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Register a new userSynchrotron spectral index and interstellar medium densities of star-forming galaxies
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The spectral index of synchrotron emission is an important parameter in understanding the properties of cosmic ray electrons (CREs) and the interstellar medium (ISM). We determine the synchrotron spectral index ($alpha_{rm nt}$) of four nearby star-forming galaxies, namely NGC 4736, NGC 5055, NGC 5236 and NGC 6946 at sub-kpc linear scales. The $alpha_{rm nt}$ was determined between 0.33 and 1.4 GHz for all the galaxies. We find the spectral index to be flatter ($gtrsim -0.7$) in regions with total neutral (atomic + molecular) gas surface density, $Sigma_{rm gas} gtrsim rm 50~M_odot pc^{-2}$, typically in the arms and inner parts of the galaxies. In regions with $Sigma_{rm gas} lesssim rm 50~M_odot pc^{-2}$, especially in the interarm and outer regions of the galaxies, the spectral index steepens sharply to $<-1.0$. The flattening of $alpha_{rm nt}$ is unlikely to be caused due to thermal free--free absorption at 0.33 GHz. Our result is consistent with the scenario where the CREs emitting at frequencies below $sim0.3$ GHz are dominated by bremsstrahlung and/or ionization losses. For denser medium ($Sigma_{rm gas} gtrsim rm 200~M_odot pc^{-2}$), having strong magnetic fields ($sim 30~mu$G), $alpha_{rm nt}$ is seen to be flatter than $-0.5$, perhaps caused due to ionization losses. We find that, due to the clumpy nature of the ISM, such dense regions cover only a small fraction of the galaxy ($lesssim5$ percent). Thus, the galaxy-integrated spectrum may not show indication of such loss mechanisms and remain a power-law over a wide range of radio frequencies (between $sim 0.1$ to 10 GHz).
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The interpretation of Galactic synchrotron observations is complicated by the degeneracy between the strength of the magnetic field perpendicular to the line of sight (LOS), $B_perp$, and the cosmic-ray electron (CRe) spectrum. Depending on the observing frequency, an energy-independent spectral energy slope $s$ for the CRe spectrum is usually assumed: $s=-2$ at frequencies below $simeq$400 MHz and $s=-3$ at higher frequencies. Motivated by the high angular and spectral resolution of current facilities such as the LOw Frequency ARray (LOFAR) and future telescopes such as the Square Kilometre Array (SKA), we aim to understand the consequences of taking into account the energy-dependent CRe spectral energy slope on the analysis of the spatial variations of the brightness temperature spectral index, $beta$, and on the estimate of the average value of $B_perp$ along the LOS. We illustrate analytically and numerically the impact that different realisations of the CRe spectrum have on the interpretation of the spatial variation of $beta$. We find that the common assumption of an energy-independent $s$ is valid only in special cases. We show that for typical magnetic field strengths of the diffuse interstellar medium ($simeq$2$-$20 $mu$G), at frequencies of 0.1$-$10 GHz, the electrons that are mainly responsible for the synchrotron emission have energies in the range $simeq$100 MeV$-$50 GeV. This is the energy range where the spectral slope, $s$, of CRe has its greatest variation. We also show that the polarisation fraction can be much smaller than the maximum value of $simeq 70%$ because the orientation of ${bf B}_perp$ varies across the telescopes beam and along the LOS. Finally, we present a look-up plot that can be used to estimate the average value of $B_perp$ along the LOS from a set of values of $beta$ measured at centimetre to metre wavelengths, for a given CRe spectrum.
We present new results from near-infrared spectroscopy with Keck/MOSFIRE of [OIII]-selected galaxies at $zsim3.2$. With our $H$ and $K$-band spectra, we investigate the interstellar medium (ISM) conditions, such as ionization states and gas metallicities. [OIII] emitters at $zsim3.2$ show a typical gas metallicity of $mathrm{12+log(O/H) = 8.07pm0.07}$ at $mathrm{log(M_*/M_odot) sim 9.0-9.2}$ and $mathrm{12+log(O/H) = 8.31pm0.04}$ at $mathrm{log(M_*/M_odot) sim 9.7-10.2}$ when using the empirical calibration method. We compare the [OIII] emitters at $zsim3.2$ with UV-selected galaxies and Ly$alpha$ emitters at the same epoch and find that the [OIII]-based selection does not appear to show any systematic bias in the selection of star-forming galaxies. Moreover, comparing with star-forming galaxies at $zsim2$ from literature, our samples show similar ionization parameters and gas metallicities as those obtained by the previous studies using the same calibration method. We find no strong redshift evolution in the ISM conditions between $zsim3.2$ and $zsim2$. Considering that the star formation rates at a fixed stellar mass also do not significantly change between the two epochs, our results support the idea that the stellar mass is the primary quantity to describe the evolutionary stages of individual galaxies at $z>2$.
The radio continuum spectra of 14 star-forming galaxies are investigated by fitting nonthermal (synchrotron) and thermal (free-free) radiation laws. The underlying radio continuum measurements cover a frequency range of ~325 MHz to 24.5 GHz (32 GHz in case of M82). It turns out that most of these synchrotron spectra are not simple power-laws, but are best represented by a low-frequency spectrum with a mean slope alpha_nth = 0.59 +/- 0.20 (S_nu ~ nu^-alpha), and by a break or an exponential decline in the frequency range of 1 - 12 GHz. Simple power-laws or mildly curved synchrotron spectra lead to unrealistically low thermal flux densities, and/or to strong deviations from the expected optically thin free-free spectra with slope alpha_th = 0.10 in the fits. The break or cutoff energies are in the range of 1.5 - 7 GeV. We briefly discuss the possible origin of such a cutoff or break. If the low-frequency spectra obtained here reflect the injection spectrum of cosmic-ray electrons, they comply with the mean spectral index of Galactic supernova remnants. A comparison of the fitted thermal flux densities with the (foreground-corrected) Halpha fluxes yields the extinction, which increases with metallicity. The fraction of thermal emission is higher than believed hitherto, especially at high frequencies, and is highest in the dwarf galaxies of our sample, which we interpret in terms of a lack of containment in these low-mass systems, or a time effect caused by a very young starburst.
We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118 star-forming galaxies at $zsim1.5$ in the Subaru Deep Field. These galaxies are selected as [OII]$lambda$3727 emitters at $zapprox$ 1.47 and 1.62 from narrow-band imaging. We detect H$alpha$ emission line in 115 galaxies, [OIII]$lambda$5007 emission line in 45 galaxies, and H$beta$, [NII]$lambda$6584, and [SII]$lambdalambda$6716,6731 in 13, 16, and 6 galaxies, respectively. Including the [OII] emission line, we use the six strong nebular emission lines in the individual and composite rest-frame optical spectra to investigate physical conditions of the interstellar medium in star-forming galaxies at $zsim$1.5. We find a tight correlation between H$alpha$ and [OII], which suggests that [OII] can be a good star formation rate (SFR) indicator for galaxies at $zsim1.5$. The line ratios of H$alpha$/[OII] are consistent with those of local galaxies. We also find that [OII] emitters have strong [OIII] emission lines. The [OIII]/[OII] ratios are larger than normal star-forming galaxies in the local Universe, suggesting a higher ionization parameter. Less massive galaxies have larger [OIII]/[OII] ratios. With evidence that the electron density is consistent with local galaxies, the high ionization of galaxies at high redshifts may be attributed to a harder radiation field by a young stellar population and/or an increase in the number of ionizing photons from each massive star.
We analyse the far-infrared properties of $sim$ 5,000 star-forming galaxies at $z<4.5$, drawn from the deepest, super-deblended catalogues in the GOODS-N and COSMOS fields. We develop a novel panchromatic SED fitting algorithm, $texttt{Stardust}$, that models the emission from stars, AGN, and infrared dust emission, without relying on energy balance assumptions. Our code provides robust estimates of the UV-optical and FIR physical parameters, such as the stellar mass ($M_*$), dust mass ($M_{rm dust}$), infrared luminosities ($L_{rm IR}$) arising from AGN and star formation activity, and the average intensity of the interstellar radiation field ($langle U rangle$). Through a set of simulations we quantify the completeness of our data in terms of $M_{rm dust}$, $L_{rm IR}$ and $langle U rangle$, and subsequently characterise the distribution and evolution of these parameters with redshift. We focus on the dust-to-stellar mass ratio ($f_{rm dust}$), which we parametrise as a function of cosmic age, stellar mass, and specific star formation rate. The $f_{rm dust}$ is found to increase by a factor of 10 from $z=0$ to $z=2$ and appears to remain flat at higher$-z$, mirroring the evolution of the gas fraction. We also find a growing fraction of warm to cold dust with increasing distance from the main sequence, indicative of more intense interstellar radiation fields, higher star formation efficiencies and more compact star forming regions for starburst galaxies. Finally, we construct the dust mass functions (DMF) of star-forming galaxies up to $z=1$ by transforming the stellar mass function to DMF through the scaling relations derived here. The evolution of $f_{rm dust}$ and the recovered DMFs are in good agreement with the theoretical predictions of the Horizon-AGN and IllustrisTNG simulations.
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