arXiv subjects

M. Lacy

Publications and source records attributed to M. Lacy.

At least 55 records · Page 3Linked to original sources

arXiv2009

The Dark Side of Reionization: Probing Cooling in the Early Universe

Probing the growth of structure from the epoch of hydrogen recombination to the formation of the first stars and galaxies is one of the most important uncharted areas of observational cosmology. Far-IR spectroscopy covering $λ$ 100-500 microns from space, and narrow partial transmission atmospheric bands available from the ground, opens up the possibility of probing the molecular hydrogen and metal fine-structure lines from primordial clouds from which the first stars and galaxies formed at 6 < z $<$ 15. Building on Spitzer observations of unexpectedly powerful H2 emission from shocks, we argue that next-generation far-IR space telescopes may open a new window into the main cloud cooling processes and feedback effects which characterized this vital, but unexplored epoch. Without this window, we are essential blind to the dominant cloud cooling which inevitably led to star formation and cosmic reionization.

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arXiv2008

Mid-Infrared Spectra of High Redshift (z>2) Radio Galaxies

We present the first mid-infrared Spitzer/Infrared Spectrograph (IRS) observations of powerful radio galaxies at z>2. These radio galaxies, 4C +23.56 (z=2.48) and 6C J1908+7220 (z=3.53), both show strong mid-infrared continua, but with 6C J1908+7220 also showing strong PAH emission at rest-frame 6.2 and 7.7um. In 4C+23.56 we see no obvious PAH features above the continuum. The PAH emission in 6C J1908+7220 is the amongst the most distant observed to date and implies that there is a large instantaneous star formation rate (SFR). This is consistent with the strong detection of 6C J1908+7220 at far-IR and sub-mm wavelengths, indicative of large amounts of cold dust, ~10^9Msun. Powerful radio galaxies at lower redshifts tend to have weak or undetectable PAH features and typically have lower far-IR luminosities. In addition, 4C 23.56 shows moderate silicate absorption as seen in less luminous radio galaxies, indicating tau_{9.7um}=0.3+/-0.05. This feature is shifted out of the observed wavelength range for 6C J1908+7220. The correlation of strong PAH features with large amounts of cold dust, despite the presence of a powerful AGN, is in agreement with other recent results and implies that star formation at high redshift is, in some cases at least, associated with powerful, obscured AGN.

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arXiv2007

Large amounts of optically-obscured star formation in the host galaxies of some type-2 quasars

We present Hubble Space Telescope images, and spectral energy distributions from optical to infrared wavelengths for a sample of six 0.3<z<0.8 type-2 quasars selected in the mid-infrared using data from the Spitzer Space Telescope. All the host galaxies show some signs of disturbance. Most seem to possess dusty, star-forming disks. The disk inclination, estimated from the axial ratio of the hosts, correlates with the depth of the silicate feature in the mid-infrared spectra, implying that at least some of the reddening towards the AGN arises in the host galaxy. The star formation rates in these objects, as inferred from the strengths of the PAH features and far-infrared continuum, range from 3-90 Msun/yr, but are mostly much larger than those inferred from the [OII]3727 emission line luminosity, due to obscuration. Taken together with studies of type-2 quasar hosts from samples selected in the optical and X-ray, this is consistent with previous suggestions that two types of extinction processes operate within the type-2 quasar population, namely a component due to the dusty torus in the immediate environment of the AGN, and a more extended component due to a dusty, star forming disk.

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arXiv2007

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

We have investigated the influence of nuclear parameters such as black hole (BH) mass and photoionizing luminosity on the FRI/FRII transition in a sample of nearby (z<0.2) 3CR radio galaxies. The sample was observed with medium-resolution, optical spectroscopy and contains some galaxies with unpublished velocity dispersion measurements and emission-line fluxes. Measured velocity dispersions are 130-340 km/s with a mean of 216 km/s. Converting to BH mass, we find that the BH mass distribution is identical for FRIs and FRIIs, with a mean of approximately 2.5x10^8 Msun. We convert [OII] and [OIII] emission-line luminosities to photoionizing luminosity 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 BH masses at lower photoionizing (and jet) luminosities. This agrees with the 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_optical^1.8, because both L_radio and L_optical relate to AGN nuclear parameters. When recasting the Ledlow-Owen relation into BH mass versus photoionizing and jet luminosity, we find that the recasted relation describes the sample quite well. The FRI/FRII transition occurs 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 consistent with the Ledlow-Owen relation, which predicts a weak BH mass dependence in the transition luminosity. We conclude that the FRI/FRII dichotomy is caused by a combination of external and nuclear factors, with the latter dominating.

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arXiv2007

An Investigation into the Effects of Luminosity on the Mid-Infrared Spectral Energy Distributions of Radio-Quiet Quasars

We present an analysis of the effects of luminosity on the shape of the mid-infrared spectral energy distributions (SEDs) of 234 radio-quiet quasars originally presented by Richards et al. In quasars without evident dust extinction, the spectrally integrated optical and infrared luminosities are linearly correlated over nearly three decades in luminosity. We find a significant (>99.99% confidence) correlation between the 1.8-8.0 micron spectral index and infrared luminosity that indicates an enhancement of the mid-infrared continuum with increasing luminosity. Coupled with strong evidence for spectral curvature in more luminous quasars, we conclude this trend is likely a manifestation of the `near-infrared (3-5 micron) bump' noticed in earlier quasar SED surveys. The strength of this feature is indicative of the contribution of emission from the hottest (>1000 K) dust to the mid-infrared spectrum; higher luminosity quasars tend to show more hot dust emission. Finally, the comparable distribution of bolometric corrections from the monochromatic 3 micron luminosity as well as its lack of sensitivity to dust extinction as compared to the standard bolometric correction from nu*L_{5100A} suggest that the former may be a more robust indicator of bolometric quasar luminosity. The close link between the power in the mid-infrared and optical and the effect of luminosity on the shape of the mid-infrared continuum indicate that considering mid-infrared emission independent of the properties of the quasar itself is inadequate for understanding the parsec-scale quasar environment.

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arXiv2006

A New Measurement of the Stellar Mass Density at z~5: Implications for the Sources of Cosmic Reionization

We present a new measurement of the integrated stellar mass per comoving volume at redshift 5 determined via spectral energy fitting drawn from a sample of 214 photometrically-selected galaxies with z'<26.5 in the southern GOODS field. Following procedures introduced by Eyles et al. (2005), we estimate stellar masses for various sub-samples for which reliable and unconfused Spitzer IRAC detections are available. A spectroscopic sample of 14 of the most luminous sources with =4.92 provides a firm lower limit to the stellar mass density of 1e6 Msun/Mpc^3. Several galaxies in this sub-sample have masses of order 10^11 Msun implying significant earlier activity occurred in massive systems. We then consider a larger sample whose photometric redshifts in the publicly-available GOODS-MUSIC catalog lie in the range 4.4 <z 5.6. Before adopting the GOODS-MUSIC photometric redshifts, we check the accuracy of their photometry and explore the possibility of contamination by low-z galaxies and low-mass stars. After excising probable stellar contaminants and using the z'-J color to exclude any remaining foreground red galaxies, we conclude that 196 sources are likely to be at z~5. The implied mass density from the unconfused IRAC fraction of this sample, scaled to the total available, is 6e6 Msun/Mpc^3. We discuss the uncertainties as well as the likelihood that we have underestimated the true mass density. Including fainter and quiescent sources the total integrated density could be as high as 1e7 Msun/Mpc^3. Using the currently available (but highly uncertain) rate of decline in the star formationhistory over 5 <z< 10, a better fit is obtained for the assembled mass at z~5 if we admit significant dust extinction at early times or extend the luminosity function to very faint limits. [abridged]

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arXiv2006

Optical spectroscopy and X-ray detections of a sample of quasars and AGN selected in the mid-infrared from two Spitzer wide-area surveys

We present optical spectroscopy of a sample of 77 luminous AGN and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and SWIRE XMM-LSS fields, with a typical 24mu flux density of 5mJy. The median redshift is 0.6, with a range of ~0.05-4. Only 33% (25/77) of these objects are normal type-1 quasars, with no obscuration. 44% (34/77) are type-2 objects, with high-ionization, narrow emission lines, 14% (11/77) are dust-reddened type-1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. 9% (7/77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra which could be of either starburst or LINER types. These latter objects are analogous to the X-ray detected population of AGN with weak or non-existent optical AGN emission (the ``XBONGs''). 21 of our objects from the SWIRE field fall within moderately-deep XMM exposures. All the unobscured quasars, and about half the obscured quasars are detected in these exposures. This sample, when taken together with other samples of Spitzer selected AGN and quasars, and results from X-ray studies, confirms that obscured AGN dominate the AGN and quasar number counts of all rapidly-accreting supermassive black hole systems, at least for z~<4. This implies a high radiative efficiency for the black hole accretion process.

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arXiv2006

The nuclear regions of NGC 7582 from [NeII] spectroscopy at 12.8 microns - an estimate of the black hole mass

We present a high-resolution (R~16,000) spectrum and a narrow-band image centered on the [NeII]12.8 micron line of the central kpc region of the starburst/Seyfert2 galaxy NGC 7582. The galaxy has a rotating circum-nuclear starburst disk, shown at great detail at a diffraction-limited resolution of 0.4 arcsec. The high spatial resolution allows us to probe the dynamics of the [NeII] gas in the nuclear regions, and to estimate the mass of the central black hole. We construct models of gas disks rotating in the combined gravitational potential from the stellar bulge and a central black hole, and derive a black hole mass of 5.5 x 10^7 solar masses with a 95% confidence interval of [3.6,8.1] x 10^7 solar masses. The black hole mass combined with stellar velocity dispersion measurements from the literature shows that the galaxy is consistent with the local M-sigma relation. This is the first time that a black hole mass in a galaxy except our own Milky Way system has been estimated from gas dynamics in the mid-infrared. We show that spatially resolved mid-infrared spectroscopy may be competitive with similar techniques in the optical and near-infrared, and may prove to be important for estimating black hole masses in galaxies with strong nuclear dust obscuration. The high spectral resolution allows us to determine the heliocentric systemic velocity of the galaxy to between 1614 and 1634 km/s. The mid-infrared image reveals several dense knots of dust-embedded star formation in the circum-nuclear disk, and we briefly discuss its morphology.

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arXiv2006

Mid-IR selected Quasars in the First Look Survey

We present a preliminary investigation of the spectral energy distributions (SEDs), and star-formation properties of a sample of Mid-IR selected Quasars. The mid-infrared SEDs of our objects are consistent with that expected from clumpy torus models. At longer wavelengths, the radio to infrared ratios of several objects are consistent with those of star-forming galaxies.

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arXiv2006

Mapping Global Star Formation in the Interacting Galaxy Pair ARP 32

A multi-wavelength set of photometric data including UV (GALEX), optical, near-IR, infrared (Spitzer) and radio (VLA 20cm) images and spectroscopic observations are used to map the dust-obscured and unobscured star formation in the galaxy pair ARP 32. The system consists of an actively star-forming galaxy and another one with depressed star formation. The most active galaxy has disrupted morphology and different sites of star formation. Spectroscopic data show hints of nuclear activity in its core, intense star formation in limited regions of the galaxy as well as an underlying population of stars witnessing a past episode of star formation. Current star formation rates are estimated from UV and bolometric IR luminosities.

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arXiv2006

The 1-1000 micron SEDs of far-infrared galaxies

Galaxies selected at 170um by the ISO FIRBACK survey represent the brightest \~10% of the Cosmic Infrared Background. Examining their nature in detail is therefore crucial for constraining models of galaxy evolution. Here we combine Spitzer archival data with previous near-IR, far-IR, and sub-mm observations of a representative sample of 22 FIRBACK galaxies spanning three orders of magnitude in infrared luminosity. We fit a flexible, multi-component, empirical SED model of star-forming galaxies designed to model the entire ~1-1000um wavelength range. The fits are performed with a Markov Chain Monte Carlo (MCMC) approach, allowing for meaningful uncertainties to be derived. This approach also highlights degeneracies such as between Td and beta, which we discuss in detail. From these fits and standard relations we derive: L_IR, L_PAH, SFR, tau_V, M_star, M_dust, Td, and beta. We look at a variety of correlations between these and combinations thereof in order to examine the physical nature of these galaxies. Our conclusions are supplemented by morphological examination of the sources, and comparison with local samples. We find the bulk of our sample to be consistent with fairly standard size and mass disk galaxies with somewhat enhanced star-formation relative to local spirals, but likely not bona fide starbursts. A few higher-z LIGs and ULIGs are also present, but contrary to expectation, they are weak mid-IR emitters and overall are consistent with star-formation over an extended cold region rather than concentrated in the nuclear regions. We discuss the implications of this study for understanding populations detected at other wavelengths, such as the bright 850um SCUBA sources or the faint Spitzer 24um sources.

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arXiv2006

The Spitzer Space Telescope Extra-Galactic First Look Survey: 24 micron data reduction, catalog, and source identification

We present the reduction of the 24 micron data obtained during the first cosmological survey performed by the Spitzer Space Telescope (First Look Survey, FLS). The survey consists of a shallow observation of 2.5x2 sq deg centered at 17h18m +59d30m (main survey) and a deeper observation of 1x0.5 sq deg centered at 17h17m +59d45m(verification survey). Issues with the reduction of the 24 micron MIPS data are discussed and solutions to attenuate instrumental effects are proposed and applied to the data. Approximately 17000 sources are extracted with a SNR greater than five. The photometry of the point sources is evaluated through PSF fitting using an empirical PSF derived from the data. Aperture corrections and the absolute calibration have been checked using stars in the field. Astrometric and photometric errors depend on the SNR of the source varying between 0.35-1 arcsec and 5-15%, respectively, for sources detected at 20-5 sigma. The flux of the 123 extended sources have been estimated through aperture photometry. The extended sources cover less than 0.3% of the total area of the survey. Based on simulations, the main and verification surveys are 50% complete at 0.3 and 0.15 mJy, respectively. Counterparts have been searched for in optical and radio catalogs. More than 80% of the 24 micron sources have a reliable optical counterpart down to R=25.5. 16% of the sources have a 20 cm counterpart down to 0.1 mJy and ~ 80% of the radio-infrared associations have a reliable optical counterpart. A residual map is obtained by subtracting point sources detected at the 3-sigma level and interpolating the regions occupied by extended sources. Several galactic clouds with low and intermediate velocities are identified by comparison with neutral Hydrogen data from this field.

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arXiv2006

Silicate Emission in the Spitzer IRS spectrum of FSC 10214+4724

We present the first MIR spectrum of the z=2.2856 ultraluminous, infrared galaxy FSC 10214+4724, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope. The spectrum spans a rest wavelength range of 2.3-11.5 microns, covering a number of key diagnostic emission and absorption features. The most prominent feature in the IRS spectrum is the silicate emission at rest-frame 10 microns. We also detect an unresolved emission line at a rest wavelength of 7.65 microns which we identify with [NeVI], and a slightly resolved feature at 5.6 microns identified as a blend of [Mg VII] and [Mg V]. There are no strong PAH emission features in the FSC 10214+4724 spectrum. We place a limit of 0.1 micron on the equivalent width of 6.2 micron PAH emission but see no evidence of a corresponding 7.7 micron feature. Semi-empirical fits to the spectral energy distribution suggest about 45% of the bolometric luminosity arises from cold 50 K dust, half arises from warm (190 K) dust, and the remainder, 5%, originates from hot (640 K) dust. The hot dust is required to fit the blue end of the steep MIR spectrum. The combination of a red continuum, strong silicate emission, little or no PAH emission, and no silicate absorption, makes FSC 10214+4724 unlike most other ULIRGs or AGN observed thus far with IRS. These apparently contradictory properties may be explained by an AGN which is highly magnified by the lens, masking a (dominant) overlying starburst with unusually weak PAH emission.

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arXiv2005

Star Formation Rates and Extinction Properties of IR-Luminous Galaxies in the Spitzer First Look Survey

We investigate the instantaneous star formation rates (SFR) and extinction properties for a large (N=274), near-infrared (2.2 micron) + mid-infrared (24 micron) selected sample of normal to ultra-luminous infrared galaxies (ULIRGs) [10^9 ~0.8 in the Spitzer Extragalactic First Look Survey. We combine Spitzer MIPS 24-micron observations with high-resolution, optical Keck Deimos spectroscopy to derive optical emission-line and infrared star formation rates (SFR_{opt} & SFR_{IR}, respectively). Direct comparison of these SFR diagnostics reveals that our sample exhibits a wide range of extinction (1.0 1, which make up a large fraction of our sample, have SFR~100 M_{sun}/yr and ~2.5 mag. This translates to a 97% mean attenuation of the [OII] forbidden line doublet, with the most extreme sources having as much as 99.7% of their [OII] line flux extinguished by dust. Based on a SFR_{IR}/SFR_{opt} diagnostic, we derive an IR-luminosity-dependent Av^{IR} function [Av^{IR}=0.75*log(L_{IR}/L_{sun})-6.35 mag] that we use to extinction correct our emission line luminosities. Application of this correction results in a correlation between SFR_{IR} and SFR_{opt} that has a dispersion of 0.2 dex. Investigation of the Av dependence on redshift reveals that for a fixed L_{IR}, there is no significant Av evolution. The mean attenuation of our sample is intermediate between that of local optical/UV- and radio-selected samples and has a marginally stronger L_{IR} dependence.

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arXiv2005

Spitzer 70 and 160-micron Observations of the Extragalactic First Look Survey

We present Spitzer 70um and 160um observations of the Spitzer extragalactic First Look Survey (xFLS). The data reduction techniques and the methods for producing co-added mosaics and source catalogs are discussed. Currently, 26% of the 70um sample and 49% of the 160um-selected sources have redshifts. The majority of sources with redshifts are star-forming galaxies at z<0.5, while about 5% have infrared colors consistent with AGN. The observed infrared colors agree with the spectral energy distribution (SEDs) of local galaxies previously determined from IRAS and ISO data. The average 160um/70um color temperature for the dust is Td~= 30+/-5 K, and the average 70um/24um spectral index is alpha~= 2.4+/-0.4. The observed infrared to radio correlation varies with redshift as expected out to z~1 based on the SEDs of local galaxies. The xFLS number counts at 70um and 160um are consistent within uncertainties with the models of galaxy evolution, but there are indications that the current models may require slight modifications. Deeper 70um observations are needed to constrain the models, and redshifts for the faint sources are required to measure the evolution of the infrared luminosity function.

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arXiv2005

The Infrared Array Camera component of the Spitzer Space Telescope Extragalactic First Look Survey

We present Infrared Array Camera (IRAC) data and source catalogs from the Spitzer Space Telescope Extragalactic First Look Survey. The data were taken in four broad bands centered at nominal wavelengths of 3.6, 4.5, 5.8 and 8.0 microns. A set of mosaics and catalogs have been produced which are ~80% complete and ~99% reliable to their chosen flux density limits. The main field survey covers 3.8 deg^2, and has flux density limits of 20muJy, 25muJy, 100muJy and 100muJy at wavelengths of 3.6, 4.5, 5.8 and 8.0 microns,respectively. The deeper ``verification'' survey covers 0.25 deg^2 with limits of 10muJy, 10muJy, 30muJy and 30muJy, respectively. We also include deep data in the ELAIS-N1 field which covers 0.041deg^2 with limits of 4muJy, 3muJy, 10muJy and 10muJy, respectively, but with only two wavelength coverage at a given sky position. The final bandmerged catalogs contain 103193 objects in the main field, 12224 in the verification field and 5239 in ELAIS-N1. Flux densities of high signal-to-noise objects are accurate to about 10%, and the residual systematic error in the absolute flux density scale is ~2-3%. We have successfully extracted sources at source densities as high as 100000 deg^-2 in our deepest 3.6 and 4.5 micron data. The mosaics and source catalogs will be made available through the Spitzer Science Center archive and the Infrared Science Archive.

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arXiv2004

Mid-infrared selection of quasar-2s in Spitzer's First Look Survey

We present early results from the spectroscopic follow-up of a sample of candidate obscured AGN selected in the mid-infrared from the Spitzer First Look Survey. Our selection allows a direct comparison of the numbers of obscured and unobscured AGN at a given luminosity for the first time, and shows that the ratio of obscured to unobscured AGN at infrared luminosities corresponding to low luminosity quasars is ~1:1 at z~0.5. Most of our optically-faint candidate obscured AGN have the high-ionization, narrow-line spectra expected from type-2 AGN. A composite spectrum shows evidence for Balmer absorption lines, indicating recent star-formation activity in the host galaxies. There is tentative evidence for a decrease in the obscured AGN fraction with increasing AGN luminosity.

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arXiv2004

Simulating the Spitzer mid-IR color-color diagrams

We use a simple parameterization of the mid-IR spectra of a wide range of galaxy types in order to predict their distribution in the Infrared Array Camera (IRAC) 3.6um, 4.5um, 5.8um, and 8.0um and Multiband Photometer for Spitzer (MIPS) 24um color-color diagrams. We distinguish three basic spectral types by the energetically dominant component in the 3--12 micron regime: stellar-dominated; PAH-dominated; and continuum-dominated. We have used a Markov chain Monte Carlo (MCMC) approach to arrive at a more systematic and robust representation of the mid-IR spectra of galaxies than more traditional approaches. We find that IRAC color-color plots are well suited to distinguishing the above spectral types, while the addition of 24 micron data allows us to suggest practical 3-color cuts which preferentially select higher redshift sources of specific type. We compare our simulations with the color-color plot obtained by the Spitzer First Look Survey (FLS) and find reasonable agreement. Lastly, we discuss other applications as well as future directions for this work.

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