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Register a new userHardness of ionizing radiation fields in MaNGA star-forming galaxies
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We investigate radiation hardness within a representative sample of 67 nearby (0.02 $lesssim $z$ lesssim$0.06) star-forming (SF) galaxies using the integral field spectroscopic data from the MaNGA survey. The softness parameter $eta$ = $frac{O^{+}/O^{2+}}{S^{+}/S^{2+}}$ is sensitive to the spectral energy distribution of the ionizing radiation. We study $eta$ via the observable quantity $etaprime$ (=$frac{[OII]/[OIII]}{[SII][SIII]}$) We analyse the relation between radiation hardness (traced by $eta$ and $etaprime$) and diagnostics sensitive to gas-phase metallicity, electron temperature, density, ionization parameter, effective temperature and age of ionizing populations. It is evident that low metallicity is accompanied by low log $etaprime$, i.e. hard radiation field. No direct relation is found between radiation hardness and other nebular parameters though such relations can not be ruled out. We provide empirical relations between log $rmeta$ and strong emission line ratios N$_2$, O$_3$N$_2$ and Ar$_3$O$_3$ which will allow future studies of radiation hardness in SF galaxies where weak auroral lines are undetected. We compare the variation of [O III]/[O II] and [S III]/[S II] for MaNGA data with SF galaxies and H II regions within spiral galaxies from literature, and find that the similarity and differences between different data set is mainly due to the metallicity. We find that predictions from photoionizaion models considering young and evolved stellar populations as ionizing sources in good agreement with the MaNGA data. This comparison also suggests that hard radiation fields from hot and old low-mass stars within or around SF regions might significantly contribute to the observed $eta$ values.
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Using results from high-resolution galaxy formation simulations in a standard Lambda-CDM cosmology and a fully conservative multi-resolution radiative transfer code around point sources, we compute the energy-dependent escape fraction of ionizing photons from a large number of star forming regions in two galaxies at five different redshifts from z=3.8 to 2.39. All escape fractions show a monotonic decline with time, from (at the Lyman-limit) ~6-10% at z=3.6 to ~1-2% at z=2.39, due to higher gas clumping at lower redshifts. It appears that increased feedback can lead to higher f_esc at z>3.4 via evacuation of gas from the vicinity of star forming regions and to lower f_esc at z<2.39 through accumulation of swept-up shells in denser environments. Our results agree well with the observational findings of citet{inoue..06} on redshift evolution of f_esc in the redshift interval z=2-3.6.
We consider the circumnuclear regions of MaNGA galaxies. The spectra are classified as AGN-like, HII-region-like (or SF-like), and intermediate (INT) spectra according to their positions on the BPT diagram. There are the following four configurations of the radiation distributions in the circumnuclear regions: 1) AGN+INT, the innermost region of the AGN-like radiation is surrounded by a ring of radiation of the intermediate type; 2) INT, the central area of radiation of the intermediate type; 3) SF+INT, the inner region of the HII-region-like radiation is surrounded by a ring of radiation of the intermediate type; and 4) SF, the HII-region-like radiation only. The LINERS of configurations 1 and 2 are examined. The spaxel spectra of the LINERs form a sequences on the BPT diagram. The line ratios change smoothly with radius, from AGN-like at the center to HII-region-like at larger distances. This is in agreement with the paradigm that the LINERs are excited by AGN activity. The AGN and INT radiation in the circumnuclear region is accompanied by an enhanced gas velocity dispersion, s_g. The radius of the area of the AGN and INT radiation is similar to the radius of the area with enhanced s_g, and the central s_g,c correlates with the luminosity of the AGN+INT area. We assume that the gas velocity dispersion can serve as an indicator of the AGN activity. The values of s_g,c for the SF-type centers partly overlap with those of the AGN-type centers. We find that there is a demarcation line between the positions of the AGN-type and SF-type objects on the s_g,c - central Halpha surface brightness diagram.
We report on the HST detection of the Lyman-continuum (LyC) radiation emitted by a galaxy at redshift z=3.794, dubbed Ion1 (Vanzella et al. 2012). The LyC from Ion1 is detected at rest-frame wavelength 820$sim$890 AA with HST WFC3/UVIS in the F410M band ($m_{410}=27.60pm0.36$ magnitude (AB), peak SNR = 4.17 in a circular aperture with radius r = 0.12) and at 700$sim$830 AA with the VLT/VIMOS in the U-band ($m_U = 27.84pm0.19$ magnitude (AB), peak SNR = 6.7 with a r = 0.6 aperture). A 20-hr VLT/VIMOS spectrum shows low- and high-ionization interstellar metal absorption lines, the P-Cygni profile of CIV and Ly$alpha$ in absorption. The latter spectral feature differs from what observed in known LyC emitters, which show strong Ly$alpha$ emission. An HST far-UV color map reveals that the LyC emission escapes from a region of the galaxy that is bluer than the rest, presumably because of lower dust obscuration. The F410M image shows that the centroid of the LyC emission is offset from the centroid of the non-ionizing UV emission by 0.12$pm$0.03, corresponding to 0.85$pm$0.21 kpc (physical), and that its morphology is likely moderately resolved. These morphological characteristics favor a scenario where the LyC photons produced by massive stars escape from low HI column-density cavities in the ISM, possibly carved by stellar winds and/or supernova. We also collect the VIMOS U-band images of a sample of 107 Lyman-break galaxies with spectroscopic redshifts at $3.40<z<3.95$, i.e. sampling the LyC, and stack them with inverse-variance weights. No LyC emission is detected in the stacked image, resulting in a 32.5 magnitude (AB) flux limit (1$sigma$) and an upper limit of absolute LyC escape fraction $f_{esc}^{abs} < 0.63%$. LyC emitters like Ion1 are very likely at the bright-end of the LyC luminosity function.
We use data from 1222 late-type star-forming galaxies in the SDSS IV MaNGA survey to identify regions in which the gas-phase metallicity is anomalously-low compared to expectations from the tight empirical relation between metallicity and stellar surface mass-density at a given stellar mass. We find anomalously low metallicity (ALM) gas in 10% of the star-forming spaxels, and in 25% of the galaxies in the sample. The incidence rate of ALM gas increases strongly with both global and local measures of the specific star-formation rate, and is higher in lower mass galaxies and in the outer regions of galaxies. The incidence rate is also significantly higher in morphologically disturbed galaxies. We estimate that the lifetimes of the ALM regions are a few hundred Myr. We argue that the ALM gas has been delivered to its present location by a combination of interactions, mergers, and accretion from the halo, and that this infusion of gas stimulates star-formation. Given the estimated lifetime and duty cycle of such events, we estimate that the time-averaged accretion rate of ALM gas is similar to the star-formation rate in late type galaxies over the mass-range M$_* sim10^9$ to 10$^{10}$ M$_{odot}$.
We describe a new method for simulating ionizing radiation and supernova feedback in the analogues of low-redshift galactic disks. In this method, which we call star-forming molecular cloud (SFMC) particles, we use a ray-tracing technique to solve the radiative transfer equation for ultraviolet photons emitted by thousands of distinct particles on the fly. Joined with high numerical resolution of 3.8 pc, the realistic description of stellar feedback helps to self-regulate star formation. This new feedback scheme also enables us to study the escape of ionizing photons from star-forming clumps and from a galaxy, and to examine the evolving environment of star-forming gas clumps. By simulating a galactic disk in a halo of 2.3e11 Msun, we find that the average escape fraction from all radiating sources on the spiral arms (excluding the central 2.5 kpc) fluctuates between 0.08% and 5.9% during a ~20 Myr period with a mean value of 1.1%. The flux of escaped photons from these sources is not strongly beamed, but manifests a large opening angle of more than 60 degree from the galactic pole. Further, we investigate the escape fraction per SFMC particle, f_esc(i), and how it evolves as the particle ages. We discover that the average escape fraction f_esc is dominated by a small number of SFMC particles with high f_esc(i). On average, the escape fraction from a SFMC particle rises from 0.27% at its birth to 2.1% at the end of a particle lifetime, 6 Myrs. This is because SFMC particles drift away from the dense gas clumps in which they were born, and because the gas around the star-forming clumps is dispersed by ionizing radiation and supernova feedback. The framework established in this study brings deeper insight into the physics of photon escape fraction from an individual star-forming clump, and from a galactic disk.
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