arXiv subjects

G. H. Rieke

Publications and source records attributed to G. H. Rieke.

At least 163 records · Page 9Linked to original sources

arXiv2004

Spitzer observations of MAMBO galaxies: weeding out active nuclei in starbursting proto-ellipticals

We present Spitzer observations in five wavebands between 3.6 and 24um of an unbiased sample of 9 luminous, dusty galaxies selected at 1200um by the MAMBO camera on the IRAM 30-m telescope, a population akin to the well-known submm or `SCUBA' galaxies (hereafter SMGs). Owing to the coarse resolution of submm/mm instrumentation, SMGs have traditionally been difficult to identify at other wavelengths. We compare our multi-wavelength catalogs to show that the overlap between 24 and 1200um must be close to complete at these flux levels. We find that all (4/4) of the most secure >=4sigma SMGs have robust >=4sigma counterparts at 1.4GHz, while the fraction drops to 7/9 using all >=3sigma SMGs. We show that combining mid-IR and marginal (>=3sigma) radio detections provides plausible identifications in the remaining cases, enabling us to identify the complete sample. Accretion onto an obscured central engine is betrayed by the shape of the mid-IR continuum emission for several sources, confirming Spitzer's potential to weed out active galaxies. We demonstrate the power of a S(24um)/S(8um) vs S(8um)/S(4.5um) color-color plot as a diagnostic for this purpose. However, we conclude that the majority (~75%) of SMGs have rest-frame mid-/far-IR SEDs commensurate with obscured starbursts. Sensitive 24-um observations are clearly a useful route to identify and characterize reliable counterparts to high-redshift far-IR-bright galaxies, complementing what is possible via deep radio imaging.

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arXiv2004

The 24-Micron View of Embedded Star Formation in NGC 7129

We present observations of the star formation region NGC 7129 taken with the Multiband Imaging Photometer for Spitzer (MIPS). A significant population of sources, likely pre-main sequence members of the young stellar cluster, is revealed outside the central photoionization region. Combining with Infrared Array Camera (IRAC) and ground-based near-infrared images, we have obtained colors and spectral energy distributions for some 60 objects. The [3.6]-[4.5] vs. [8]-[24] color-color plane shows sources clustered at several different loci, which roughly correspond to the archetypal evolutionary sequence Class 0, I, II, and III. We obtain preliminary classifications for 36 objects, and find significant numbers of both Class I and II objects. Most of the pre-main sequence candidates are associated with the densest part of the molecular cloud surrounding the photoionization region, indicating active star formation over a broad area outside the central cluster. We discuss three Class II candidates that exhibit evidence of inner disk clearing, which would be some of the youngest known examples of a transition from accretion to optically thin quiescent disks.

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arXiv2004

Spatially Resolved Ultraviolet, H-alpha, Infrared, and Radio Star Formation in M81

We present Multiband Imaging Photometer for Spitzer (MIPS) observations of M81 at 24, 70, and 160 micron. The grand design nature of M81 is clearly seen, showing two well resolved spiral arms containing numerous bright star forming regions. The MIPS images reveal a significant amount of cold dust associated with the spiral arms. We investigate the variation of the ultraviolet (UV), H-alpha, and infrared (IR) luminosities and star formation rate (SFR) indicators across the face of M81 using the MIPS images and archival UV and H-alpha images. For regions in M81, we find that UV and H-alpha SFRs (uncorrected for dust attenuation) are always lower than the IR SFR. The cause of this behavior is dust attenuation and/or using SFR calibrations appropriate for entire galaxies, not regions in galaxies. The characteristics of the dust attenuation for the regions indicate the dust grains and/or geometry are different from those in starburst galaxies. The behavior of the infrared-radio correlation in M81 is seen to vary from the global average, with variations correlated with the morphology of M81.

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arXiv2004

IRAC Imaging of Lockman Hole

IRAC imaging of a 4'7x4'7 area in the Lockman Hole detected over 400 galaxies in the IRAC 3.6 micron and 4.5 micron bands, 120 in the 5.8 micron, and 80 in the 8 micron bandin 30 minutes of observing time. Color-color diagrams suggest that about half of these galaxies are at redshifts 0.6 1.3). We also detect IRAC counterparts for 6 of the 7 SCUBA sources and all 9 XMM sources in this area. The detection of the counterparts of the SCUBA sources and galaxies at z>1.3 demonstrates the ability of IRAC to probe the universe at very high redshifts.

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arXiv2004

24 micron Properties of X-ray Selected AGN

We examine the 24 micron to X-ray color of 157 X-ray--selected AGN as a function of X-ray obscuration and optical classification in the Chandra Deep Field South. The sample consists of the Chandra hard--band detections with hard flux >10^-15 erg/s/cm^2. A deep 24 micron mosaic obtained with Spitzer provides mid--infrared fluxes for the sample. Since obscured AGN locally have higher 24 micron / 2-8 keV ratios than unobscured AGN, and since X-ray background models predict a large population of obscured AGN, we expect to find many X-ray--hard, IR--bright AGN. Instead, we find that the 24 micron to X-ray flux ratio does not depend on X-ray hardness in the full sample, nor does it differ between narrow--line and broad--line AGN. We identify 5 nearly Compton--thick AGN, and find they have similar 24 micron to X-ray flux ratios compared to the full sample. We consider AGN in the narrow redshift spikes at z~0.7; for these AGN, there is some evidence that the flux ratio increases with X-ray hardness. The redshift slice also shows an odd trend, which is also prominent in the full sample: a group of X-ray--hard AGN with very low 24 micron to X-ray flux ratios.

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arXiv2004

PAHs contribution to the infrared output energy of the Universe at z~2

We present an updated phenomenological galaxy evolution model to fit the Spitzer 24, 70 and 160 microns number counts as well as all the previous mid and far infrared observations. Only a minor change of the co-moving luminosity density distribution in the previous model (Lagache, Dole, Puget 2003), combined with a slight modification of the starburst template spectra mainly between 12 and 30 microns, are required to fit all the data available. We show that the peak in the MIPS 24 micron counts is dominated by galaxies with redshift between 1 and 2, with a non negligible contribution from the z>2 galaxies (~30% at S=0.2 mJy). The very close agreement between the model and number counts at 15 and 24 microns strikingly implies that (1) the PAHs (Policyclic Aromatic Hydrocarbons) features remain prominent in the redshift band 0.5 to 2.5 and (2) the IR energy output has to be dominated by ~3 10^11 Lo to ~3 10^12 Lo galaxies from redshift 0.5 to 2.5. Combining Spitzer with the Infrared Space Observatory (ISO) deep cosmological surveys gives for the first time an unbiased view of the infrared Universe from z=0 to z=2.5.

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arXiv2004

Confusion in the infrared: Spitzer and beyond

We use the source counts measured with the Multiband Imaging Photometer for Spitzer (MIPS) at 24, 70, and 160 microns to determine the 5-sigma confusion limits due to extragalactic sources: 56 micro-Jy, 3.2 and 40 mJy at 24, 70 and 160 microns, respectively. We also make predictions for confusion limits for a number of proposed far infrared missions of larger aperture (3.5 to 10m diameter).

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arXiv2004

Sub-millimeter detections of Spitzer Space Telescope galaxy populations

We present sub-millimeter statistical detections of galaxies discovered in the 5'x5' Spitzer Early Release Observations (to 4-15 microJy 5 sigma at 3.6-8 microns, 170 microJy at 24 microns) through a stacking analysis of our reanalysed SCUBA 8mJy survey maps, and a Spitzer identification of a new sub-millimeter point source in the 8mJy survey region. For sources detected at 5.8 or 8 microns (154 and 111 sources respectively), we detect positive skews in the sub-millimeter flux distributions at 99.2-99.8% confidence using Kolmogorov-Smirnov tests, at both 850 microns and 450 microns. We also marginally detect the Spitzer 24 micron galaxies at 850 microns at 97% confidence, and place limits on the mean sub-millimeter fluxes of the 3.6 and 4.5 micron sources. Integrating the sub-millimeter fluxes of the Spitzer populations, we find the 5.8 micron galaxies contribute 0.12 +/- 0.05 nW/m^2/sr to the 850 micron background, and 2.4 +/- 0.7 nW/m^2/sr to the 450 micron background; similar contributions are made by the 8 micron-selected sample. We infer that the populations dominating the 5.8 and 8 micron extragalactic background light also contribute around a quarter of the 850 micron background and the majority of the 450 micron background.

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arXiv2004

The 24 Micron Source Counts in Deep Spitzer Surveys

Galaxy source counts in the infrared provide strong constraints on the evolution of the bolometric energy output from distant galaxy populations. We present the results from deep 24 micron imaging from Spitzer surveys, which include approximately 50,000 sources to an 80% completeness of 60 uJy. The 24 micron counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 - 4 mJy, and converge below 0.3 mJy. The 24 micron counts exceed expectations from non-evolving models by a factor >10 at 0.1 mJy. The peak in the differential number counts corresponds to a population of faint sources that is not expected from predictions based on 15 micron counts from ISO. We argue that this implies the existence of a previously undetected population of infrared-luminous galaxies at z ~ 1-3. Integrating the counts to 60 uJy, we derive a lower limit on the 24 micron background intensity of 1.9 +/- 0.6 nW m-2 sr-1 of which the majority (~ 60%) stems from sources fainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60 uJy contribute 0.8 {+0.9/-0.4} nW m-2 sr-1 to the background, which provides an estimate of the total 24 micron background of 2.7 {+1.1/-0.7} nW m-2 sr-1.

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arXiv2004

Extremely Red Objects in The Lockman Hole

We investigate Extremely Red Objects (EROs) using near- and mid-infrared observations in five passbands (3.6 to 24 micron) obtained from the Spitzer Space Telescope, and deep ground-based R and K imaging. The great sensitivity of the IRAC camera allows us to detect 64 EROs in only 12 minutes of IRAC exposure time, by means of an R-[3.6] color cut (analogous to the traditional red R-K cut). A pure infrared K-[3.6] red cut detects a somewhat different population and may be more effective at selecting z > 1.3 EROs. We find 17% of all galaxies detected by IRAC at 3.6 or 4.5 micron to be EROs. These percentages rise to about 40% at 5.8 micron, and about 60% at 8.0 micron. We utilize the spectral bump at 1.6 micron to divide the EROs into broad redshift slices using only near-infrared colors (2.2/3.6/4.5 micron). We conclude that two-thirds of all EROs lie at redshift z > 1.3. Detections at 24 micron imply that at least 11% of 0.6 < z < 1.3 EROs and at least 22% of z > 1.3 EROs are dusty star-forming galaxies.

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arXiv2004

Missing Massive Stars in Starbursts: Stellar Temperature Diagnostics and the IMF

Determining the properties of starbursts requires spectral diagnostics of their ultraviolet radiation fields, to test whether very massive stars are present. We test several such diagnostics, using new models of line ratio behavior combining Cloudy, Starburst99 and up-to-date spectral atlases. For six galaxies we obtain new measurements of HeI 1.7 um / Brackett 10, a difficult to measure but physically simple (and therefore reliable) diagnostic. We obtain new measurements of HeI 2.06 um / Brackett gamma in five galaxies, and find that it and [OIII]/Hbeta are generally unreliable diagnostics in starbursts. The heteronuclear and homonuclear mid--infrared line ratios (notably [NeIII] 15.6 um / [NeII] 12.8 um) consistently agree with each other and with HeI 1.7 um / Br10; this argues that the mid-infrared line ratios are reliable diagnostics of spectral hardness. In a sample of 27 starbursts, [NeIII]/[NeII] ratios are significantly less excited than model predictions for a Salpeter IMF extending to 100 solar masses. Plausible model alterations strengthen this conclusion. By contrast, the low-mass and low-metallicity galaxies II Zw 40 and NGC 5253 show relatively high neon line ratios, compatible with a Salpeter slope extending to at least 40--60 Msol. One solution for the low neon line ratios in the high--metallicity starbursts would be that they are deficient in >40 Msol stars compared to a Salpeter IMF. An alternative explanation, which we prefer, is that massive stars in high--metallicity starbursts spend much of their lives embedded within ultra--compact HII regions that prevent the near- and mid-infrared nebular lines from forming and escaping. This hypothesis has important consequences for starburst modelling and interpretation.

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arXiv2003

Diffuse X-ray emission in spiral galaxies

We compare the soft diffuse X-ray emission from Chandra images of 12 nearby intermediate inclination spiral galaxies to the morphology seen in Halpha, molecular gas, and mid-infrared emission. We find that diffuse X-ray emission is often located along spiral arms in the outer parts of spiral galaxies but tends to be distributed in a rounder morphology in the center. The X-ray morphology in the spiral arms matches that seen in the mid-infrared or Halpha and so implies that the X-ray emission is associated with recent active star formation. We see no strong evidence for X-ray emission trailing the location of high mass star formation in spiral arms. However, population synthesis models predict a high mechanical energy output rate from supernovae for a time period that is about 10 times longer than the lifetime of massive ionizing stars, conflicting with the narrow appearance of the arms in X-rays. The fraction of supernova energy that goes into heating the ISM must depend on environment and is probably higher near sites of active star formation. The X-ray estimated emission measures suggest that the volume filling factors and scale heights are high in the galaxy centers but low in the outer parts of these galaxies. The differences between the X-ray properties and morphology in the centers and outer parts of these galaxies suggest that galactic fountains operate in outer galaxy disks but that winds are primarily driven from galaxy centers.

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arXiv2003

SINGS: The SIRTF Nearby Galaxies Survey

The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies, and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multi-wavelength library of ancillary and complementary observations, including radio continuum, HI, CO, submillimeter, BVRIJHK, H-alpha, Paschen-alpha, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

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arXiv2003

A Census of the Young Cluster IC 348

We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42'x28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015 M_sun by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A_V<=4) for a 16'x14' field centered on the cluster, we construct an IMF that is unbiased in mass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and then declines into the substellar regime. In comparison, the similarly-derived IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.

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arXiv2002

Massive star formation in Luminous Infrared Galaxies

We present HST/NICMOS observations of a sample of LIRGs. We show that active star formation appears to be occurring not only in the bright nuclei of these galaxies, but also in luminous super-star clusters and giant HII regions with ages of up to 20-40Myr. This population of bright clusters and HII regions is unprecedented in normal galaxies and emphasizes the effects of the extreme star formation in LIRGs.

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arXiv2000

NGC1614: A Laboratory for Starburst Evolution

The modest extinction and reasonably face-on viewing geometry make the luminous IR galaxy NGC1614 an ideal laboratory for study of a powerful starburst. HST/NICMOS observations show: 1.) deep CO stellar absorption, tracing a starburst nucleus about 45 pc in diameter; 2.) surrounded by a ~600 pc diameter ring of supergiant HII regions revealed in Pa$α$ line emission; 3.) lying within a molecular ring indicated by its extinction shadow in H-K; 4.) all at the center of a disturbed spiral galaxy. The luminosities of the giant HII regions in the ring are extremely high, an order of magnitude brighter than 30 Doradus. The relation of deep stellar CO bands to surrounding ionized gas ring to molecular gas indicates that the luminous starburst started in the nucleus and is propagating outward into the surrounding molecular ring. This hypothesis is supported by evolutionary starburst modeling that shows that the properties of NGC1614 can be fitted with two short-lived bursts of star formation separated by 5 Myr. The total dynamical mass of the starburst region of 1.3x10^9 Msol is mostly accounted for by the old pre-star burst stellar population. Although our starburst models use a modified Salpeter IMF, the tight mass budget suggests that the IMF may contain relatively more 10-30 Msol stars and fewer low mass stars than the Salpeter function. The dynamical mass is nearly 4 times smaller than the mass of molecular gas estimated from the standard ratio of 12CO(1-0) to H2. A number of arguments place the mass of gas in the starburst region at ~25% of the dynamical mass, nominally about 1/15 and with an upper limit of 1/10 of the amount estimated from 12CO and the standard ratio. (Abridged)

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arXiv2000

The Multitude of Unresolved Continuum Sources at 1.6 microns in Hubble Space Telescope images of Seyfert Galaxies

We examine 112 Seyfert galaxies observed by the Hubble Space Telescope (HST) at 1.6 microns. We find that ~50% of the Seyfert 2.0 galaxies which are part of the Revised Shapeley-Ames (RSA) Catalog or the CfA redshift sample contain unresolved continuum sources at 1.6 microns. All but a couple of the Seyfert 1.0-1.9 galaxies display unresolved continuum sources. The unresolved sources have fluxes of order a mJy, near-infrared luminosities of order 10^41 erg/s and absolute magnitudes M_H ~-16. Comparison non-Seyfert galaxies from the RSA Catalog display significantly fewer (~20%), somewhat lower luminosity nuclear sources, which could be due to compact star clusters. We find that the luminosities of the unresolved Seyfert 1.0-1.9 sources at 1.6 microns are correlated with [OIII] 5007A and hard X-ray luminosities, implying that these sources are non-stellar. Assuming a spectral energy distribution similar to that of a Seyfert 2 galaxy, we estimate that a few percent of local spiral galaxies contain black holes emitting as Seyferts at a moderate fraction, 10^-1 to 10^-4, of their Eddington luminosities. With increasing Seyfert type the fraction of unresolved sources detected at 1.6 microns and the ratio of 1.6 microns to [OIII] fluxes tend to decrease. These trends are consistent with the unification model for Seyfert 1 and 2 galaxies.

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arXiv2000

The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters

We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R=800) and existing spectral types and photometry and the models of D'Antona & Mazzitelli, we find that the distributions of ages of comparable samples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of \~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass IMFs in these sites of clustered star formation are similar over a wide range of stellar densities and other environmental conditions. With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 Msun, and then rolls over into a power law that continues from about 1 Msun to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 Msun and probably, after a modest completeness correction, even from 0.01-0.02 Msun. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against various properties of the observed IMF.

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