The radio-far-infrared correlation in the faintest star-forming dwarf galaxies (original) (raw)
Related papers
The Effect of Star Formation on the Far-Infrared-Radio Correlation within Galaxies
The Astrophysical Journal, 2006
Using data obtained for twelve galaxies as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope (WSRT)-SINGS radio continuum survey, we study how star formation activity affects the far-infrared (FIR)-radio correlation within galaxies by testing a phenomenological model which describes the radio image as a smeared version of the FIR image. The physical basis of this description is that cosmic-ray (CR) electrons will diffuse measurably farther than the mean free path of dust-heating photons before decaying by synchrotron radiation. This description works well in general. Galaxies with higher infrared surface brightnesses have best-fit smoothing scale-lengths of a few hundred parsecs, substantially shorter than those for lower surface brightness galaxies. We interpret this result to suggest that galaxies with higher disk averaged star formation rates have had a recent episode of enhanced star formation and are characterized by a higher fraction of young CR electrons that have traveled only a few hundred parsecs from their acceleration sites in supernova remnants compared to galaxies with lower star formation activity.
The Astrophysical Journal, 2006
We present an initial look at the far infrared-radio correlation within the starforming disks of four nearby, nearly face-on galaxies (NGC 2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging, observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS), and Westerbork Synthesis Radio Telescope (WSRT) radio continuum data, taken for the WSRT SINGS radio continuum survey, we are able to probe variations in the logarithmic 24 µm/22 cm (q 24 ) and 70 µm/22 cm (q 70 ) surface brightness ratios across each disk at sub-kpc scales. We find general trends of decreasing q 24 and q 70 with declining surface brightness and with increasing radius. We also find that the dispersion in q 24 is generally a bit larger than what is found for q 70 within galaxies, and both are comparable to what is measured globally among galaxies at around 0.2 dex. The residual dispersion around the 2 trend of q 24 and q 70 versus surface brightness is smaller than the residual dispersion around the trend of q 24 and q 70 versus radius, on average by ∼0.1 dex, indicating that the distribution of star formation sites is more important in determining the infrared/radio disk appearance than the exponential profiles of disks. We have also performed preliminary phenomenological modeling of cosmic ray electron (CRe − ) diffusion using an image-smearing technique, and find that smoothing the infrared maps improves their correlation with the radio maps. We find that exponential smoothing kernels work marginally better than Gaussian kernels, independent of projection for these nearly face-on galaxies. This result suggests that additional processes besides simple random-walk diffusion in three dimensions must affect the evolution of CRe − s. The best fit smoothing kernels for the two less active star-forming galaxies (NGC 2403 and NGC 3031) have much larger scale-lengths than those of the more active star-forming galaxies (NGC 5194 and NGC 6946). This difference may be due to the relative deficit of recent CRe − injection into the interstellar medium (ISM) for the galaxies having largely quiescent disks.
Far-infrared properties of submillimeter and optically faint radio galaxies
Astronomy and Astrophysics, 2010
We use deep observations obtained with the Photodetector Array Camera & Spectrometer (PACS) onboard the Herschel space observatory to study the far-infrared (FIR) properties of submillimeter and optically faint radio galaxies (SMGs and OFRGs). From literature we compiled a sample of 35 securely identified SMGs and nine OFRGs located in the GOODS-N and the A2218 fields. This sample is cross-matched with our PACS 100 µm and 160 µm multi-wavelength catalogs based on sources-extraction using prior detections at 24 µm. About half of the galaxies in our sample are detected in at least the PACS 160 µm bandpass. The dust temperatures and the infrared luminosities of our galaxies are derived by fitting their PACS and SCUBA 850 µm (only the upper limits for the OFRGs) flux densities with a single modified (β = 1.5) black body function. The median dust temperature of our SMG sample is T dust = 36 ± 8 K while for our OFRG sample it is T dust = 47 ± 3 K. For both samples, median dust temperatures derived from Herschel data agree well with previous estimates. In particular, Chapman et al. found a dust temperature of T dust = 36 ± 7 K for a large sample of SMGs assuming the validity of the FIR/radio correlation (i.e., q = log 10 (L FIR [W]/L 1.4 GHz [W Hz −1 ]/3.75 × 10 12 )). The agreement between our studies confirms that the local FIR/radio correlation effectively holds at high redshift even though we find q = 2.17 ± 0.19, a slightly lower value than that observed in local systems. The median infrared luminosities of SMGs and OFRGs are 4.6×10 12 L ⊙ and 2.6×10 12 L ⊙ , respectively. We note that for both samples the infrared luminosity estimates from the radio part of the spectral energy distribution (SED) are accurate, while estimates from the mid-IR are considerably (∼ ×3) more uncertain. Our observations confirm the remarkably high luminosities of SMGs and thus imply median star-formation rates of 960 M ⊙ yr −1 for SMGs with S (850 µm) > 5 mJy and 460 M ⊙ yr −1 for SMGs with S (850 µm) > 2 mJy, assuming a Chabrier IMF and no dominant AGN contribution to the far-infrared luminosity.
BLAST: the far-infrared/radio correlation in distant galaxies
Monthly Notices of the Royal Astronomical Society, 2010
We investigate the correlation between far-infrared (FIR) and radio luminosities in distant galaxies, a lynchpin of modern astronomy. We use data from the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), Spitzer, the Large Apex BOlometer Cam-erA (LABOCA), the Very Large Array (VLA) and the Giant Metre-wave Radio Telescope (GMRT) in the Extended Chandra Deep Field South (ECDFS). For a catalogue of BLAST 250-µm-selected galaxies, we re-measure the 70-870-µm flux densities at the positions of their most likely 24-µm counterparts, which have a median [interquartile] redshift of 0.74 [0.25, 1.57]. From these, we determine the monochromatic flux density ratio, q 250 (= log 10 [S 250µm /S 1,400MHz ]), and the bolometric equivalent, q IR . At z ≈ 0.6, where our 250-µm filter probes rest-frame 160-µm emission, we find no evolution relative to q 160 for local galaxies. We also stack the FIR and submm images at the positions of 24-µm-and radio-selected galaxies. The difference between q IR seen for 250-µm-and radio-selected galaxies suggests star formation provides most of the IR luminosity in < ∼ 100-µJy radio galaxies, but rather less for those in the mJy regime. For the 24-µm sample, the radio spectral index is constant across 0 < z < 3, but q IR exhibits tentative evidence of a steady decline such that q IR ∝ (1 + z) −0.15±0.03 -significant evolution, spanning the epoch of galaxy formation, with major implications for techniques that rely on the FIR/radio correlation. We compare with model predictions and speculate that we may be seeing the increase in radio activity that gives rise to the radio background.
The Radio-FIR Correlation in the Milky Way
Publications of the Astronomical Society of Australia, 2010
We investigate the scale on which the correlation arises between the 843 MHz radio and the 60 μm far-infrared (FIR) emission from star forming regions in the Milky way. The correlation, which exists on the smallest scales investigated (down to ≈ 4 pc), becomes noticeably tight on fields of size 30′, corresponding to physical scales of ≈ 20–50 pc. The FIR to radio flux ratio on this scale is consi stent with the radio emission being dominated by thermal emission. We also investigate the location dependence ofqmean, a parameter measuring the mean FIR to radio flux ratio, of a sample of star forming regions. We show thatqmeandisplays a modest dependence on galactic latitude. If this is interpreted as a dependence on the intensity of star formation activity, the result is consistent with studies of the Large Magellanic Cloud (LMC) and other near by galaxies that show elevated values forqin regions of enhanced star formation.
The Astrophysical Journal, 2010
We have measured the near-infrared colors and the fluxes of individual pixels in 68 galaxies common to the Spitzer Infrared Nearby Galaxies Survey and the Large Galaxy Atlas Survey. Each galaxy was separated into regions of increasingly red near-infrared colors. In the absence of dust extinction and other non-stellar emission, stellar populations are shown to have relatively constant NIR colors, independent of age. In regions of high star formation, the average intensity of pixels in red-excess regions (at 1.25 µm, 3.6 µm, 4.5 µm, 5.6 µm, 8.0 µm and 24 µm) scales linearly with the intrinsic intensity of Hα emission, and thus with the star-formation rate within the pixel. This suggests that most NIR-excess regions are not red because their light is being depleted by absorption. Instead, they are red because additional infrared light is being contributed by a process linked to star-formation. This is surprising because the shorter wavelength bands in our study (1.25 µm-5.6 µm) do not probe emission from cold (10-20 K) and warm (50-100 K) dust associated with star-formation in molecular clouds. However, emission from hot dust (700-1000 K) and/or Polycyclic Aromatic Hydrocarbon molecules can explain the additional emission seen at the shorter wavelengths in our study. The contribution from hot dust and/or PAH emission at 2 µm-5 µm and PAH emission at 5.6 µm and 8.0 µm scales linearly with warm dust emission at 24 µm and the intrinsic Hα emission. Since both are tied to the star-formation rate, our analysis shows that the NIR excess continuum emission and PAH emission at ∼ 1 − 8 µm can be added to spectral energy distribution models in a very straight-forward way, by simply adding an additional component to the models that scales linearly with star-formation rate. Recent work by and shows that a good estimator for the starformation rate emerges from the linear combination of a galaxy's near-UV or visible-wavelength emission (either UV continuum or line fluxes, attenuated by dust) and its
Monthly Notices of the Royal Astronomical Society, 2003
The radio counterparts to the 15-µm sources in the European Large Area ISO Survey southern fields are identified in 1.4-GHz maps down to ∼ 80 µJy. The radio -MIR correlation is investigated and derived for the first time at these flux densities for a sample of this size. Our results show that radio and MIR luminosities correlate almost as well as radio and FIR, at least up to z ≃ 0.6. Using the derived relation and its spread together with the observed 15-µm counts, we have estimated the expected contribution of the 15-µm extragalactic populations to the radio source counts and the role of MIR starburst galaxies in the well known 1.4-GHz source excess observed at sub-mJy levels. Our analysis demonstrates that IR emitting starburst galaxies do not contribute significantly to the 1.4-GHz counts for strong sources, but start to become a significant fraction of the radio source population at flux densities < ∼ 0.5 − 0.8 mJy. They are expected to be responsible for more than 60% of the observed radio counts at < ∼ 0.05 mJy. These results are in agreement with the existing results on optical identifications of faint radio sources.
Monthly Notices of the Royal Astronomical Society
The infrared-radio correlation (IRRC) underpins many commonly used radio luminosity–star formation rate (SFR) calibrations. In preparation for the new generation of radio surveys, we revisit the IRRC of low-z galaxies by (a) drawing on the best currently available infrared (IR) and 1.4 GHz radio photometry, plus ancillary data over the widest possible area, and (b) carefully assessing potential systematics. We compile a catalogue of ∼9500, z < 0.2 galaxies and derive their 1.4 GHz radio (L1.4), total IR, and monochromatic IR luminosities in up to seven bands, allowing us to parametrize the wavelength dependence of monochromatic IRRCs from 22–500 µm. For the first time for low-z samples, we quantify how poorly matched IR and radio survey depths bias measured median IR/radio ratios, overlineqmathrmTIR\overline{q}_{\mathrm{TIR}}overlineqmathrmTIR, and discuss the level of biasing expected for low-z IRRC studies in ASKAP/MeerKAT fields. For our subset of ∼2000 high-confidence star-forming galaxies, we find a median $...
INFRARED PROPERTIES OF A COMPLETE SAMPLE OF STAR-FORMING DWARF GALAXIES
2010
We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the Spitzer Space Telescope. The relationships between metallicity, star formation rate (SFR) and mid-infrared color in these systems show that the galaxies span a wide range of properties. However, the galaxies do show a deficit of 8.0 µm polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0 µm luminosity which is only 0.004 L * 8.0 while the median B-band luminosity is 0.05 L * B . Despite many of the galaxies being 8.0 µm deficient, there is about a factor of 4 more extremely red galaxies in the [3.6] − [8.0] color than for a sample of normal galaxies with similar optical colors. We show correlations between the [3.6] − [8.0] color and luminosity, metallicity, and to a lesser extent SFRs that were not evident in the original, smaller sample studied previously. The luminosity-metallicity relation has a flatter slope for dwarf galaxies as has been indicated by previous work. We also show a relationship between the 8.0 µm luminosity and the metallicity of the galaxy which is not expected given the competing effects (stellar mass, stellar population age, and the hardness of the radiation field) that influence the 8.0 µm emission. This larger sample plus a well-defined selection function also allows us to compute the 8.0 µm luminosity function and compare it with the one for the local galaxy population. Our results show that below 10 9 L ⊙ , nearly all the 8.0 µm luminosity density of the local universe arises from dwarf galaxies that exhibit strong Hα emission -i.e., 8.0 µm and Hα selection identify similar galaxy populations despite the deficit of 8.0 µm emission observed in these dwarfs.
Breaking FIR-Radio Correlation: The Case of Interacting Galaxies
Far-infrared (FIR)--radio correlation is a well-established empirical connection between continuum radio and dust emission of star-forming galaxies, used as a tool in determining star-formation rates. Here we point out that in the case of interacting star-forming galaxies this tool might break. Galactic interactions and mergers have been known to give rise to tidal shocks and disrupt morphologies especially in the smaller of the interacting components. Moreover, these shocks can also heat the gas and dust and accelerate particles leading to tidal cosmic-ray population in addition to standard galactic cosmic rays. Both heating and additional non-thermal radiation will obviously affect the FIR-radio correlation of these systems. To test this hypothesis we have analyzed a sample of 43 infrared bright star-forming interacting galaxies at different merger stages. We have found that their FIR-radio correlation parameter and radio emission spectral index vary over different merger stages a...