Multi-wavelength Studies Of Interstellar Medium In Nearby Galaxies

The interplay between interstellar medium(ISM)and star formation is a key driver of galaxy formation and evolution.Due to the ISM extinction,the observations carried out at infrared,(sub)millimeter and radio wavelengths offer key tracers of ISM and study the ISM physics in details.The thermal infrared emission of galaxy comes from the re-radiation of dust that absorbs the UV photons from the massive OB star.The un-thermal radio emission comes from the synchrotron radiation of the relativistic cosmic-ray electrons,which were accelerated by the shock of supernova remains(SNR).The origin of both emis-sion roots on star formation.And thus the correlation between infrared and radio emis-sion is powerful way to understanding the interplay between star formation and ISM.Existing studies mainly focus on metal-rich galaxies.To remedy this situation and to gain insights into the ISM physics at low metallicity,we have compiled a sample of 26 metal-poor galaxies with 12+log(O/H)?8.1 with IR photometry and radio 1.4 GHz continuum data.With a comparison sample of metal-rich galaxies at higher metallicity,we have investigated the dependence on the metallicity of the IR/radio relationship at 24 ?m,70?m,100?m,160?m and the integrated IR luminosity.It is found the de-pendence of qIR=log(fIR/f1.4GHz)on the metallicity changes with the IR wavelength.Metal-poor galaxies have on average lower qIR as compared to the metal-rich ones with larger offsets at longer IR wavelengths,from-0.03 at q24?m to-0.6 at q160?m.The qIR of metal-poor galaxies at 70,100 and 160 ?m decreases with the decreasing metallic-ity and IR-to-FUV ratio,and the qIR at 100 and 160?m is also correlated with the IR color especially the far-IR color(f100?m/f160?m).While qIR of metal-poor galaxies is lower,their mean SFR-to-radio ratio is the same as the metal-rich ones once the SFR includes both obscured and unobscured parts,indicating the 1.4 GHz radio emission is still an effective tracer of SFRs for metal-poor galaxies.We proposed a mechanism that invokes a combined effects of low dust-to-SFR ratio and warm dust temperature of metal-poor galaxies to interpret the behavior of qIR of metal-poor galaxies,in which the low dust-to-SFR ratio results in a low fraction of radiation from massive stars that has been reprocessed and re-emitted In the IR,and warm dust temperature enhances the IR emission at short IR wavelength.Other mechanisms related to the radio emission including the enhanced magnetic field strength and increased thermal radio contribu-tion are unable to reconcile the IR-wavelength-dependent differences of qIR between metal-poor and metal-rich galaxies.The infrared and sub-millimeter emission of a galaxy traces the radiation from dust grains.Some studies argue that dwarf galaxies show submm-excess that is the excess above the black-body fitting to the full infrared/submm SEDs.We present sub-mm maps of one of the nearest dwarf irregular galaxies-NGC6822 at 450 and 850?m with the Sub-millimeter Common-User Bolometer Array(SCUBA2)at the James Clerk Maxwell Telescope,with the main goal to investigate if there is a sub-mm excess.In total three star-forming regions in the galaxy have solid detection at 850?m.We com-bined the SCUBA data with Spitzer and Herschel photometry at lower IR wavelengths and carried out a series of two-modified black-body fittings with different emissivity indexes.It is found that none of these regions show any sub-mm excess within the observed errors.By comparing with other galaxies,the presence of the submm-excess seems not to be related with the metallicity,indicating the metallicity is not a unique parameter in resulting in the change of the dust emissivity index.The(sub)millimeter wavelength contains rich molecular lines that can be used to probe the physical and chemical properties of molecular ISM.Deep(sub)mm molec-ular line observations of nearby galaxies are important to understanding the properties of molecular ISM in these galaxies.We present results of deep millimeter line observa-tions towards the center of nearby gas-rich Seyfert 2 galaxy NGC 1068 with the IRAM 30m telescope.We detected thirty-two lines in this galaxy,among which fifteen lines were firstly detected.With about a factor of 4 better sensitivity than previous obser-vations for this source at 3 mm band,we detected several weak lines at the first time in this source,such as lines from CH3CCH,CH3OCH3,and HC18O+,and confirmed some marginal detections,such as HN13(1-0).Lines from several carbon chain molecules and shock related molecules were also detected in this source.We investi-gate the reason why some of the molecules were detected at 3 mm but not at 2 mm and 1mm,and vice versa.With the different line width and profile,we constraint the physi-cal and chemistry properties of the CND.We inferred the C2H,c-C3H2 are distributed both the CND and star forming rings.The CND region will be a potential smelter to discover the much longer carbon molecules.The CND region has a complex dynamic structure.