Nanyao Lu

Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near-Infrared Continuum, and Mid-Infrared Emission Features

We present ISOPHOT spectra of the regions 2.5-4.9 μm and 5.8-11.6 μm for a sample of 45 disk galaxies from the US Infrared Space Observatory Key Project on Normal Galaxies. The galaxies were selected to span the range in global properties of normal, star-forming disk galaxies in the local universe. The spectra can be decomposed into three spectral components: (1) continuum emission from stellar photospheres, which dominates the near-infrared (NIR; 2.5-4.9 μm) spectral region; (2) a weak NIR excess continuum, which has a color temperature of ~103 K, carries a luminosity of a few percent of the total far-infrared (FIR) dust luminosity LFIR and most likely arises from the interstellar medium (ISM); and (3) the well-known broad emission features at 6.2, 7.7, 8.6, and 11.3 μm, which are generally attributed to aromatic carbon particles. These aromatic features in emission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 μm) part of the spectrum and resemble the so-called type A spectra observed in many nonstellar sources and the diffuse ISM in our own Galaxy. The few notable exceptions include NGC 4418, where a dust continuum replaces the AFEs in MIR, and NGC 1569, where the AFEs are weak and the strongest emission feature is [S IV] 10.51 μm. The relative strengths of the AFEs vary by 15%-25% among the galaxies. However, little correlation is seen between these variations and either IRAS 60 μm/100 μm flux density ratio R(60/100) or the FIR/blue luminosity ratio LFIR/LB, two widely used indicators of the current star formation activity, suggesting that the observed variations are not a consequence of the radiation field differences among the galaxies. We demonstrate that the NIR excess continuum and AFE emission are correlated, suggesting that they are produced by similar mechanisms and similar (or the same) material. On the other hand, as the current star formation activity increases, the overall strengths of the AFEs and the NIR excess continuum drop significantly with respect to that of the FIR emission from large dust grains. In particular, the summed luminosity of the AFEs falls from ~0.2 LFIR for the most IR-quiescent galaxies to ~0.1 LFIR for the most IR-active galaxies. This is likely a consequence of the preferential destruction in intense radiation fields of the small carriers responsible for the NIR/AFE emission.

The Herschel Dwarf Galaxy Survey - I. Properties of the low-metallicity ISM from PACS spectroscopy

Context. The far-infrared (FIR) lines are important tracers of the cooling and physical conditions of the interstellar medium (ISM) and are rapidly becoming workhorse diagnostics for galaxies throughout the universe. There are clear indications of a different behavior of these lines at low metallicity that needs to be explored. Aims. Our goal is to explain the main differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies, and how this translates in ISM properties. Methods. We present Herschel/PACS spectroscopic observations of the [Cu2009ii] 157 μm, [Ou2009i] 63 and 145 μm, [Ou2009iii] 88 μm, [Nu2009ii] 122 and 205 μm, and [Nu2009iii] 57 μm fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models. Results. We find that the FIR lines together account for up to 3 percent of LTIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of [Ou2009iii]88/[Nu2009ii]122 and [Nu2009iii]57/[Nu2009ii]122 are high, indicative of hard radiation fields. In the photodissociation region (PDR), the [Cu2009ii]157/[Ou2009i]63 ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the [Ou2009i]145/[Ou2009i]63 ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The [Ou2009iii]88/[Ou2009i]63 ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found to be ~4 times higher in the dwarfs than in metal-rich galaxies. The high [Cu2009ii]/LTIR, [Ou2009i]/LTIR, and [Ou2009iii]/LTIR ratios, which decrease with increasing LTIR and LTIR/LB, are interpreted as a combination of moderate far-UV fields and a low PDR covering factor. Harboring compact phases of a low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that of metal-rich galaxies.

The Local Galaxy 8 μm Luminosity Function

A Spitzer Space Telescope survey in the NOAO Deep Wide Field in Bootes provides a complete, 8 μm-selected sample of galaxies to a limiting (Vega) magnitude of 13.5. In the 6.88 deg2 field sampled, 79% of the 4867 galaxies have spectroscopic redshifts, allowing an accurate determination of the local (z < 0.3) galaxy luminosity function. Stellar and dust emission can be separated on the basis of observed galaxy colors. Dust emission (mostly PAH) accounts for 80% of the 8 μm luminosity, stellar photospheres account for 19%, and AGN emission accounts for roughly 1%. A subsample of the 8 μm-selected galaxies have blue, early-type colors, but even most of these have significant PAH emission. The luminosity functions for the total 8 μm luminosity and for the dust emission alone are both well fit by Schechter functions. For the 8 μm luminosity function, the characteristic luminosity is νL(8.0 μm) = 1.8 × 1010 L☉, while for the dust emission alone it is 1.6 × 1010 L☉. The average 8 μm luminosity density at z < 0.3 is 3.1 × 107 L☉ Mpc-3, and the average luminosity density from dust alone is 2.5 × 107 L☉ Mpc-3. This luminosity arises predominantly from galaxies with 8 μm luminosities (νLν) between 2 × 109 and 2 × 1010 L☉, i.e., normal galaxies, not luminous or ultraluminous infrared galaxies (LIRGs/ULIRGs).

Linking dust emission to fundamental properties in galaxies: The low-metallicity picture

Aims. In this work, we aim to provide a consistent analysis of the dust properties from metal-poor to metal-rich environments by linking them to fundamental galactic parameters. Methods. We consider two samples of galaxies: the Dwarf Galaxy Survey (DGS) and the Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel (KINGFISH), totalling 109 galaxies, spanning almost 2 dex in metallicity. We collect infrared (IR) to submillimetre (submm) data for both samples and present the complete data set for the DGS sample. We model the observed spectral energy distributions (SED) with a physically-motivated dust model to access the dust properties: dust mass, total-IR luminosity, polycyclic aromatic hydrocarbon (PAH) mass fraction, dust temperature distribution, and dust-to-stellar mass ratio. Results. Using a different SED model (modified black body), different dust composition (amorphous carbon in lieu of graphite), or a different wavelength coverage at submm wavelengths results in differences in the dust mass estimate of a factor two to three, showing that this parameter is subject to non-negligible systematic modelling uncertainties. We find half as much dust with the amorphous carbon dust composition. For eight galaxies in our sample, we find a rather small excess at 500 μm (≤1.5σ). We find that the dust SED of low-metallicity galaxies is broader and peaks at shorter wavelengths compared to more metal-rich systems, a sign of a clumpier medium in dwarf galaxies. The PAH mass fraction and dust temperature distribution are found to be driven mostly by the specific star formation rate, sSFR, with secondary effects from metallicity. The correlations between metallicity and dust mass or total-IR luminosity are direct consequences of the stellar mass-metallicity relation. The dust-to-stellar mass ratios of metal-rich sources follow the well-studied trend of decreasing ratio for decreasing sSFR. The relation is more complex for low-metallicity galaxies with high sSFR, and depends on the chemical evolutionary stage of the source (i.e. gas-to-dust mass ratio). Dust growth processes in the ISM play a key role in the dust mass build-up with respect to the stellar content at high sSFR and low metallicity. Conclusions. We conclude that the evolution of the dust properties from metal-poor to metal-rich galaxies derives from a complex interplay between star formation activity, stellar mass, and metallicity.

High-resolution, 3D radiative transfer modeling - I. The grand-design spiral galaxy M 51

Context. Dust reprocesses about half of the stellar radiation in galaxies. The thermal re-emission by dust of absorbed energy is considered to be driven merely by young stars so is often applied to tracing the star formation rate in galaxies. Recent studies have argued that the old stellar population might be responsible for a non-negligible fraction of the radiative dust heating. n nAims. In this work, we aim to analyze the contribution of young (≲100 Myr) and old (~10 Gyr) stellar populations to radiative dust heating processes in the nearby grand-design spiral galaxy Mu200951 using radiative transfer modeling. High-resolution 3D radiative transfer (RT) models are required to describe the complex morphologies of asymmetric spiral arms and clumpy star-forming regions and to model the propagation of light through a dusty medium. n nMethods. In this paper, we present a new technique developed to model the radiative transfer effects in nearby face-on galaxies. We construct a high-resolution 3D radiative transfer model with the Monte-Carlo code SKIRT to account for the absorption, scattering, and non-local thermal equilibrium (NLTE) emission of dust in Mu200951. The 3D distribution of stars is derived from the 2D morphology observed in the IRACu20093.6u2009μm, GALEX FUV, Hα, and MIPSu200924u2009μm wavebands, assuming an exponential vertical distribution with an appropriate scale height. The dust geometry is constrained through the far-ultraviolet (FUV) attenuation, which is derived from the observed total-infrared-to-far-ultraviolet luminosity ratio. The stellar luminosity, star formation rate, and dust mass have been scaled to reproduce the observed stellar spectral energy distribution (SED), FUV attenuation, and infrared SED. n nResults. The dust emission derived from RT calculations is consistent with far-infrared and submillimeter observations of Mu200951, implying that the absorbed stellar energy is balanced by the thermal re-emission of dust. The young stars provide 63% of the energy for heating the dust responsible for the total infrared emission (8−1000u2009μm), while 37% of the dust emission is governed through heating by the evolved stellar population. In individual wavebands, the contribution from young stars to the dust heating dominates at all infrared wavebands but gradually decreases towards longer infrared and submillimeter wavebands for which the old stellar population becomes a non-negligible source of heating. Upon extrapolation of the results for Mu200951, we present prescriptions for estimating the contribution of young stars to the global dust heating based on a tight correlation between the dust heating fraction and specific star formation rate.

A Herschel Survey of the [N II] 205 μm Line in Local Luminous Infrared Galaxies: The [N II] 205 μm Emission as a Star Formation Rate Indicator

We present, for the first time, a statistical study of [N II] 205 mu m line emission for a large sample of local luminous infrared galaxies using Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (SPIRE FTS) data. For our sample of galaxies, we investigate the correlation between the [N II] luminosity (L-[N II]) and the total infrared luminosity (L-IR), as well as the dependence of L-[N II]/L-IR ratio on L-IR, far-infrared colors (IRAS f(60)/f(100)), and the [O III] 88 mu m to [N II] luminosity ratio. We find that L-[N II] correlates almost linearly with L-IR for non-active galactic nucleus galaxies (all having L-IR < 10(12) L-circle dot) in our sample, which implies that L-[N II] can serve as a star formation rate tracer which is particularly useful for high-redshift galaxies that will be observed with forthcoming submillimeter spectroscopic facilities such as the Atacama Large Millimeter/submillimeter Array. Our analysis shows that the deviation from the mean L-[N II]-L-IR relation correlates with tracers of the ionization parameter, which suggests that the scatter in this relation is mainly due to the variations in the hardness, and/or ionization parameter, of the ambient galactic UV field among the sources in our sample.

Water Vapor in nearby Infrared Galaxies as Probed by Herschel

We report the first systematic study of the submillimeter water vapor rotational emission lines in infrared (IR) galaxies based on the Fourier Transform Spectrometer (FTS) data of Herschel SPIRE. Among the 176 galaxies with publicly available FTS data, 45 have at least one H2O emission line detected. The H2O line luminosities range from similar to 1 x 10(5) L-circle dot to similar to 5 x 10(7) L-circle dot while the total IR luminosities (L-IR) have a similar spread (similar to 1-300 x 10(10) L-circle dot). In addition, emission lines of H2O+ and (H2O)-O-18 are also detected. H2O is found, for most galaxies, to be the strongest molecular emitter after CO in FTS spectra. The luminosity of the five most important H2O lines is near-linearly correlated with L-IR, regardless of whether or not strong active galactic nucleus signature is present. However, the luminosity of H2O(2(11)-2(02)) and H2O(2(20)-2(11)) appears to increase slightly faster than linear with L-IR. Although the slope turns out to be slightly steeper when z similar to 2-4 ULIRGs are included, the correlation is still closely linear. We find that L-H2O/L-IR decreases with increasing f(25)/f(60), but see no dependence on f(60)/f(100), possibly indicating that very warm dust contributes little to the excitation of the submillimeter H2O lines. The average spectral line energy distribution (SLED) of the entire sample is consistent with individual SLEDs and the IR pumping plus collisional excitation model, showing that the strongest lines are H2O(2(02)-1(11)) and H2O(3(21)-3(12)).

The data processing pipelines for the Herschel/SPIRE imaging Fourier transform spectrometer

We present an update to the data processing pipelines that generate calibrated spectral data products from the Spectral and Photometric Imaging Receiver (SPIRE), one of three scientific instruments onboard the European Space Agencys Herschel Space Observatory launched on 14 May 2009. The pipelines process telemetry from SPIREs imaging Fourier Transform Spectrometer (FTS) taken in point source, jiggle- and raster-map observing modes, producing calibrated spectra in low-, medium-, high-, and mixed low- and high-spectral resolution. While the order and algorithms of the data processing modules in the spectrometer pipelines remain for the most part unchanged compared to their pre-launch status, some improvements and optimizations have been realized through the analysis of data from the performance verification and science demonstration phases of the mission. The data processing pipelines for the SPIRE FTS as of the beginning of the routine phase of the Herschel mission are presented in their entirety, with more detailed descriptions reserved for those elements that have changed since launch, in particular the first- and second-level correction steps for glitches, the step that corrects for clipped samples, and the process by which Level-1 spectral data are converted to Level-2 products. In addition, we discuss some of the challenging aspects still faced by the automated processing pipelines, such as the removal of the contributions from the Herschel telescope and SPIRE instrument, and the relative spectral response correction and flux conversion steps.

High-J CO Versus Far-Infrared Relations in Normal and Starburst Galaxies

We present correlations between 9 CO transition (

Status of the SPIRE photometer data processing pipelines during the early phases of the Herschel Mission

J=4-3