arXiv subjects

D. Rigopoulou

Publications and source records attributed to D. Rigopoulou.

At least 145 records · Page 8Linked to original sources

arXiv2009

On the Stellar Masses of IRAC detected Lyman Break Galaxies at z~3

We present results of a large survey of the mid--IR properties of 248 Lyman Break Galaxies with confirmed spectroscopic redshift using deep Spitzer/IRAC observations in six cosmological fields. We model the Spectral Energy Distributions (SEDs) employing a revised version of the Bruzual and Charlot synthesis population code that incorporates a new treatment of the TP--AGB phase (CB07). Our primary aim is to investigate the impact of the AGB phase in the stellar masses of the LBGs, and compare our new results with previous stellar mass estimates. Based on the new CB07 code we find that the stellar masses of LBGs are smaller on average by a factor of ~1.4 compared to previous estimates. LBGs with 8um and/or 24um detections show higher masses (M~10^11 Mo) than LBGs faint in the IRAC bands (M~10^9 Mo). The ages of these massive LBGs are considerably higher than the rest of the population, indicating that they have been star-forming for at least ~1 Gyr. We also show how the addition of the IRAC bands, improves the accuracy of the estimated stellar masses and reduced the scatter on the derived M/L ratios. In particular, we present a tight correlation between the 8um IRAC band (rest-frame K for galaxies at z~3) and the stellar mass. We calculate the number density of massive (M > 10^11 Mo) LBGs and find it to be $Φ$= (1.12 $\pm$ 0.4) x 10^(-5) Mpc^(-3), ~1.5 times lower than that found by previous studies. Finally, based on UV-corrected SFRs we investigate the SFR-stellar mass correlation at z~3, find it similar to the one observed at other redshifts and show that our data place the peak of the evolution of the specific star formation rate at z~3.

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arXiv2009

Spectral Energy Distributions of type 2 QSOs: obscured star formation at high redshifts

We present new mid-infrared and submillimetre observations for a sample of eight high redshift type-2 QSOs located in the Chandra Deep Field South. The sources are X-ray absorbed with luminosities in excess of 10^44 erg/s. Two of the targets have robust detections, S/N > 4, while a further three targets are marginally detected with S/N > =2.5. All sources are detected in multiple mid-infrared bands with the Spitzer Space Telescope. The multiwavelength spectral energy distributions (SEDs) of the type-2 QSOs are compared to those of two local ultraluminous galaxies (Arp220 and IR22491) in order to assess contributions from a star-forming component in various parts of the SED. We suggest that their submillimetre emission is possibly due to a starburst while a large fraction of the mid-infrared energy is likely to originate in the obscured central quasar. Using the mid-infrared and submm observations we derive infrared luminosities which are found to be in excess of L>10^12Lsun. The submillimetre (850micron) to X-ray (2 keV) spectral indices (alpha_SX) span a wide range. About half of the type-2 QSOs have values typical for a Compton-thick AGN with only 1 per cent of the nuclear emission seen through scattering and, the remaining with values typical of submm-bright galaxies. Combining the available observational evidence we outline a possible scenario for the early stages of evolution of these sources.

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arXiv2009

IRS Spectroscopy and Multi-wavelength Study of Luminous Star-forming Galaxies at z~1.9

We analyze a sample of galaxies chosen to have F24>0.5 mJy and satisfy a certain IRAC color criterion. IRS spectra yield redshifts, spectral types, and PAH luminosities, to which we add multi-wavelength broadband photometry. Stellar population modeling and IRS spectra together demonstrate that the double criteria used to select this sample have efficiently isolated massive star-forming galaxies at z~1.9. This is the first starburst-dominated ULIRG sample at high redshift with total infrared luminosity measured directly from FIR and millimeter photometry, and as such gives us the first accurate view of broadband SEDs for starburst galaxies at extremely high luminosity and at all wavelengths. Similar broadband data are assembled for three other galaxy samples -- local starburst galaxies, local AGN/ULIRGS, and a second 24mu-luminous z~1.9 sample dominated by AGN. L(PAH)/L(IR) for the new z~1.9 starburst sample is the highest ever seen, some three times higher than in local starbursts, whereas in AGNs this ratio is depressed below the starburst trend, often severely. Several pieces of evidence imply that AGNs exist in this starburst dominated sample, except two of which even host very strong AGN, while they still have very strong PAH emission. The ACS images show most objects have very extended morphologies in the rest-frame UV band, thus extended distribution of PAH molecules. Such an extended distribution prevents further destruction PAH molecules by central AGNs. We conclude that objects in this sample are ULIRGs powered mainly by starburst; and the total infrared luminosity density contributed by this type of objects is 0.9-2.6x 10^7 Lsol/Mpc^3.

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arXiv2009

Mid-infrared spectroscopy of infrared-luminous galaxies at z~0.5-3

We present results on low-resolution mid-infrared (MIR) spectra of 70 infrared-luminous galaxies obtained with the Infrared Spectrograph (IRS) onboard Spitzer. We selected sources from the European Large Area Infrared Survey (ELAIS) with S15 > 0.8 mJy and photometric or spectroscopic z > 1. About half of the sample are QSOs in the optical, while the remaining sources are galaxies, comprising both obscured AGN and starbursts. We classify the spectra using well-known infrared diagnostics, as well as a new one that we propose, into three types of source: those dominated by an unobscured AGN (QSOs), obscured AGN, and starburst-dominated sources. Starbursts concentrate at z ~ 0.6-1.0 favored by the shift of the 7.7-micron PAH band into the selection 15 micron band, while AGN spread over the 0.5 < z < 3.1 range. Star formation rates (SFR) are estimated for individual sources from the luminosity of the PAH features. An estimate of the average PAH luminosity in QSOs and obscured AGN is obtained from the composite spectrum of all sources with reliable redshifts. The estimated mean SFR in the QSOs is 50-100 Mo yr^-1, but the implied FIR luminosity is 3-10 times lower than that obtained from stacking analysis of the FIR photometry, suggesting destruction of the PAH carriers by energetic photons from the AGN. The SFR estimated in obscured AGN is 2-3 times higher than in QSOs of similar MIR luminosity. This discrepancy might not be due to luminosity effects or selection bias alone, but could instead indicate a connection between obscuration and star formation. However, the observed correlation between silicate absorption and the slope of the near- to mid-infrared spectrum is compatible with the obscuration of the AGN emission in these sources being produced in a dust torus.

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arXiv2008

A Millimetre Survey of Starburst Dominated Ultraluminous Infrared Galaxies at z~2

We present millimetre observations of a sample of 12 high redshift ultraluminous infrared galaxies (ULIRGs) in the Extended Growth Strip (EGS). These objects were initially selected on the basis of their observed mid--IR colours (0.0 < [3.6]-[4.5] < 0.4 and -0.7 < [3.6]-[8.0] < 0.5) to lie at high redshift 1.5 < z < 3, and subsequent 20-38 micron mid-IR spectroscopy confirms that they lie in a narrow redshift window centered on z=2. We detect 9/12 of the objects in our sample at high significance (>3 sigma) with a mean 1200\micron flux of = 1.6+/-0.1 mJy. Our millimetre photometry, combined with existing far-IR photometry from the Far-IR Deep Extragalactic Legacy (FIDEL) Survey and accurate spectroscopic redshifts, places constraints both sides of the thermal dust peak. This allows us to estimate the dust properties, including the far--IR luminosity, dust temperature, and dust mass. We find that our sample is similar to other high-z and intermediate-z ULIRGs, and local systems, but has a different dust selection function than submillimeter-selected galaxies. Finally, we use existing 20cm radio continuum imaging to test the far-IR/radio correlation at high redshift. We find that our sample is consistent with the local relation, implying little evolution. Furthermore, this suggests that our sample selection method is efficient at identifying ultraluminous, starburst--dominated systems within a very narrow redshift range centered at z~2.

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arXiv2008

IRAC Photometric Analysis and the Mid-IR Photometric Properties of Lyman Break Galaxies

We present photometric analysis of deep mid-infrared observations obtained by Spitzer/IRAC covering the fields Q1422+2309, Q2233+1341, DSF2237a,b, HDFN, SSA22a,b and B20902+34, giving the number counts and the depths for each field. In a sample of 751 LBGs lying in those fields, 443, 448, 137 and 152 are identified at 3.6microns, 4.5microns, 5.8microns, 8.0microns IRAC bands respectively, expanding their spectral energy distribution to rest-near-infrared and revealing that LBGs display a variety of colours. Their rest-near-infrared properties are rather inhomogeneous, ranging from those that are bright in IRAC bands and exhibit [R]-[3.6] > 1.5 colours to those that are faint or not detected at all in IRAC bands with [R]-[3.6] < 1.5 colours and these two groups of LBGs are investigated. We compare the mid-IR colours of the LBGs with the colours of star-forming galaxies and we find that LBGs have colours consistent with star-foming galaxies at z~3. The properties of the LBGs detected in the 8microns IRAC band (rest frame K-band) are examined separately, showing that they exhibit redder [R]-[3.6] colours than the rest of the population and that IRAC 8microns band can be used as a diagnostic tool, to separate AGN dominated objects from normal star-forming galaxies at z~3

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arXiv2008

The properties of 70micron selected high-redshift galaxies in the Extended Groth Strip

We examine the infrared properties of 43 high redshift (0.1 35 and for a large fraction (~50 per cent) the SED turns over into the Rayleigh-Jeans regime at wavelengths longward of 90 microns. For comparison, we also fit semi-empirical templates based on local galaxy data, however, these underestimate the far-infrared SED shape by a factor of at least 2 and in extreme cases up to 10 for the majority (~70 per cent) of the sources. Further investigation of SED characteristics reveals that the mid-infrared (70/24 microns) continuum slope is decoupled from various galaxy properties such as the total infrared luminosity and far-infrared peak, quantified by the L_160/L_70 ratio. In view of these results, we propose that these high-redshift galaxies have different properties to their local counterparts, in the sense that large amounts of dust cause heavy obscuration and are responsible for an additional cold emissive component, appearing as a far-infrared excess in their SEDs.

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arXiv2007

The Host Galaxies and Black Holes of Typical z~0.5-1.4 AGN

We study the stellar and star formation (SF) properties of the host galaxies of 58 X-ray selected AGN in the GOODS portion of the Chandra Deep Field South (CDF-S) region at z~0.5-1.4. The AGN are selected such that their rest-frame UV to near-IR SEDs are dominated by stellar emission, i.e., they show a prominent 1.6micron bump, thus minimizing the AGN emission 'contamination'. This AGN population comprises approximately 50% of the X-ray selected AGN at these redshifts. Using models of stellar and dust emission we model their SEDs to derive stellar masses (M*) and total (UV+IR) star formation rates (SFR). AGN reside in the most massive galaxies at the redshifts probed here. Their characteristic stellar masses (M*~7.8x10^10 Msun and M*~1.2x10^11 Msun at median z of 0.67 and 1.07, respectively) appear to be representative of the X-ray selected AGN population at these redshifts, and are intermediate between those of local type 2 AGN and high redshift (z~2) AGN. The inferred black hole masses (MBH~ 2x10^8 Msun) of typical AGN are similar to those of optically identified quasars at similar redshifts. Since the AGN in our sample are much less luminous (L(2-10keV)<10^44 erg/s) than quasars, typical AGN have low Eddington ratios (eta~0.01-0.001). This suggests that, at least at intermediate z, the cosmic AGN 'downsizing' is due to both a decrease in the characteristic stellar mass of typical host galaxies, and less efficient accretion. Finally there is no strong evidence in AGN host galaxies for either highly suppressed SF (expected if AGN played a role in quenching SF) or elevated SF when compared to mass selected (i.e., IRAC-selected) galaxies of similar stellar masses and redshifts. This may be explained by the fact that galaxies with M*~5x 10^10 - 5x10^11 Msun are still being assembled at the redshifts probed here.

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arXiv2007

The Local Galaxy 8 micron Luminosity Function

A SST survey in the NOAO Deep-Wide Field in Boötes provides a complete, 8-micron-selected sample of galaxies to a limiting (Vega) magnitude of 13.5. In the 6.88 deg$^2$ field sampled, 79% of the 4867 galaxies have spectroscopic redshifts, allowing an accurate determination of the local (z<0.3) galaxy luminosity function. Stellar and dust emission can be separated on the basis of observed galaxy colors. Dust emission (mostly PAH) accounts for 80% of the 8 micron luminosity, stellar photospheres account for 19%, and AGN emission accounts for roughly 1 %. A sub-sample of the 8 micron-selected galaxies have blue, early-type colors, but even most of these have significant PAH emission. The luminosity functions for the total 8 micron luminosity and for the dust emission alone are both well fit by Schechter functions. For the 8 micron luminosity function, the characteristic luminosity is νL_ν^*(8.0 \micron) = 1.8 \times 10^{10}$ \Lsun while for the dust emission alone it is 1.6 x 10^{10}$ \Lsun\null. The average 8 \micron luminosity density at z<0.3 is 3.1 x 10^7 \Lsun Mpc^{-3}, and the average luminosity density from dust alone is 2.5 x 10^7 \Lsun Mpc^{-3}. This luminos ity arises predominantly from galaxies with 8 \micron luminosities ($νL_ν$) between $2\times 10^9$ and $2 x 10^{10}$ \Lsun, i.e., normal galaxies, not LIRGs or ULIRGs.

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arXiv2006

AEGIS: Infrared Spectral Energy Distributions of MIPS 70micron selected sources

We present 0.5 -160 micron Spectral Energy Distributions (SEDs) of galaxies, detected at 70microns with the Multiband Imaging Photometer for Spitzer (MIPS), using broadband imaging data from Spitzer and ground-based telescopes. Spectroscopic redshifts, in the range 0.2 0.5) and high redshift (0.5<z<1.2) bins. Using this unique ``far-infrared'' selected sample we derive an empirical mid to far-infrared relationship that can be used to estimate the infrared energy budget of galaxies in the high-redshift universe. Our sample can be used as a template to translate far-infrared luminosities into bolometric luminosities for high redshift objects.

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arXiv2006

A 15um selected sample of high-z starbursts and AGNs

We report results from a Spitzer GO-1 program of IRS spectroscopy of a large sample of Luminous Infrared Galaxies and quasars selected from the European Large Area ISO Survey (ELAIS) which have a wide multiwavelength coverage, including ISOCAM, ISOPHOT, IRAC and MIPS (from SWIRE), and optical photometry. We present the sample selection and results from the IRS spectroscopy.

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arXiv2006

AEGIS: Infrared Spectroscopy of An Infrared Luminous Lyman Break Galaxy at z=3.01

We report the detection of rest--frame 6.2 and 7.7 \micron emission features arising from Polycyclic Aromatic Hydrocarbons (PAH) in the Spitzer/IRS spectrum of an infrared-luminous Lyman break galaxy at z=3.01. This is currently the highest redshift galaxy where these PAH emission features have been detected. The total infrared luminosity inferred from the MIPS 24 \micron and radio flux density is 2$\times10^{13}$ L$_{\odot}$, which qualifies this object as a so--called hyperluminous infrared galaxy (HyLIRG). However, unlike local HyLIRGs which are generally associated with QSO/AGNs and have weak or absent PAH emission features, this HyLIRG has very strong 6.2 and 7.7 \micron PAH emission. We argue that intense star formation dominates the infrared emission of this source, although we cannot rule out the presence of a deeply obscured AGN. This LBG appears to be a distorted system in the HST ACS F606W and F814W images, possibly indicating that a significant merger or interaction is driving the large IR luminosity.

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arXiv2006

AEGIS: A Panchromatic Study of IRAC-selected Extremely Red Objects with Confirmed Spectroscopic Redshifts

We study 87 Extremely Red Objects (EROs), selected both to have color redder than R-[3.6] = 4.0, and to have confirmed spectroscopic redshifts. Together, these two constraints result in this sample populating a fairly narrow redshift range at 0.76 < z < 1.42. The key new ingredient included here is deep Spitzer Space Telescope InfraRed Array Camera (IRAC) data. Based on [3.6]-[8.0] color, we demonstrate that it is possible to classify EROs into early-type, dusty starburst, or power-law (AGN) types. We present ultraviolet to mid-infrared spectral energy distributions (SEDs) and Advanced Camera for Surveys (ACS) images, both of which support our simple IRAC color classification.

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arXiv2006

The All-wavelength Extended Groth Strip International Survey (AEGIS) Data Sets

In this the first of a series of Letters, we present a description of the panchromatic data sets that have been acquired in the Extended Groth Strip region of the sky. Our survey, the All-wavelength Extended Groth strip International Survey (AEGIS), is intended to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray (0.5 - 10 keV), GALEX ultraviolet (1200 - 2500 Angstrom), CFHT/MegaCam Legacy Survey optical (3600 - 9000 Angstroms), CFHT/CFH12K optical (4500 - 9000 Angstroms), Hubble Space Telescope/ACS optical (4400 - 8500 Angstroms), Palomar/WIRC near-infrared (1.2 - 2.2 microns), Spitzer/IRAC mid-infrared (3.6 - 8.0 microns), Spitzer/MIPS far-infrared (24 - 70 microns), and VLA radio continuum (6 - 20 cm). In addition, this region of the sky has been targeted for extensive spectroscopy using the DEIMOS spectrograph on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

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arXiv2006

SPITZER Observations of z~3 Lyman Break Galaxies: stellar masses and mid-infrared properties

We describe the spectral energy distributions (SEDs) of Lyman Break Galaxies (LBGs) at z~3 using deep mid-infrared and optical observations of the Extended Groth Strip, obtained with IRAC and MIPS on board Spitzer and from the ground, respectively. We focus on LBGs with detections at all four IRAC bands, in particular the 26 galaxies with IRAC 8 micron band (rest--frame K-band) detections. We use stellar population synthesis models and probe the stellar content of these galaxies. Based on best--fit continuous star-formation models we derive estimates of the stellar mass for these LBGs. As in previous studies, we find that a fraction of LBGs have very red colors and large estimated stellar masses (M_stellar > 5 x 10 ^{10} M_solar): the present Spitzer data allow us, for the first time, to study these massive LBGs in detail. We discuss the link between these LBGs and submm-luminous galaxies.We find that the number density of these massive LBGs at high redshift is higher than predicted by current semi-analytic models of galaxy evolution.

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arXiv2006

Mid-Infrared Identifications of SCUBA Galaxies in the CUDSS 14-Hour Field with the Spitzer Space Telescope

We identify 17 possible 8.0 micron-selected counterparts to the submillimeter galaxies in the CUDSS 14-hour field, derived from deep imaging carried out with the IRAC and MIPS instruments aboard the Spitzer Space Telescope. Ten of the 17 counterparts are not the same as those previously identified at shorter wavelengths. We argue that 8.0 micron selection offers a better means for identifying counterparts to submillimeter galaxies than near-infrared or optical selection. Based on the panchromatic SEDs, most counterparts appear to be powered by ongoing star formation. Power-law fits to the SEDs suggest that five objects in the 8.0 micron-selected counterpart sample harbor dominant AGNs; a sixth object is identified as a possible AGN. The 3.6 to 8.0 micron colors of the infrared-selected counterparts are significantly redder than the general IRAC galaxy population in the CUDSS 14-hour field.

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arXiv2005

Infrared power-law galaxies in the Chandra Deep Field South: AGN and ULIRGs

We investigate the nature of a sample of 92 Spitzer/MIPS 24 micron selected galaxies in the CDFS, showing power law-like emission in the Spitzer/IRAC 3.6-8 micron bands. The main goal is to determine whether the galaxies not detected in X-rays (47% of the sample) are part of the hypothetical population of obscured AGN not detected even in deep X-ray surveys. The majority of the IR power-law galaxies are ULIRGs at z>1, and those with LIRG-like IR luminosities are usually detected in X-rays. The optical to IR spectral energy distributions (SEDs) of the X-ray detected galaxies are almost equally divided between a BLAGN SED class (similar to an optically selected QSO) and a NLAGN SED (similar to the BLAGN SED but with an obscured UV/optical continuum). A small fraction of SEDs resemble warm ULIRG galaxies (e.g., Mrk231). Most galaxies not detected in X-rays have SEDs in the NLAGN+ULIRG class as they tend to be optically fainter, and possibly more obscured. Moreover, the IR power-law galaxies have SEDs significantly different from those of high-z (z_sp>1) IR (24 micron) selected and optically bright (VVDS I_AB<=24) star-forming galaxies whose SEDs show a very prominent stellar bump at 1.6 micron. The galaxies detected in X-rays have 2-8 keV rest-frame luminosities typical of AGN. The galaxies not detected in X-rays have global X-ray to mid-IR SED properties that make them good candidates to contain IR bright X-ray absorbed AGN. If all these sources are actually obscured AGN, we would observe a ratio of obscured to unobscured 24 micron detected AGN of 2:1, whereas models predict a ratio of up to 3:1. Additional studies using Spitzer to detect X-ray-quiet AGN are likely to find more such obscured sources.

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arXiv2005

Follow-Up Near-infrared Spectroscopy of Ultraluminous Infrared Galaxies observed by ISO

We present low resolution near-infrared spectroscopy of an unbiased sample of 24 ultraluminous infrared galaxies (ULIRGs), selected from samples previously observed spectroscopically in the mid-infrared with the Infrared Space Observatory (ISO). Qualitatively, the near-infrared spectra resemble those of starbursts. Only in one ULIRG, IRAS 04114-5117E, do we find spectroscopic evidence for AGN activity. The spectroscopic classification in the near-infrared is in very good agreement with the mid-infrared one. For a subset of our sample for which extinction corrections can be derived from Pa-alpha and Br-gamma, we find rather high Pa-alpha luminosities, in accordance with the powering source of these galaxies being star formation.[Fe] emission is strong in ULIRGs and may be linked to starburst and superwind activity. Additionally, our sample includes two unusual objects. The first, IRAS F00183-7111, exhibits extreme [Fe] emission and the second, IRAS F23578-5307, is according to our knowledge one of the most luminous infrared galaxies in H2 rotation-vibration emission.

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