Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userInfrared line ratios revealing starburst conditions in galaxies
70
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The physical conditions in typical starburst galaxies are investigated through critical infrared (IR) line ratios, as previously suggested by Lutz et al. (1998, A&A, 333, L75). The calculations by a composite model which consistently accounts for the coupled effect of shock and photoionization by hot stars definitely fit the observed line ratios of single objects and explain the observed relation between [OIV]/([NeII]+0.44[NeIII]) and [NeIII]/[NeII]. The shock velocity and the gas density are the critical parameters. Most of the shocks are produced in low density-velocity (n_0 = 100 cm-3 and V_s = 50 - 100 km/s) clouds which represent the bulk of the ionized gas in starburst galaxies. However, though they are by many orders less numerous, high-velocity (= 400 - 600 km/s) shocks in dense (= 500 - 800 cm-3) clouds are necessary to reproduce the critical IR line ratios observed in the low-excitation Starburst Nucleus Galaxies (SBNGs: M82, M83, NGC 253, NGC 3256, NGC 3690, and NGC 4945). These model predictions are in good agreement with the powerful starburst-driven superwinds and highly pressured ISM observed in SBNGs. On the contrary, the high-excitation HII galaxies (II Zw 40 and NGC 5253) do not show any clear signature of large scale outflows of gas. This difference between HII galaxies and SBNGs can be interpreted in terms of temporal evolution of their starbursts.
rate research
Read More
We conducted systematic observations of the H I Br$alpha$ (4.05 $mu$m) and Br$beta$ (2.63 $mu$m) lines in 52 nearby ($z<0.3$) ultraluminous infrared galaxies (ULIRGs) with AKARI. Among 33 ULIRGs wherein the lines are detected, three galaxies show anomalous Br$beta$/Br$alpha$ line ratios ($sim1.0$), which are significantly higher than those for case B (0.565). Our observations also show that ULIRGs have a tendency to exhibit higher Br$beta$/Br$alpha$ line ratios than those observed in Galactic H II regions. The high Br$beta$/Br$alpha$ line ratios cannot be explained by a combination of dust extinction and case B since dust extinction reduces the ratio. We explore possible causes for the high Br$beta$/Br$alpha$ line ratios and show that the observed ratios can be explained by a combination of an optically thick Br$alpha$ line and an optically thin Br$beta$ line. We simulated the H II regions in ULIRGs with the Cloudy code, and our results show that the high Br$beta$/Br$alpha$ line ratios can be explained by high-density conditions, wherein the Br$alpha$ line becomes optically thick. To achieve a column density large enough to make the Br$alpha$ line optically thick within a single H II region, the gas density must be as high as $nsim10^8$ $mathrm{cm}^{-3}$. We therefore propose an ensemble of H II regions, in each of which the Br$alpha$ line is optically thick, to explain the high Br$beta$/Br$alpha$ line ratio.
We study the Polycyclic Aromatic Hydrocarbons (PAH) bands, ionic emission lines, and Mid-infrared continuum properties, in a sample of 171 emission line galaxies taken from literature plus 15 new active galactic nuclei (AGN) Spitzer spectra. The continuum shape steeply rises for longer wavelengths and can be fitted with a warm blackbody distribution of T=150-300K. The brightest PAH spectral bands (6.2, 7.7, 8.6, 11.3, and 12.7$mu$m) and the forbidden emission lines of [Si II] 34.8$mu$m, [Ar II] 6.9, [S III] 18.7 and 33.4 were detected in all the Starbursts and in ~80% of the Seyfert~2. Taking under consideration only the PAH bands at 7.7$mu$m, 11.3$mu$m, and 12.7$mu$m we find they are present in ~80% of the Seyfert 1, while only half of this type of activity show the 6.2$mu$m and 8.6 PAH bands. The observed intensities ratios for neutral and ionized PAHs (6.2/7.7 x 11.3/7.7) were compared to theoretical intensity ratios, showing that AGNs have higher ionization fraction and larger PAH (> 180 carbon atoms) than SB galaxies. The ratio between the ionized (7.7) and the neutral PAH bands (8.6 and 11.3) are distributed over different ranges for AGNs and SB galaxies, suggesting that these ratios could depend on the ionization fraction, as well as on the hardness of the radiation field. The ratio between the 7.7 and 11.3 bands is nearly constant with the increase of [Ne III]15.5/[Ne II], indicating that the fraction of ionized to neutral PAH bands does not depend on the hardness of the radiation field. The equivalent width of both PAH features show the same dependence with [Ne III]/[Ne II], suggesting that the PAH, emitting either ionized (7.7) or neutral (11.3) bands, may be destroyed with the increase of the hardness of the radiation field.
For studies of galaxy formation and evolution, one of the major benefits of the James Webb Space Telescope is that space-based IFUs like those on its NIRSpec and MIRI instruments will enable spatially resolved spectroscopy of distant galaxies, including spectroscopy at the scale of individual star-forming regions in galaxies that have been gravitationally lensed. In the meantime, there is only a very small subset of lensed sources where work like this is possible even with the Hubble Space Telescopes Wide Field Camera 3 infrared channel grisms. We examine two of these sources, SDSS J1723+3411 and SDSS J2340+2947, using HST WFC3/IR grism data and supporting spatially-unresolved spectroscopy from several ground-based instruments to explore the size of spatial variations in observed strong emission line ratios like O32, R23, which are sensitive to ionization parameter and metallicity, and the Balmer decrement as an indicator of reddening. We find significant spatial variation in the reddening and the reddening-corrected O32 and R23 values which correspond to spreads of a few tenths of a dex in ionization parameter and metallicity. We also find clear evidence of a negative radial gradient in star formation in SDSS J2340+2947 and tentative evidence of one in SDSS J1723+3411, though its star formation is quite asymmetric. Finally, we find that reddening can vary enough spatially to make spatially-resolved reddening corrections necessary in order to characterize gradients in line ratios and the physical conditions inferred from them, necessitating the use of space-based IFUs for future work on larger, more statistically robust samples.
(Abridged) We present new K-band spectroscopy for a sample of 48 starburst galaxies, obtained using UKIRT in Hawaii. This constitutes a fair sample of the most common types of starburst galaxies found in the nearby Universe. The variety of near infrared spectral features shown by these galaxies implies different bursts characteristics, which suggests that we survey galaxies with different star formation histories or at different stages of their burst evolution. Using synthetic starburst models, we conclude that the best ensemble of parameters which describe starburst galaxies in the nearby universe are a constant rate of star formation, a Salpeter IMF with an upper mass cutoff equal to 30 solar mass and bursts ages between 10 Myr and 1 Gyr. The model is fully consistent with the differences observed in the optical and FIR between the different types of starbursts. It suggests that HII galaxies have younger bursts and lower metallicities than SBNGs, while LIRGs have younger bursts but higher metallicities. Our observations suggest that the starburst phenomenon must be a sustained or self--sustained phenomenon: either star formation is continuous in time or multiple bursts happen in sequence over a relatively long period of time. The generality of our observations implies that this is a characteristic of starburst galaxies in the nearby Universe.
We study the impact of deviations from the Kennicutt-Schmidt relation (quantified by the `burstiness parameter $kappa_s$), gas metallicity ($Z$), and density ($n$) on the observed [OIII]88$mu$m/[CII]158$mu$m surface brightness ratios ($Sigma_{[OIII]}/Sigma_{[CII]}$) in nine galaxies at $zapprox6-9$. We first discuss possible biases in the measured $Sigma_{[OIII]}/Sigma_{[CII]}$ ratios by comparing the data with zoom-in cosmological simulations, and then use a Markov Chain Monte Carlo algorithm to derive the best fit values of ($kappa_s, Z, n$). We find that (i) the strongest dependence of $Sigma_{[OIII]}/Sigma_{[CII]}$ is on $kappa_s$; (ii) high ratios identify starburst galaxies with short gas depletion times ($t_{dep}=6-49,rm Myr$); (iii) a secondary dependence on density is found, with $Sigma_{[OIII]}/Sigma_{[CII]}$ anticorrelating with $n$ as a result of the lower [OIII] critical density, (iv) the ratio only weakly depends on $Z$. The nine galaxies are significantly enriched (Z=0.2-0.5 $Z_odot$), and dense ($n=10^{1-3} {rm cm}^{-3}$). This lends further support to the starburst scenario in which a rapid enrichment of the interstellar medium is expected.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
