Observations of ULIRGs with the IRS on Spitzer

Ultraluminous Infrared Galaxies (ULIRGs), those systems with L_(IR) ≥ 10^(12)L_⊙, have the power output of quasars, yet (re-)emit the vast majority of their energy in the far-infrared part of the spectrum. Because this energy is reprocessed by dust, the origin of the power is often hidden from view. With the Infrared Spectrograph on the Spitzer Space Telescope, we have observed a large number of low-redshift ULIRGs in order to better understand the source of their power, the conditions in the dusty interstellar medium, and their relationship to optically selected quasi-stellar objects (QSOs), effectively increasing the number of ULIRGs with rest-frame mid-infrared spectra by two orders of magnitude. The IRS has also been used to obtain the first mid-infrared spectra of samples of ULIRGs at redshifts of z > 2. In this paper, we summarize the key findings from the IRS studies of ULIRGs at low and high redshifts, and look toward the future of far-infrared spectroscopy from space

Starbursts and AGN in Luminous Infrared Galaxies

Luminous Infrared Galaxies (LIRGs), which emit a significant fraction of their bolometric luminosity in the far-infrared, have L_(IR) ≥ 10^(11)L_ʘ. LIRGs are a mixture of single galaxies, disk galaxy pairs, interacting systems, and advanced mergers which exhibit enhanced star-formation rates and AGN activity compared to normal galaxies. At the highest luminosities (L_(IR) ≥ 10^(12)L_ʘ), Ultraluminous Infrared Galaxies (ULIRGs) may represent an important stage in the formation of QSOs. If the growth of the central black hole preferentially occurs during periods of enhanced star formation, LIRGs and ULIRGs are important laboratories in which to understand both processes. The Great Observatories All-Sky LIRG Survey (GOALS) is aimed at measuring the properties of a large, complete sample of low-redshift LIRGs across the electromagnetic spectrum. With its ability to probe the stars, dust and ionized and molecular gas in LIRGs through mid-infrared imaging and spectroscopy, Spitzer is providing a key component of the GOALS data. In this paper, we summarize the key findings from Spitzer Infrared Spectrograph (IRS) studies of ULIRGs at low and high redshifts, and present early results from GOALS in the context of the co-evolution of starbursts and AGN

GOODS-Herschel: a population of 24 μm dropout sources at z<2

Using extremely deep PACS 100- and 160 μm Herschel data from the GOODS-Herschel program, we identify 21 infrared bright galaxies previously missed in the deepest 24   μm surveys performed by Spitzer/MIPS. These MIPS dropouts are predominantly found in two redshift bins, centred at z ~ 0.4 and ~1.3. Their S_(100)/S_(24) flux density ratios are similar to those of local (ultra-) luminous infrared galaxies (LIRGs and ULIRGs), whose silicate absorption features at 18 μm (at z ~ 0.4) and 9.7 μm (at z ~ 1.3) are shifted into the 24 μm MIPS band at these redshifts. The high-z sub-sample consists of 11 infrared luminous sources, accounting for ~2% of the whole GOODS-Herschel sample and putting strong upper limits on the fraction of LIRGs/ULIRGs at 1.0 43 colour cut selects galaxies with a redshift distribution similar to that of the MIPS dropouts and when combined with a second colour cut, S_(16)/S_8 > 4, isolates sources at 1.0 < z < 1.7. We show that these sources have elevated specific star formation rates (sSFR) compared to main sequence galaxies at these redshifts and are likely to be compact starbursts with moderate/strong 9.7 μm silicate absorption features in their mid-IR spectra. Herschel data reveal that their infrared luminosities extrapolated from the 24 μm flux density are underestimated, on average, by a factor of ~3. These silicate break galaxies account for 16% (8%) of the ULIRG (LIRG) population in the GOODS fields, indicating a lower limit in their space density of 2.0 × 10^(-5) Mpc^(-3). Finally, we provide estimates of the fraction of z < 2 MIPS dropout sources as a function of the 24-, 100-, 160-, 250- and 350   μm sensitivity limits, and conclude that previous predictions of a population of silicate break galaxies missed by the major 24   μm extragalactic surveys have beenoverestimated

The influence of AGN nuclear parameters on the FRI/FRII dichotomy

We have investigated the influence of nuclear parameters such as black hole mass and photoionizing luminosity on the FRI/FRII transition in a sample of nearby (z<0.2) radio galaxies from the 3CR catalogue. The sample was observed with medium-resolution, optical spectroscopy and contains some galaxies with unpublished velocity dispersion measurements and emission-line fluxes. The measured velocity dispersions for the sample lie in the range 130-340 km s^(-1) with a mean of 216 km s^(-1). Using the M-σ relation, we convert to black hole mass and find that the black hole mass distribution is identical for FRI and FRII galaxies, with a mean of ≈ 2.5 × 10^8 M_☉. We determine narrow emission-line luminosities from [O III] and [O III] in our spectra, as well as from the literature, and convert them to photoionizing luminosities under the assumption that the gas is ionized by the nuclear UV continuum. Most of the galaxies with FRI morphology and/or low-excitation emission-line spectra have progressively lower black hole masses at lower photoionizing (and jet) luminosities. This agrees with the well-known Ledlow-Owen relation which states that the radio luminosity at the FRI/FRII transition depends on the optical luminosity of the host, L_(radio) ∝ L^(1.8)_(optical), because these two luminosities relate to AGN nuclear parameters. When recasting the Ledlow-Owen relation into black hole mass versus photoionizing luminosity and jet luminosity, we find that the recasted relation describes the sample quite well. Furthermore, the FRI/FRII transition is seen to occur at approximately an order of magnitude lower luminosity relative to the Eddington luminosity than the soft-to-hard transition in X-ray binaries. This difference is also consistent with the Ledlow-Owen relation, which predicts a weak black hole mass dependence in the transition luminosity in Eddington units. We conclude that the FRI/FRII dichotomy is caused by a combination of external and nuclear factors, with the latter dominating

Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: The KINGFISH Sample

Interstellar dust and starlight are modeled for the galaxies of the project "Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel." The galaxies were observed by the Infrared Array Camera and the Multiband Imaging Photometer for Spitzer on Spitzer Space Telescope, and the Photodetector Array Camera and Spectrometer and the Spectral and Photometric Imaging Receiver on Herschel Space Observatory. With data from 3.6 to 500 μm, dust models are strongly constrained. Using a physical dust model, for each pixel in each galaxy we estimate (1) dust surface density, (2) dust mass fraction in polycyclic aromatic hydrocarbons (PAHs), (3) distribution of starlight intensities heating the dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR luminosity originating in subregions with high starlight intensity. The dust models successfully reproduce the observed global and resolved spectral energy distributions. With the angular resolution of Herschel, we obtain well-resolved maps (available online) for the dust properties. As in previous studies, we find the PAH fraction q_(PAH) to be an increasing function of metallicity, with a threshold oxygen abundance Z/Z⊙ ≈ 0.1, but we find the data to be fitted best with q_(PAH) increasing linearly with log(O/H) above a threshold value of 0.15(O/H)⊙. We obtain total dust masses for each galaxy by summing the dust mass over the individual map pixels; these "resolved" dust masses are consistent with the masses inferred from a model fit to the global photometry. The global dust-to-gas ratios obtained from this study are found to correlate with galaxy metallicities. Systems with Z/Z⊙ ≳ 0.5 have most of their refractory elements locked up in dust, whereas in systems with Z/Z⊙ ≾ 0.3 most of these elements tend to remain in the gas phase. Within galaxies, we find that q_(PAH) is suppressed in regions with unusually warm dust with vL_v(70 μm) ≳ 0.4L_(dust). With knowledge of one long-wavelength flux density ratio (e.g., f₁₆₀/f₅₀₀), the minimum starlight intensity heating the dust (U_(min)) can be estimated to within ~50%, despite a variation in U_(min) of more than two orders of magnitude. For the adopted dust model, dust masses can be estimated to within ~0.2 dex accuracy using the f₁₆₀/f₅₀₀ flux ratio and the integrated dust luminosity, and to ~0.07 dex accuracy using the 500 μm luminosity vL_v(500 µm) alone. There are additional systematic errors arising from the choice of dust model, but these are hard to estimate. These calibrated prescriptions for estimating starlight heating intensity and dust mass may be useful for studies of high-redshift galaxies

The Influence of Black Hole Mass and Accretion Rate on the FRI/FRII Radio Galaxy Dichotomy

We use medium resolution optical spectra of 3CR radio galaxies to estimate their black hole masses and accretion rates. Black hole masses are found from central stellar velocity dispersions and accretion rates are derived from narrow emission-line luminosities. The sample covers both Fanaroff-Riley (FR) classes; the more powerful FRIIs and the less powerful FRIs. We find that FRIs and FRIIs separate in diagrams of black hole mass versus radio luminosity and narrow-line luminosity. This suggests that, at a given black hole mass, the FRIIs accrete more efficiently, or accrete more matter, than FRIs.Comment: 2 pages, 1 figure. Contribution to the proceedings of the conference "Growing Black Holes", Garching, Germany, June 21-24, 2004. Eds. A. Merloni, S. Nayakshin, R. Sunyae

Observations of Ultraluminous Infrared Galaxies with the IRS on Spitzer

We present spectra taken with the Infrared Spectrograph on Spitzer covering the 5−38μm region of the ten Ultraluminous Infrared Galaxies (ULIRGs) found in the IRAS Bright Galaxy Sample. Among the BGS ULIRGs, we find a factor of 50 spread in the spectral slope from 5.5−60μm in the rest frame. There is evidence for water ice and hydrocarbon absorption (from 5.5−6.5μm) in 7/10 BGS ULIRGs, as well as absorption features of C2H2 and HCN in four and possibly six of the 10 BGS ULIRGs, indicating shielded molecular clouds and a warm, dense ISM. We have detected [NeV] emission at 14.3μm and 24.3μm in three out of the 10 BGS ULIRGs. The AGN fractions implied by either the [NeV]/[NeII] or [OIV]/[NeII] line flux ratios (or their upper limits) are significantly lower than implied by the MIR slope or strength of the 6.2μm PAH EQW feature

High Resolution WFPC2 Imaging of IRAS 09104+4109

With a infrared luminosity of nearly 10^13 Lsuns, IRAS 09104+4109 is the most luminous galaxy with z<0.5 in the IRAS All Sky Survey. A radio-loud Seyfert 2 type optical spectrum, a cD host galaxy in a rich cluster, and a massive cooling flow make IRAS 09104+4109 unique among ultraluminous infrared galaxies. Cannibalized cluster members and the cooling intercluster medium may contribute both the fuel and the dust needed to re-radiate the power of IRAS 09104+4109 into the far-infrared. We have imaged IRAS 09104+4109 in the WFPC2 F622W, F814W, and FR680N filters on the HST to obtain rest frame 4300A, 5700A, and [OIII] emission line images on sub-kpc scales. IRAS 09104+4109 displays a complex morphology on the smallest scales, with radiating filaments, an asymmetric [OIII] nebula, and a number of very faint, irregular blue objects surrounding the cD galaxy. We discuss the nature and possible interplay between the enshrouded QSO nucleus, the cD host galaxy and the irregular cluster.Comment: LaTex, 6 pages with 2 postscript and 1 jpg figure. To appear in the proc. of the Ringberg workshop "Ultraluminous Galaxies: Monsters or Babies" (Ringberg castle, Sept. 1998), Ap&SS, in pres

A Study of Heating and Cooling of the ISM in NGC 1097 with Herschel-PACS and Spitzer-IRS

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar, and an active nucleus. We present a detailed study of the spatial variation of the far-infrared (FIR) [C II]158 μm and [O I]63 μm lines and mid-infrared H_2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star-forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([C II]158 μm+[O I]63 μm)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7 μm PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [C II]158 μm/PAH(5.5-14 μm) is found. PAHs in the ring are responsible for a factor of two more [C II]158 μm and [O I]63 μm emission per unit mass than PAHs in the Enuc S. spectral energy distribution (SED) modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high-intensity photodissociation regions (PDRs), in which case G_0 ~ 10^(2.3) and n_H ~ 10^(3.5) cm^(–3) in the ring. For these values of G_0 and n_H, PDR models cannot reproduce the observed H2 emission. Much of the H2 emission in the starburst ring could come from warm regions in the diffuse interstellar medium that are heated by turbulent dissipation or shocks

Near Infrared Observations of a Redshift 5.34 Galaxy: Further Evidence for Dust Absorption in the Early Universe

Imaging at 1.25 and 2.20 microns has been obtained of the field containing the galaxy (RD1) found at redshift 5.34 by Dey et al.(1998). This galaxy has been detected at 1.25 microns, while the lower redshift (z=4.02) galaxy also found in the same field by Dey et al. was detected at both 1.25 and 2.20 microns. Comparison to stellar population synthesis models indicates that if RD1 is a young ( 0.5 mag) is indicated. Combined with observations of other high redshift systems, these data show that dust is likely to be an important component of young galaxies even at redshifts of z > 5. The extinction-corrected monochromatic luminosity of RD1 at 1500 angstroms is then a factor of about three larger than L(1500)* as determined by Dickinson (1998) for z ~ 3 starburst galaxies. The implied star formation rate in RD1, corrected for extinction, is ~ 50-100 solar masses per year.Comment: plain LaTex with 1 postscript figure. ApJ Letters, accepte