Mid-infrared Luminosity Research Articles

The Ages of Elliptical Galaxies from Mid‐Infrared Emission

The mid-infrared (10-20 μm) luminosity of elliptical galaxies is dominated by the integrated emission from circumstellar dust in red giant stars. As a single stellar population evolves, the rate of dusty mass loss from red giant stars decreases with time, so the mid-infrared luminosity should also decline with stellar age. To seek such a correlation, we have used archival Infrared Space Observatory (ISO) observations to determine surface brightness profiles and central fluxes at 15 μm in 17 early-type galaxies for which stellar ages have been determined from optical spectral indices. The radial surface brightness distributions at 15 μm generally follow the stellar de Vaucouleurs profile, as expected. We find that the surface brightness ratio μ15 μm/μI band is systematically higher in elliptical galaxies with ages 5 Gyr and in galaxies that exhibit evidence of recent mergers. Within the accuracy of our observations, μ15 μm/μI band shows no age dependence for ages 5 Gyr. The corresponding flux ratios F15 μm/FI band within apertures scaled to the effective radius (Re/8) are proportional to the μ15 μm/μI band ratios at larger galactic radii, indicating that no 15 μm emission is detected from central dust clouds visible in optical images in some of our sample galaxies. Emission at 15 μm is observed in noncentral massive clouds of dust and cold gas in NGC 1316, an elliptical galaxy that is thought to have had a recent merger. Recent Spitzer Space Telescope data also indicate the presence of polycyclic aromatic hydrocarbon (PAH) emission at 8 μm. Several ellipticals have extended regions of 15 μm emission that have no obvious counterparts at other frequencies.

Mid-infrared luminosity as an indicator of the total infrared luminosity of galaxies

The infrared (IR) emission plays a crucial role for understanding the star formation in galaxies hidden by dust. We first examined four estimators of the IR luminosity of galaxies, L_fir (Helou et al. 1988), L_tir (Dale et al. 2001), revised version of L_tir (Dale & Helou 2002) (we denote L_tir2), and L_ir (Sanders & Mirabel 1996) by using the observed SEDs of well-known galaxies. We found that L_ir provides excellent estimates of the total IR luminosity for a variety of galaxy SEDs. The performance of L_tir2 was also found to be very good. Using L_ir, we then statistically analyzed the IRAS PSCz galaxy sample (Saunders et al. 2000) and found useful formulae relating the MIR monochromatic luminosities [L(12um) and L(25um)], and L_ir. For this purpose we constructed a subsample of 1420 galaxies with all IRAS four band (12, 25, 60, and 100um) flux densities. We found linear relations between L_ir and MIR luminosities, L(12um) and L(25um). The prediction error with 95-% confidence level is a factor of 4-5. Hence, these formulae are useful for the estimation of the total IR luminosity only from 12um or 25um observations. We further tried to make an `interpolation' formula for galaxies at 0<z<1. For this purpose we construct the formula of the relation between 15-um luminosity and the total IR luminosity. We conclude that the 15-um formula can be used as an estimator of the total IR luminosity from 24um observation of galaxies at z \simeq 0.6.

Obscured and Unobscured Active Galactic Nuclei in the Spitzer Space Telescope First Look Survey

Selection of active galactic nuclei (AGNs) in the infrared facilitates the discovery of AGNs whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess the fraction of AGNs with mid-infrared (MIR) luminosities that are comparable to quasars and that are missed in optical quasar surveys because of dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg^2 extragalactic FLS. These quasars occupy a distinct region in MIR color space by virtue of their strong, red continua. This has allowed us to define an MIR color criterion for selecting AGN candidates. About 2000 FLS objects have colors that are consistent with them being AGNs, but most are much fainter in the MIR than the SDSS quasars, which typically have 8 μm flux densities S_(8.0) ~ 1 mJy. We have investigated the properties of 43 objects with S_(8.0) ≥ 1 mJy that satisfy our AGN color selection. This sample should contain both unobscured quasars as well as AGNs that are absent from the SDSS survey because of extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects that are likely to be obscured quasars or luminous Seyfert 2 galaxies. This suggests that the numbers of obscured and unobscured AGNs are similar in samples selected in the MIR at S_(8.0) ~ 1 mJy.

The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: Scatter and unification aspects

We use mid-infrared spectral decomposition to separate the 6 μm mid-infrared AGN continuum from the host emission in the ISO low resolution spectra of 71 active galaxies and compare the results to observed and intrinsic 2-10 keV hard X-ray fluxes from the literature. We find a correlation between mid-infrared luminosity and absorption corrected hard X-ray luminosity, but the scatter is about an order of magnitude, significantly larger than previously found with smaller statistics. Main contributors to this scatter are likely variations in the geometry of absorbing dust, and AGN variability in combination with non-simultaneous observations. There is no significant difference between type 1 and type 2 objects in the average ratio of mid-infrared and hard X-ray emission, a result which is not consistent with the most simple version of a unified scheme in which an optically and geometrically thick torus dominates the mid-infrared AGN continuum. Most probably, significant non-torus contributions to the AGN mid-IR continuum are masking the expected difference between the two types of AGN.

Cold Dust in Early‐Type Galaxies. I. Observations

We describe far infrared observations of early-type galaxies selected from the ISO archive. This rather inhomogeneous sample includes 39 giant elliptical galaxies and 14 S0 (or later) galaxies. These galaxies were observed using a variety of different observing modes, each of which requires different and often rather elaborate photometric reduction procedures. The ISO background data agrees well with COBE-DIRBE results to which we have renormalized our calibrations. As a further check, the ISO fluxes from galaxies at 60 and 100 microns agree very well with those previously observed with IRAS at these wavelengths. The spatial resolution of ISO is several times greater than that of IRAS and the ISO observations extend out to 200 microns which views a significantly greater mass of colder dust not assessable to IRAS. Most of the galaxies are essentially point sources at ISO resolution, but a few are clearly extended at FIR wavelengths with image sizes that increase with FIR wavelength. The integrated far infrared luminosities do not correlate with optical luminosities, suggesting that the dust may have an external, merger-related origin. In general the far infrared spectral energy distributions can be modeled with dust at two temperatures, ~ 43 K and ~ 20 K, which probably represent limits of a continuous range of temperatures. The colder dust component dominates the total mass of dust, which is typically more than ten times larger than the dust masses previously estimated for the same galaxies using IRAS observations. For S0 galaxies we find that the optically normalized far-infrared luminosity $L_{FIR} / L_B$ correlates strongly with the mid-infrared luminosity $L_{15\mu\rm{m}} / L_B$, but that correlation is weaker for elliptical galaxies.

The radio-mid-infrared correlation and the contribution of 15-μm galaxies to the 1.4-GHz source counts

The radio counterparts to the 15-µm sources in the European Large Area ISO Survey southern fields are identified in 1.4-GHz maps down to ∼80 µJy. The radio‐mid-infrared correlation is investigated and derived for the first time at these flux densities for a sample of this size. Our results show that radio and mid-infrared (MIR) luminosities correlate almost as well as radio and far-infrared (FIR), at least up to z � 0.6. Using the derived relation and its spread together with the observed 15-µm counts, we have estimated the expected contribution of the 15-µm extragalactic populations to the radio source counts and the role of MIR starburst galaxies in the well-known 1.4-GHz source excess observed at sub-mJy levels. Our analysis demonstrates that IR emitting starburst galaxies do not contribute significantly to the 1.4-GHz counts for strong sources, but start to become a significant fraction of the radio source population at flux densities 0.5‐0.8 mJy. They are expected to be responsible for more than 60 per cent of the observed radio counts at 0.05 mJy. These results are in agreement with the existing results on optical identifications of faint radio

Mid‐Infrared Imaging of the Protostellar Binary L1448N IRS 3(A,B)

Mid-infrared (10-25 μm) imaging of the protostellar binary system L1448N IRS 3(A,B) is presented. Only one source, IRS 3(A), was detected at mid-infrared wavelengths—all of the mid-infrared emission from IRS 3(A,B) emanates from IRS 3(A). The mid-infrared luminosity of IRS 3(A) is Lmidir = 1.3 2 L☉, which yields a central source mass, depending on the mass infall rate, of M* = 0.2 M☉ . The envelope mass surrounding IRS 3(A) is ~0.15 M☉, suggesting that the central source and the envelope are of comparable mass. The locations of IRS 3(A) and IRS 3(B) on an Menv-Lbol diagram indicate that IRS 3(A) and IRS 3(B) appear to be Class I and Class 0 protostars, respectively.

The bulk of the cosmic infrared background resolved by ISOCAM

Deep extragalactic surveys with ISOCAM revealed the presence of a large density of faint mid-infrared (MIR) sources. We have computed the 15m integrated galaxy light produced by these galaxies above a sensitivity limit of 50 Jy. It sets a lower limit to the 15m extragalactic background light of (2:4 0:5) nW m 2 sr 1 .T he redshift distribution of the ISOCAM galaxies is inferred from the spectroscopically complete sample of galaxies in the Hubble Deep Field North (HDFN). It peaks around z 0.8 in agreement with studies in other elds. The rest-frame 15m and bolometric infrared (8{1000m) luminosities of ISOCAM galaxies are computed using the correlations that we establish between the 6.75, 12, 15m and infrared (IR) luminosities of local galaxies. The resulting IR luminosities were double-checked using radio (1.4 GHz) flux densities from the ultra-deep VLA and WSRT surveys of the HDFN on a sample of 24 galaxies as well as on a sample of 109 local galaxies in common between ISOCAM and the NRAO VLA Sky Survey (NVSS). This comparison shows for the rst time that MIR and radio luminosities correlate up to z 1. This result validates the bolometric IR luminosities derived from MIR luminosities unless both the radio-far infrared (FIR) and the MIR-FIR correlations become invalid around z 1. The fraction of IR light produced by active nuclei was computed from the cross-correlation with the deepest X-ray surveys from the Chandra and XMM-Newton observatories in the HDFN and Lockman Hole respectively. We nd that at most 20% of the 15m integrated galaxy light is due to active galactic nuclei (AGNs) unless a large population of AGNs was missed by Chandra and XMM-Newton. About 75% of the ISOCAM galaxies are found to belong to the class of luminous infrared galaxies (LIR 10 11 L). They exhibit star formation rates of the order of100 M yr 1 . The comoving density of infrared light due to these luminous IR galaxies was more than 40 times larger at z 1 than today. The contribution of ISOCAM galaxies to the peak of the cosmic infrared background (CIRB) at 140m was computed from the MIR-FIR correlations for star forming galaxies and from the spectral energy distribution of the Seyfert 2, NGC 1068, for AGNs. We nd that the galaxies unveiled by ISOCAM surveys are responsible for the bulk of the CIRB, i.e. (16 5) nW m 2 sr 1 as compared to the (25 7) nW m 2 sr 1 measured with the COBE satellite, with less than 10% due to AGNs. Since the CIRB contains most of the light radiated over the history of star formation in the universe, this means that a large fraction of present-day stars must have formed during a dusty starburst event similar to those revealed by ISOCAM.

Models for Multiband Infrared Surveys

Empirical backward galaxy evolution models for IR bright galaxies are constrained using multiband IR surveys. A new Monte Carlo algorithm is developed for this task. It exploits a large library of realistic spectral energy distributions (SEDs) of 837 local IR galaxies (IRAS 25 ?m selected) from the UV (1000 ?) to the radio (20 cm), including Infrared Space Observatory (ISO) measured 3-13 ?m unidentified broad features (UIBs). The basic assumption is that the local correlation between SEDs and mid-infrared (MIR) luminosities can be applied to earlier epochs of the universe, an assumption that will be strongly tested by SIRTF. By attaching an SED appropriately drawn from the SED library to every source predicted by a given model, the algorithm enables simultaneous comparisons with multiple surveys in a wide range of wave bands. Three populations of IR sources are considered in the evolution models. These include (1) starburst galaxies, (2) normal late-type galaxies, and (3) galaxies with active galactic nuclei (AGNs). Constrained by data from the literature, our best-fit model (peak model) predicts that since z = 1.5 the population of starburst galaxies undergoes a very strong luminosity evolution [L = L0(1 + z)4.2] and also strong density evolution [? = ?0(1 + z)2], the normal late-type galaxy population undergoes a passive luminosity evolution [L = L0(1 + z)1.5], and the galaxies with an AGN undergo a pure luminosity evolution similar to that of optical QSOs [L = L0(1 + z)3.5]. Prior at z ? 1.5 all evolution rates drop as (1 + z)-3. The luminosity evolution results in evolution of SEDs of IR bright sources because of the luminosity dependence of the SEDs. Predictions for number counts, confusion limits, redshift distributions, and color-color diagrams are made for multiband surveys using the upcoming SIRTF satellite. A ? cosmology (?? = 0.7, ?m = 0.3, H0 = 75 km s-1 Mpc-1) is assumed throughout the paper.

Interpreting the Cosmic Infrared Background: Constraints on the Evolution of the Dust‐enshrouded Star Formation Rate

The mid-infrared local luminosity function is evolved with redshift to —t the spectrum of the cosmic infrared background (CIRB) at j[ 5 km and the galaxy counts from various surveys at mid-infrared, far-infrared, and submillimeter wavelengths. A variety of evolutionary models provide satisfactory —ts to the CIRB and the number counts. The degeneracy in the range of models cannot be broken by current observations. However, the diUerent evolutionary models yield approximately the same comoving number density of infrared luminous galaxies as a function of redshift. Since the spectrum of the cosmic background at j[ 200 km is quite sensitive to the evolution at high redshift, i.e., z [ 1, all models that —t the counts require a —attening at z D 0.8 to avoid overproducing the CIRB. About 80% of the 140 km CIRB is produced at 0 z 1.5, while only about 30% of the 850 km background is produced within the same redshift range. The nature of the evolution is then translated into a measure of the dustenshrouded star formation rate (SFR) density as a function of redshift and compared with estimates from rest-frame optical/ultraviolet surveys. The dust-enshrouded SFR density appears to peak at z 0.8 ^ 0.1, much sooner than previously thought, with a value of yr~1 Mpc~3, and remains almost 0.25 ~0.10.12 M _ constant up to z D 2. At least 70% of this star formation takes place in infrared luminous galaxies with The long-wavelength observations that constrain our evolutionary models do not strongL IR [ 1011 L _ . ly trace the evolution at z [ 2 and a drop-oU in the dust-enshrouded SFR density is consistent with both the CIRB spectrum and the number counts. However, a comparison with the infrared luminosity function derived from extinction-corrected rest-frame optical/ultraviolet observations of the Lyman break galaxy population at z D 3 suggests that the almost —at comoving SFR density seen between redshifts of 0.8 and 2 extends up to a redshift of z D 4. (%)

Modelling ISO galaxy counts with luminosity and merger rate evolution

We model galaxy evolution in the 6.75- and 15-μm passbands of the ISO satellite, by combining models of galaxy evolution at optical wavelengths (which are consistent with the optical galaxy counts) with observed spectral energy distributions in the infrared. Our model is consistent with the local 12-μm galaxy luminosity function (from IRAS), including the bright end if we place a small fraction (∼3.5 per cent at z=0) of the model's spiral galaxies in interacting pairs with mid-infrared luminosities enhanced by major starbursts. Source counts from deep ISO surveys in the 6.75- and 15-μm passbands significantly exceed non-evolving predictions. We find the number counts, redshift distributions and the wide range of mid-infrared/optical flux ratios of the detected galaxies to be reasonably consistent with our evolving model. Our models suggest that the steep number count of 6.75-μm sources can be explained primarily by the evolving E/S0 galaxies. About one-third of 6.75-μm sources do appear to be E/S0s or early-type spirals, but starburst galaxies at 0.6<z<1.1 are more prominent in this passband than we expected. In the 15-μm passband the main contributors appear to be evolving, star-forming spirals and starbursting galaxy mergers, as in our model. Visibly interacting galaxies form ∼31 per cent of the 15-μm detections to 200 μJy, and have high mid-infrared/optical flux ratios consistent with our models for major starbursts with high dust extinction (up to ∼1.8 mag in the rest-frame blue band). The numbers and high mean redshift (0.68±0.08) of merging/interacting galaxies detected in these ISO surveys are only consistent with the local mid-infrared luminosity function if the merger-triggered starbursts undergo significant luminosity evolution, of ∼(1+z)2 to z∼1, in addition to the ∼(1+z)2 number evolution reflecting the increase with redshift in the fraction of merging/interacting galaxies (as estimated from optical surveys).

Properties of hot stars in the Wolf-Rayet galaxy NGC 5253 from ISO-SWS spectroscopy

ISO-SWS spectroscopy of the Wolf—Rayet galaxy NGC 5253 is presented, and analysed to provide estimates of its hot young star population. Our approach differs from previous investigations in that we are able to distinguish between the regions in which different infrared fine-structure lines form, using complementary ground-based observations. The high-excitation nebular [S iv] emission is formed in a very compact region, which we attribute to the central super-star nucleus, and lower excitation [Ne iii] nebular emission originates in the galactic core. We use photoionization modelling coupled with the latest theoretical O-star flux distributions to derive effective stellar temperatures and ionization parameters of Teff≥ 38 kK, log Q ∼ 8.25 for the compact nucleus, with Teff∼ 35 kK, log Q ≤ 8 for the larger core. Results are supported by more sophisticated calculations using evolutionary synthesis models. We assess the contribution that Wolf—Rayet stars may make to highly ionized nebular lines (e.g. [O IV]). From our Brα flux, the 2-arcsec nucleus contains the equivalent of approximately 1000 O7 V star equivalents and the starburst there is 2–3 Myr old; the 20-arcsec core contains about 2500 O7 V star equivalents, with a representative age of ∼5 Myr. The Lyman ionizing flux of the nucleus is equivalent to that of the 30 Doradus region. These quantities are in good agreement with the observed mid-infrared dust luminosity of 7.8 × 108 L⊙. Since this structure of hot clusters embedded in cooler emission may be common in dwarf starbursts, observing a galaxy solely with a large aperture may result in confusion. Neglecting the spatial distribution of nebular emission in NGC 5253 implies ‘global’ stellar temperatures (or ages) of 36 kK (4.8 Myr) and 39 kK (2.9 or 4.4 Myr) from the observed [Ne iii/ii] and [S iv/iii] line ratios, assuming log Q=8.

Mid-Infrared Images of Luminous Infrared Galaxies in a Merging Sequence

We report mid-infrared observations of several luminous infrared galaxies (LIGs) carried out with the Infrared Space Observatory. Our sample was chosen to represent different phases of a merger sequence of galaxy-galaxy interaction with special emphasis on early/intermediate stages of merging. The mid-infrared emission of these LIGs shows extended structures for the early and intermediate mergers, indicating that most of the mid-infrared luminosities are not from a central active galactic nucleus (AGN). Both the infrared hardness (indicated by the IRAS 12, 25, and 60 $\micron$ flux density ratios) and the peak-to-total flux density ratios of these LIGs increase as projected separation of these interacting galaxies become smaller, consistent with increasing star formation activities that are concentrated to a smaller area as the merging process advances. These observations provide among the first observational constraint of largely theoretically based scenarios.

A Three‐dimensional Decomposition of the Infrared Emission from Dust in the Milky Way

We have constructed a three-dimensional model of the Galactic large-scale infrared emission from dust associated with the molecular neutral atomic (H I), and extended low-density cm~3) (H 2 ), (n e D 1E100 ionized (H II) gas phases of the interstellar medium. The model incorporates a three-dimensional map of the molecular and neutral atomic hydrogen gas distributions, derived from available 12CO and H I surveys by using the radial velocity information in the spectral lines as a distance indicator, and available 5 and 19 GHz radio continuum surveys to trace the column density of ionized gas. We use the model to decompose the Di†use Infrared Background Experiment (DIRBE) 12E240 km obserCOBE5 vations of the Galactic plane region ( o b o „ 5i), from which the zodiacal light and stellar emission have been subtracted, into distinct emission components associated with each gas phase within selected ranges of Galactocentric distance. An interstellar dust model is ‐tted to the resulting infrared spectra to derive the following quantities within each Galactocentric distance interval: (1) the abundance and equilibrium temperature of the large dust grain component within each gas phase; (2) estimates of the abundance of very small (\200 transiently heated dust grains and polycyclic aromatic hydrocarbon (PAH) mol”) ecules; and (3) constraints on various model parameters, such as the energy density of the ambient interstellar radiation ‐eld, which heats the dust within the H I gas phase. Our results show steep negative Galactocentric gradients in the equilibrium temperature of the large dust grain component within the H I, and H II gas phases, the GalaxyIs ambient interstellar radiH 2 , ation ‐eld, and the dust-to-gas mass ratio for each gas phase. The intensity of the ambient interstellar radiation ‐eld increases by a factor of D3 between the solar circle (8.5 kpc) and the molecular ring at a Galactocentric distance of D5 kpc. The dust abundance gradient of ([0.05 ^ 0.03) dex kpc~1 is equivalent, within the uncertainties, to the metallicity gradient in the Galactic disk. The derived emission spectra are consistent with a model in which very small transiently heated dust grains and PAHs are abundant and the dominant contributors to the mid-infrared (5 km \j 40 km) luminosity from a Galactocentric distance of 2 kpc out to a Galactocentric distance of at least 12 kpc, and indicate that the relative abundance of the PAHs is signi‐cantly higher in the outer region of the Galactic disk than inside the solar circle. We combine the results of our decomposition algorithm with the results of a study of optical extinction at high Galactic latitude to derive the radial distribution of optical opacity in the Galactic disk and ‐nd that our Galaxy would be e†ectively transparent Galaxy) 0.2 mag] to an external obser[A B (total ver viewing it at a low inclination (i 30i). All of the Galactic infrared emission observed by the DIRBE can be accounted for by dust associated with gas that is detected by current radio surveys, refuting the recent suggestion that a large fraction of the dynamically inferred hidden mass in spiral galaxies may be due to unseen gas and stars in the disk of the galaxies. Subject headings: di†use radiation E dust, extinction E Galaxy: structure E infrared: ISM: continuum E ISM: abundances E ISM: molecules