy is representative of star forming galaxies at higher redshifts, it may be difficult to distinguish them from AGNs which also show relatively featureless flat spectra in the mid-IR.
In a spectroscopic survey of the Fornax cluster to Bj=17.5 using the FLAIR spectrograph on the UK Schmidt Telescope we have discovered seven new compact dwarf cluster members. These were previously thought to be giant background spirals. These new members are among the most compact, high surface brightness dwarf galaxies known with absolute magnitudes of M_B=-14 and scale lengths of alpha=400 pc. One in particular may be the first high (normal) surface brightness dwarf spiral discovered. Three of the new dwarfs are blue compact dwarfs (BCDs); their inclusion in the cluster increases the faint end of the BCD luminosity function by a factor of 2 or more. We extended the survey 2.5 mag fainter with the 2dF spectrograph and in our first field found 7 bright emission line galaxies beyond the Fornax cluster which were unresolved on the sky survey plates. Galaxies of this type would be missed in most existing galaxy surveys.
(Abridged) We perform integral field spectroscopy of a sample of Blue compact dwarf (BCD) galaxies with the aim of analyzing their morphology, the spatial distribution of some of their physical properties (excitation, extinction, and electron density) and their relationship with the distribution and evolutionary state of the stellar populations. Integral field spectroscopy observations of the sample galaxies were carried out with the Potsdam Multi-Aperture Spectrophotometer (PMAS) at the 3.5 m telescope at Calar Alto Observatory. An area 16 arcsec x 16 arcsec in size was mapped with a spatial sampling of 1 arcsec x 1 arcsec. We obtained data in the 3590-6996 Angstroms spectral range, with a linear dispersion of 3.2 Angstroms per pixel. From these data we built two-dimensional maps of the flux of the most prominent emission lines, of two continuum bands, of the most relevant line ratios, and of the gas velocity field. Integrated spectra of the most prominent star-forming regions and of whole objects within the FOV were used to derive their physical parameters and the gas metal abundances. Six galaxies display the same morphology both in emission line and in continuum maps; only in two objects, Mrk 32 and Tololo 1434+032, the distributions of the ionized gas and of the stars differ considerably. In general the different excitation maps for a same object display the same pattern and trace the star-forming regions, as expected for objects ionized by hot stars; only the outer regions of Mrk 32, I Zw 123 and I Zw 159 display higher [SII]/Halpha values, suggestive of shocks. Six galaxies display an inhomogeneous dust distribution. Regarding the kinematics, Mrk 750, Mrk 206 and I Zw 159 display a clear rotation pattern, while in Mrk 32, Mrk 475 and I Zw 123 the velocity fields are flat.
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