The Effect of Star Formation on the Far-Infrared-Radio Correlation within Galaxies (original) (raw)
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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.
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-far-infrared correlation in the faintest star-forming dwarf galaxies
Monthly Notices of the Royal Astronomical Society: Letters, 2012
We study the radio -far-infrared (FIR) correlation in a sample of faint dwarf irregular galaxies using NVSS data for 1.4 GHz radio flux, Spitzer MIPS 70µm data for FIR flux, and GALEX FUV data to estimate the star formation rates (SFR). Since our target galaxies are extremely faint, we stack images of many galaxies together to estimate the average radio and FIR fluxes. We find that for a given SFR both 70µm and 1.4 GHz fluxes are low compared to the calibration for large spirals. Nonetheless, the ratio of 70µm to 1.4 GHz flux agrees within errorbars with that seen for large galaxies. The radio-FIR correlation thus appears to be the result of a 'conspiracy'. We use the SFR to estimate the non-thermal fraction of the 1.4 GHz radio emission and find it to be around 50%, much smaller than the 90% typical for spirals. We also estimate the equipartition magnetic field and find it to be ∼ 2 µG, about five times smaller than that typical for spirals.
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
Far infrared and radio emission in dusty starburst galaxies
Arxiv preprint astro-ph/0206029, 2002
We revisit the nature of the far infrared (FIR)/Radio correlation by means of the most recent models for star forming galaxies, focusing in particular on the case of obscured starbursts. We model the IR emission with our population synthesis code, GRASIL (Silva et al. 1998). As for the radio emission, we revisit the simple model of . We find that a tight FIR/Radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electrons cooling time is shorter than the fading time of the supernova (SN) rate. Observations indicate that both these conditions are met in star forming galaxies, from normal spirals to obscured starbursts. However, since the radio non thermal (NT) emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the poststarburst phase, when the bulk of the NT component originates from less massive stars. We show that this delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation (SF) indicators. We suggest to complement the analysis of the deviations from the FIR/Radio correlation with the radio slope (q-Radio slope diagram) to obtain characteristic parameters of the burst, e.g. its intensity, age and fading time scale. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q-radio slope diagram to asses the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. Finally we discuss how deviations from the correlation, due to the evolutionary status of the starburst, affect the technique of photometric redshift determination widely used for the high-z sources.
Monthly Notices of the Royal Astronomical Society
Using infrared data from the Herschel Space Observatory and Karl G. Jansky Very Large Array (VLA) 3 GHz observations in the COSMOS field, we investigate the redshift evolution of the infrared-radio correlation (IRRC) for star-forming galaxies (SFGs) we classify as either spheroid-or disc-dominated based on their morphology. The sample predominantly consists of disc galaxies with stellar mass 10 10 M , and residing on the star-forming main sequence (MS). After the removal of AGN using standard approaches, we observe a significant difference between the redshift-evolution of the median IR/radio ratio q TIR of (i) a sample of ellipticals, plus discs with a substantial bulge component ('spheroid-dominated' SFGs) and, (ii) virtually pure discs and irregular systems ('disc-dominated' SFGs). The spheroid-dominated population follows a declining q TIR vs. z trend similar to that measured in recent evolutionary studies of the IRRC. However, for disc-dominated galaxies, where radio and IR emission should be linked to star formation in the most straightforward way, we measure very little change in q TIR. This suggests that low-redshift calibrations of radio emission as an SFR-tracer may remain valid out to at least z 1 − 1.5 for pure star-forming systems. We find that the different redshift-evolution of q TIR for the spheroid-and discdominated sample is mainly due to an increasing radio excess for spheroid-dominated galaxies at z 0.8, hinting at some residual AGN activity in these systems. This finding demonstrates that in the absence of AGN the IRRC is independent of redshift, and that radio observations can therefore be used to estimate SFRs at all redshifts for genuinely star-forming galaxies.
We use deep panchromatic datasets in the GOODS-N field, from GALEX to the deepest Herschel far-infrared and VLA radio continuum imaging, to explore, using mass-complete samples, the evolution of the star formation activity and dust attenuation of star-forming galaxies to z ≃4. Our main results can be summarized as follows: i) the slope of the SFR-M * correlation is consistent with being constant ≃0.8 at least to z ≃1.5, while its normalization keeps increasing with redshift; ii) for the first time here we are able to explore the FIR-Radio correlation for a mass-selected sample of star-forming galaxies: the correlation does not evolve up to z ≃4; iii) we confirm that galaxy stellar mass is a robust proxy for UV dust attenuation in star-forming galaxies, with more massive galaxies being more dust attenuated, strikingly we find that this attenuation relation evolves very weakly with redshift, the amount of dust attenuation increasing by less than 0.3 magnitudes over the redshift range [0.5-4] for a fixed stellar mass, as opposed to a tenfold increase of star formation rate; iv) the correlation between dust attenuation and the UV spectral slope evolves in redshift, with the median UV spectral slope of star-forming galaxies becoming bluer with redshift. By z ≃3, typical UV slopes are inconsistent, given the measured dust attenuation, with the predictions of commonly used empirical laws. Finally, building on existing results, we show that gas reddening is marginally larger (by a factor of around 1.3) than stellar reddening at all redshifts probed, and also that the amount of dust attenuation at a fixed ISM metallicity increases with redshift. We speculate that our results support evolving ISM conditions of typical star-forming galaxies such that at z ≥1.5 Main Sequence galaxies have ISM conditions getting closer to those of local starbursts.
Herschel/HerMES: the X-ray-infrared correlation for star-forming galaxies at z∼1
Monthly Notices of the Royal Astronomical Society, 2011
For the first time, we investigate the X-ray/infrared (IR) correlation for star-forming galaxies at z ∼ 1, using SPIRE submm data from the recently-launched Herschel Space Observatory and deep X-ray data from the 2 Ms Chandra deep field north (CDFN) survey. We examine the X-ray/IR correlation in the soft X-ray (SX, 0.5-2 keV) and hard X-ray (HX, 2-10 keV) bands by comparing our z ∼ 1 SPIRE-detected star-forming galaxies (SFGs) to equivalently IR-luminous (L IR > 10 10 L ⊙) samples in the local/low redshift Universe. Our results suggest that the X-ray/IR properties of the SPIRE SFGs are on average similar to those of their local counterparts, as we find no evidence for evolution in the L SX /L IR and L HX /L IR ratios with redshift. We note however, that at all redshifts, both L SX /L IR and L HX /L IR are strongly dependent on IR luminosity, with luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs, L IR > 10 11 L ⊙) having up to an order of magnitude lower values than normal infrared galaxies (L IR < 10 11 L ⊙). We derive a L SX-L IR relation and confirm the applicability of an existing L HX-L IR relation for both local and distant LIRGs and ULIRGs, consistent with a scenario where X-ray luminosity is correlated with the star-formation
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...
Measures of Star Formation Rates within Galaxies: the Impact of Diffuse Stellar Populations
2014
Thanks to the Spitzer Space Telescope, the infrared (IR) emission have been better studied in the past decades, as well as its relations to star formation rates (SFR) and the calibrations of IR emissions as SFR indicators. However, the far infrared (FIR) emission are still understudied, especially at the sub-galactic region scales. In this dissertation, I present new ground-based observations in the light of the infrared hydrogen recombination line Brγ (2.16 µm) of a sample of KINGFISH (Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel) galaxies. The Brγ emission line offers the double advantage of directly tracing ionizing photons and of being relatively insensitive to the effects of dust attenuation. It is therefore a relatively unbiased reference star formation rate (SFR) indicator that I use to establish the calibrations of the Herschel PACS 70, 100 and 160 µm emission as SFR tracers on sub-galactic scales. I confirm with higher statistical accuracy the dependence vii of the SFR calibrations on metallicity and star formation timescale. I also provide calibrations for SFR indicators for HII regions that combine the observed Hα with one of the three IR bands. I find significant diffuse IR emission not related to current star formation activity in galaxies, implying major impact of diffuse stellar population on the dust heating in nearby galaxies. I make comparisons among different SFR(IR) indicators and provide interpretations on the comparisons. viii