Daniel W. Weedman

A Significant Population of Very Luminous Dust-Obscured Galaxies at Redshift z ~ 2

The Spitzer Space Telescope has revealed a significant population of high-redshift (z ~ 2) dust-obscured galaxies with large mid-infrared to ultraviolet luminosity ratios. Due to their optical faintness, these galaxies have been previously missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-redshift population based solely on the ratio of the observed mid-infrared 24 μm to optical R-band flux density. We apply this method to observations of the ≈8.6 deg^2 NOAO Deep Wide-Field Survey Bootes field, and uncover ≈2600 dust-obscured galaxy candidates [i.e., 0.089 arcmin^−2) with 24 μm flux densities F24 μm ≥ 0.3 mJy and (R − [24]) ≥ 14 (i.e., Fν(24 μm)/Fν(R)≳1000]. These galaxies have no counterparts in the local universe. They represent 7% ± 0.6% of the 24 μm source population at F24 μm ≥ 1 mJy but increase to ≈13% ± 1% of the population at ≈0.3 mJy. These galaxies exhibit evidence of both star formation and AGN activity, with the brighter 24 μm sources being more AGN-dominated. We have measured spectroscopic redshifts for 86 of these galaxies, and find a broad redshift distribution centered at z ≈ 1.99±0.05. The space density of this population is ΣDOG(F24μ m ≥ 0.3 mJy) = (2.82 ± 0.05) × 10^−5^h3 70 Mpc^−3, similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median νLν(8 μm)≈4 × 10^11 L⊙. The infrared luminosity density contributed by this relatively rare dust-obscured galaxy population is log (IRLD) ≈8.23^+0.18 −0.30. This is ≈60^+40 −15% of that contributed by z ~ 2 ultraluminous infrared galaxies (ULIRGs, with LIR > 10^12 L⊙); our simple selection thus identifies a significant fraction of z ~ 2 ULIRGs. This IRLD is ≈26% ± 14% of the total contributed by all z ~ 2 galaxies. We suggest that these dust-obscured galaxies are the progenitors of luminous (~4L*) present-day galaxies, seen undergoing an extremely luminous, short-lived phase of both bulge and black hole growth. They may represent a brief evolutionary phase between submillimeter-selected galaxies and less obscured quasars or galaxies.

NGC 7714 - The prototype star-burst galactic nucleus

New X-ray, ultraviolet (IUE), optical, and radio (VLA) observations are presented for the bright nucleus galaxy NGC 7714. All data are explainable as resulting from intense star-formation activity in this nucleus - a star burst. The X-ray and radio luminosities are explained by supernova remnants arising from the massive stars, and approximately 10,000 supernova remnants are required in a volume of 280 pc radius. It is thought that NGC 7714 is typical of a significant class of emission-line galaxies. It is pointed out that individual supernovae should be observable in this, or similar nuclei, because they are expected to occur at rates approaching one per year.

A Spitzer high resolution mid-infrared spectral atlas of starburst galaxies

We present an atlas of Spitzer/IRS high-resolution (R ~ 600) 10-37 μm spectra for 24 well known starburst galaxies. The spectra are dominated by fine-structure lines, molecular hydrogen lines, and emission bands of polycyclic aromatic hydrocarbons (PAHs). Six out of the eight objects with a known active galactic nucleus (AGN) component show emission of the high excitation [Ne V] line. This line is also seen in one other object (NGC 4194) with, a priori, no known AGN component. In addition to strong PAH emission features in this wavelength range (11.3, 12.7, 16.4 μm), the spectra reveal other weak hydrocarbon features at 10.6, 13.5, 14.2 μm, and a previously unreported emission feature at 10.75 μm. An unidentified absorption feature at 13.7 μm is detected in many of the starbursts. We use the fine-structure lines to derive the abundance of neon and sulfur for 14 objects where the H I 7-6 line is detected. We further use the molecular hydrogen lines to sample the properties of the warm molecular gas. Several basic diagrams characterizing the properties of the sample are also shown. We have combined the spectra of all the pure starburst objects to create a high signal-to-noise ratio template, which is available to the community.

STRONG POLYCYCLIC AROMATIC HYDROCARBON EMISSION FROM z ≈ 2 ULIRGs ∗

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 0.5 mJy; (2) R – [24]>14 Vega mag; and (3) a prominent rest frame 1.6 μm stellar photospheric feature redshifted into Spitzers 3-8 μm IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 = 1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 μm stellar bump can be efficiently used to select highly obscured star-forming galaxies at z ≈ 2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 μm flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 μm detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously use observed spectral energy distributions to explain the broader redshift distribution of Spitzer-detected ULIRGs that are dominated by an active galactic nucleus (AGN). Finally, we conclude that z ≈ 2 sources with a detectable 1.6 μm stellar opacity feature lack sufficient AGN emission to veil the 7.7 μm PAH band.Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6 micron stellar photospheric feature redshifted into Spitzers 3-8 micron IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 =1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 micron stellar bump can be efficiently used to select highly obscured starforming galaxies at z~2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 micron flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 micron detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on the shapes of their SEDs. Finally, we conclude that z~2 sources with a detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to veil the 7.7 micron PAH band.

A Photometric Study of Markarian Galaxies

Photoelectric UBV observations were obtained for 92 Markarian galaxies and an additional five Seyfert galaxies to study the colors of the continuous spectra in these objects. A total of 28 Seyfert galaxies, 42 other galaxies with bright blue nuclei, and 27 diffuse galaxies were included. Multiaperture observations are presented for seven of these galaxies. It was found that galaxies spectroscopically classified as Seyfert galaxies because of broad nuclear emission lines have UBV colors corresponding to nonthermal continua, but the numerous Markarian galaxies with bright nuclei having narrow emission lines do not have such colors; instead, the ultraviolet continua of these galaxies apparently arise from hot stars. New spectroscopic and photographic observations are also presented for some Markarian galaxies of particular interest, including Markarian 3 and 205. (auth)

Infrared Spectra and Spectral Energy Distributions for Dusty Starbursts and Active Galactic Nuclei

We present spectroscopic results for all galaxies observed with the Spitzer Infrared Spectrograph (IRS) that also have total infrared fluxes f IR measured with the Infrared Astronomical Satellite (IRAS), also using AKARI photometry when available. Infrared luminosities and spectral energy distributions (SEDs) from 8 μm to 160 μm are compared to polycyclic aromatic hydrocarbon emission from starburst galaxies or mid-infrared dust continuum from active galactic nuclei (AGNs) at rest-frame wavelengths ~8 μm. A total of 301 spectra are analyzed for which IRS and IRAS include the same unresolved source, as measured by the ratio f ν(IRAS 25 μm)/f ν(IRS 25 μm). Sources have 0.004 < z < 0.34 and 42.5 < log L IR < 46.8 (erg s–1) and cover the full range of starburst galaxy and AGN classifications. Individual spectra are provided electronically, but averages and dispersions are presented. We find that log [L IR/νL ν(7.7 μm)] = 0.74 ± 0.18 in starbursts, log [L IR/νL ν(7.7 μm)] = 0.96 ± 0.26 in composite sources (starburst plus AGN), log [L IR/νL ν(7.9 μm)] = 0.80 ± 0.25 in AGNs with silicate absorption, and that log [L IR/νL ν(7.9 μm)] = 0.51 ± 0.21 in AGNs with silicate emission. L IR for the most luminous absorption and emission AGNs are similar and 2.5 times larger than for the most luminous starbursts. AGNs have systematically flatter SEDs than starbursts or composites, but their dispersion in SEDs overlaps starbursts. Sources with the strongest far-infrared luminosity from cool dust components are composite sources, indicating that these sources may contain the most obscured starbursts.

Star formation rates from [C II] 158 μm and mid-infrared emission lines for starbursts and active galactic nuclei

A summary is presented for 130 galaxies observed with the Herschel Photodetector Array Camera and Spectrometer instrument to measure fluxes for the [C II] 158 ?m emission line. Sources cover a wide range of active galactic nucleus to starburst classifications, as derived from polycyclic aromatic hydrocarbon strength measured with the Spitzer Infrared Spectrograph. Redshifts from [C II] and line to continuum strengths (equivalent width (EW) of [C II]) are given for the full sample, which includes 18 new [C II] flux measures. Calibration of L([C II)]) as a star formation rate (SFR) indicator is determined by comparing [C II] luminosities with mid-infrared [Ne II] and [Ne III] emission line luminosities; this gives the same result as determining SFR using bolometric luminosities of reradiating dust from starbursts: log SFR = log L([C II)]) ? 7.0, for SFR in M ??yr?1 and L([C II]) in L ?. We conclude that L([C II]) can be used to measure SFR in any source to a precision of ~50%, even if total source luminosities are dominated by an active galactic nucleus (AGN) component. The line to continuum ratio at 158 ?m, EW([C II]), is not significantly greater for starbursts (median EW([C II]) = 1.0 ?m) compared to composites and AGNs (median EW([C II]) = 0.7 ?m), showing that the far-infrared continuum at 158 ?m scales with [C II] regardless of classification. This indicates that the continuum at 158 ?m also arises primarily from the starburst component within any source, giving log SFR = log ?L ?(158 ?m) ? 42.8 for SFR in M ??yr?1 and ?L ?(158 ?m) in?erg?s?1.

The digitized first Byurakan survey - DFBS

Context. The First Byurakan Survey (FBS), also known as the Markarian Survey, is the largest low resolution spectroscopic survey of the sky and led to the discovery of 1500 UV-excess (UVX) galaxies and starburst galaxies. The FBS plates have also been used to search for UVX stellar objects, late-type stars, and for the identification of unusual infrared sources. Aims. The Digitized First Byurakan Survey (DFBS) provides the astronomical community with a digitized version of the FBS images and with the extracted spectra for the objects present in the plates. Methods. Nearly 2000 plates have been scanned and stored and programs were developed to compute the astrometric solution, extract the spectra, and apply wavelength and photometric calibration for the objects present in the plates. Results. The DFBS database and catalog of objects has been assembled. The DFBS database contains data for 20 000 000 objects present in the survey and provides tools for accessing the DFBS. Conclusions. New scientific projects as well as existing surveys will benefit by the digitized images and the ready-to-use extracted spectra which will allow an efficient computer-based analysis of the dataset.

The Most Luminous Starbursts in the Universe

A summary of starburst luminosities based on polycyclic aromatic hydrocarbon (PAH) features is given for 243 starburst galaxies with 0 < z < 2.5, observed with the Spitzer Infrared Spectrograph. Luminosity ν Lν(7.7 μm) for the peak luminosity of the 7.7 μm PAH emission feature is found to scale as log [ ν Lν(7.7 μm) ] = 44.63(± 0.09) + 2.48(± 0.28) log (1 + z) for the most luminous starbursts observed. Empirical calibrations of ν Lν(7.7 μm) are used to determine bolometric luminosity LIR and the star formation rate (SFR) for these starbursts. The most luminous starbursts found in this sample have log LIR = 45.4(± 0.3) + 2.5(± 0.3) log (1 + z) , in ergs s−1, and the maximum star formation rates for starbursts in units of M☉ yr−1 are log (SFR) = 2.1(± 0.3) + 2.5(± 0.3) log (1 + z) , up to z = 2.5. The exponent for pure luminosity evolution agrees with optical and radio studies of starbursts but is flatter than previous results based in infrared source counts. The maximum star formation rates are similar to the maxima determined for submillimeter galaxies; the most luminous individual starburst included within the sample has log LIR = 46.9, which gives an SFR = 3.4 × 103 M☉ yr−1.

Infrared Classification and Luminosities for Dusty Active Galactic Nuclei and the Most Luminous Quasars

Mid-infrared spectroscopic measurements from the Infrared Spectrometer (IRS) on Spitzer are given for 125 hard X-ray active galactic nuclei (AGNs; 14-195?keV) from the Swift Burst Alert Telescope (BAT) sample and for 32 AGNs with black hole masses (BHMs) from reverberation mapping. The 9.7 ?m silicate feature in emission or absorption defines an infrared AGN classification describing whether AGNs are observed through dust clouds, indicating that 55% of the BAT AGNs are observed through dust. The mid-infrared dust continuum luminosity is shown to be an excellent indicator of intrinsic AGN luminosity, scaling closely with the hard X-ray luminosity, log ?L ?(7.8 ?m)/L(X) = ?0.31 ? 0.35, and independent of classification determined from silicate emission or absorption. Dust luminosity scales closely with BHM, log ?L ?(7.8 ?m) = (37.2 ? 0.5) + 0.87 log BHM for luminosity in erg?s?1 and BHM in M ?. The 100 most luminous type 1 quasars as measured in ?L ?(7.8 ?m) are found by comparing Sloan Digital Sky Survey (SDSS) optically discovered quasars with photometry at 22 ?m from the Wide-Field Infrared Survey Explorer (WISE), scaled to rest frame 7.8 ?m using an empirical template determined from IRS spectra. The most luminous SDSS/WISE quasars have the same maximum infrared luminosities for all 1.5?< z?< 5, reaching total infrared luminosity L IR = 1014.4 L ?. Comparing with dust-obscured galaxies from Spitzer and WISE surveys, we find no evidence of hyperluminous obscured quasars whose maximum infrared luminosities exceed the maximum infrared luminosities of optically discovered quasars. Bolometric luminosities L bol estimated from rest-frame optical or ultraviolet luminosities are compared to L IR. For the local AGN, the median log L IR/L bol = ?0.35, consistent with a covering factor of 45% for the absorbing dust clouds. For the SDSS/WISE quasars, the median log L IR/L bol?= 0.1, with extremes indicating that ultraviolet-derived L bol can be seriously underestimated even for type 1 quasars.