arXiv subjects

Almudena Alonso-Herrero

Publications and source records attributed to Almudena Alonso-Herrero.

At least 109 records · Page 6Linked to original sources

arXiv2008

Typical AGN at intermediate redshifts

We study the host galaxies and black holes of typical X-ray selected AGN at intermediate redshifts (z~0.5-1.4). The AGN are selected such that their spectral energy distributions 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. AGN reside in the most massive galaxies at the redshifts probed here, with characteristic stellar masses that are intermediate between those of local type 2 AGN and high redshift (z~2) AGN. The inferred black hole masses of typical AGN are similar to those of optically identified quasars at similar redshifts. Since the AGN in our sample are much less luminous than quasars, typical AGN have low Eddington ratios. This suggests that, at least at intermediate redshifts, the cosmic AGN 'downsizing' is due to both a decrease in the characteristic stellar mass of the host galaxies, and less efficient accretion. Finally there is no strong evidence in AGN host galaxies for either highly suppressed star formation, expected if AGN played a role in quenching star formation, or elevated star formation when compared to mass selected galaxies of similar stellar masses and redshifts.

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arXiv2008

X-Ray Sources in the Star Forming Galaxies NGC 4194 and NGC 7541

We examine the X-ray point source population and 2-10 keV luminosity for two galaxies with high star formation rates (SFRs), NGC 4194 and NGC 7541. The X-ray point source luminosity function (XLF) for these two galaxies is consistent with the XLF found by Grimm et al. (2003) for a sample of star-forming galaxies. Combining our results with a sample of galaxies with SFRs above 1 solar mass/year, we find that the number of X-ray point sources above a luminosity of 2E38 erg/s is N = (1.8 +/- 0.4) SFR/(solar mass/year). This number is lower than previously inferred by Grimm et al. based on a sample of galaxies with lower SFRs. We find that the ratio of X-ray luminosity in the 2-10 keV band to SFR is L_X/(10E40 erg/s) = (0.37 +/- 0.08) SFR/(solar mass/year). This value may serve as a calibration in attempts to use X-ray luminosity to measure the SFR of galaxies at cosmological distances. The ratio of mass accreted onto compact objects versus mass used to form stars is near 1E-6. This ratio may be useful in constraining population synthesis models of X-ray binary formation in actively star forming systems.

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arXiv2008

Understanding the 8 micron vs. Pa-alpha relationship on sub-arcsecond scales in Luminous Infrared Galaxies

This work explores in detail the relation between the 8 micron and the Pa-alpha emissions for 122 HII regions identified in a sample of 10 low-z LIRGs with nearly constant metallicity (12 + log (O/H) ~ 8.8). We use Gemini/T-ReCS high-spatial resolution (<~ 0.4" ~ 120 pc for the average distance of 60 Mpc of our sample) mid-infrared imaging (at 8.7 micron or 10.3 micron) together with HST/NICMOS continuum and Pa-alpha images. The LIRG HII regions extend the L_8micron vs. L_Pa-alpha relation found for HII knots in the high-metallicity SINGS galaxies by about two orders of magnitude to higher luminosities. Since the metallicity of the LIRG sample is nearly constant, we can rule out this effect as a cause for the scatter seen in the relationship. In turn, it is attributed to two effects: age and PAH features. The L_8micron/L_Pa-alpha ratio, which varies by a factor of ten for the LIRG HII regions, is reproduced by a model with instantaneous star formation and ages ranging from ~ 4 to 7.5 Myr. The remaining dispersion around the model predictions for a given age is probably due to differential contributions of the PAH features (the 8.6 micron, in our case) to the 8 micron emission from galaxy to galaxy.

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arXiv2007

VLT-VIMOS integral field spectroscopy of luminous and ultraluminous infrared galaxies. I. The sample and first results

(Ultra)Luminous Infrared Galaxies [(U)LIRGs] are much more numerous at cosmological distances than locally, and are likely precursors of elliptical galaxies. Our goal is to carry out an integral field spectroscopic survey of a representative sample of low redshift (z < 0.26) (U)LIRGs, covering different morphologies from spirals to mergers over the entire infrared luminosity range. The present study is based on optical IFS obtained with the VIMOS instrument on the VLT. The first results of this survey are presented here with the study of two galaxies representative of the two major morphological types observed in (U)LIRGs, interacting pairs and morphologically- regular, weakly-interacting spirals, respectively. We have found that IRAS 06076-2139 consists of two low-intermediate mass (0.15 and 0.4 m*) galaxies with relative velocities of ~ 550 km/s and, therefore, it is unlikely that they will merge. The VIMOS IFS has also discovered the presence of a rapidly expanding and rotating ring of gas in the southern galaxy (Gs), likely due to a nearly central head-on passage of an intruder about 140 million years ago. IRASF 12115-4656 is a spiral for which we have found a mass of 1.2 m*. The neutral gas (traced by the NaI doublet) and ionized gas components are spatially and kinematically decoupled. The analysis presented here illustrates the full potential of IFS in two important aspects: (i) the study of the kinematics and ionization structure of complex interacting/merging systems, and (2i) the study of the kinematics of the different gas phases, neutral (cool) and ionized (warm), traced by the NaD and Halpha lines, respectively. (abbreviated)

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arXiv2007

Spitzer IRS mapping of the central kpc of Centaurus A

We report on the results of infrared spectroscopic mapping observations carried out in the nuclear region of Centaurus A (NGC5128). The 500 pc bipolar dust shell discovered by Quillen et al.(2006) is even more clearly seen in the 11.3 micron dust emission feature than previous broad band imaging. The pure rotational lines of molecular hydrogen other than the S(0) line are detected above the dusty disk and associated with the oval dust shell. The molecular hydrogen transitions indicate the presence of warm gas at temperatures 250--720K. The ratio of the dust emission features at 7.7 and 11.3 micron and the ratio of the [NeII](12.8) and 11.3 dust emission feature are lower in the 500 pc dust shell than in the star forming disk. The clearer shell morphology at 11.3 micron, warm molecular hydrogen emission in the shell, and variation in line ratios in the shell compared to those in the disk, confirm spectroscopically that this shell is a separate coherent entity and is unlikely to be a chance superposition of dust filaments. The physical conditions in the shell are most similar to Galactic supernova remnants where blast waves encounter molecular clouds. The lines requiring the highest level of ionization, [NeV] and [OIV], are detected 20--25 arcsec north-east and south-west of the nucleus and at position angles near the radio jet axis. Fine structure line ratios and limits from this region suggest that the medium is low density and illuminated by a hard radiation field at low ionization parameter. These higher S molecular hydrogen pure rotational transitions are also particularly bright in the same region as the [OIV] and [NeV] emission. This suggests that the gas associated with the dust shell has been excited near the jet axis and is part of an ionization cone.

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arXiv2007

Supermassive black holes in the Sbc spiral galaxies NGC 3310, NGC 4303 and NGC 4258

We present new Space Telescope Imaging Spectrograph (STIS) observations of three spiral galaxies, NGC 4303, NGC 3310 and NGC 4258. The bright optical emission lines H$α$ $λ$ $6564 Å$, [NII] $λ$$λ$ $6549,6585 Å$ and [SII] $λ$$λ$ $ 6718,6732 Å$ were used to study the kinematics of the ionized gas in the nuclear region of each galaxy with a $\sim 0.07\arcsec$ spatial resolution. In NGC 3310, the observed gas kinematics is well matched by a circularly rotating disk model but we are only able to set an upper limit to the BH mass which, taking into account the allowed disk inclinations, varies in the range $5.0 \times 10^{6} - 4.2 \times 10^{7} M_{\odot}$ at the 95% confidence level. In NGC 4303 the kinematical data require the presence of a BH with mass $M_{BH}=(5.0)^{+0.87}_{-2.26}\times 10^{6}M_{\odot}$ (for a disk inclination $i=70$ deg).In NGC 4258, the observed kinematics require the presence of a black hole with $M_{BH}= (7.9)^{+6.2}_{-3.5} \times 10^{7}M_{\odot}$ ($i=60$ deg). This result is in good agreement with the published value $(3.9 \pm 0.1) \times 10^{7} M_{\odot}$, derived from $H_{2}O$-maser observations. Our attempt at measuring BH masses in these 3 late type Sbc spiral galaxies has shown that these measurements are very challenging and at the limit of the highest spatial resolution currently available. Nonetheless our estimates are in good agreement with the scaling relations between black holes and their host spheroids suggesting that (i) they are reliable and (ii) black holes in spiral galaxies follows the same scaling relations as those in more massive early-type galaxies. A crucial test for the gas kinematical method, the correct recovery of the known BH mass in NGC 4258, has been successful. [abridged]

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arXiv2007

Resolving the Stellar Populations in the Circumnuclear Ring of NGC 7469

We investigate the stellar populations in the star forming ring of the luminous infrared galaxy NGC 7469. We use Hubble Space Telescope multi-wavelength (UV through NIR) imaging complemented with new K-band ground-based long-slit spectroscopy, and mid-IR and radio maps. Spectral energy distributions (SEDs) and evolutionary synthesis models have been used to characterize the star formation at different scales from those of individual star clusters (tens of pc) to that of the entire star-forming ring (kpc scale). At the smallest scales two different populations of massive (1-10 x 10^6 Msun) clusters are identified. About 25% of the clusters are young (1-3 Myr) and extincted (A_V ~ 3 mag), whereas the vast majority are of intermediate age (~9 to 20 Myr) and less obscured (A_V ~ 1 mag). At larger (hundreds of pc) scale, an analysis of the integrated SED and spectroscopic data of the ring indicates the presence of two stellar populations. The young (5-6 Myr) and obscured stellar population accounts for the Br_gamma emission and most of the IR luminosity, and about one-third of the stellar mass of the ring. The much less obscured intermediate-age population has properties similar to those of the majority of the (older) 1.1um-selected star clusters. These two populations are spatially anti-correlated. The UV-optical-NIR continuum (including the majority of the clusters) of the ring traces mostly the mildly obscured intermediate-age population, while the MIR and radio peaks mark the location of the youngest and obscured star-forming regions. This study emphasizes the need for multi-wavelength, high-angular resolution observations to characterize the star formation in the dust-obscured regions commonly present in LIRGs.

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arXiv2006

The mid-infrared emission of local Luminous Infrared Galaxies

In this paper we use the complementary imaging capabilities of Spitzer (sensitivity) and Gemini-South/T-ReCS (spatial resolution) to study the mid-IR properties of local (d < 75Mpc) LIRGs. The T-ReCS 8-10micron imaging observations of LIRGs have allowed us to spatially resolve the nuclear emission (star formation and/or AGN) and that of HII regions in the central 3-7kpc regions of LIRGs. From the comparison of the 8micron/Pa-alpha ratios of the integrated vs. resolved HII regions of LIRGs, we infer the existence of an 8micron diffuse component, not directly related to the ionizing stars, that can be as luminous as that from the resolved HII regions. We conclude that although the mid-IR integrated luminosity of galaxies undergoing dusty, intense star formation is a good indicator of the star formation rate (SFR), the empirical calibrations should be based on the integrated emission of nearby galaxies, not that of HII regions alone. To this end we provide a calibration of the SFR in terms of the integrated 24micron luminosity that can be used for distant dusty galaxies.

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arXiv2006

The quest for obscured AGN at cosmological distances: Infrared Power-Law Galaxies

We summarize multiwavelength properties of a sample of galaxies in the CDF-N and CDF-S whose Spectral Energy Distributions (SEDs) exhibit the characteristic power-law behavior expected for AGN in the Spitzer/IRAC 3.6-8micron bands. AGN selected this way tend to comprise the majority of high X-ray luminosity AGN, whereas AGN selected via other IRAC color-color criteria might contain more star-formation dominated galaxies. Approximately half of these IR power-law galaxies in the CDF-S are detected in deep (1Ms) Chandra X-ray imaging, although in the CDF-N (2Ms) about 77% are detected at the 3sigma level. The SEDs and X-ray upper limits of the sources not detected in X-rays are consistent with those of obscured AGN, and are significantly different from those of massive star-forming galaxies. About 40% of IR power-law galaxies detected in X-rays have SEDs resembling that of an optical QSO and morphologies dominated by bright point source emission. The remaining 60% have SEDs whose UV and optical continuum are much steeper (obscured) and more extended morphologies than those detected in X-rays. Most of the IR power-law galaxies not detected in X-rays have IR (8-1000micron above 10^12Lsun, and X-ray (upper limits) to mid-IR ratios similar to those of local warm (ie, hosting an AGN) ULIRGs. The SED shapes of power-law galaxies are consistent with the obscured fraction (4:1) as derived from the X-ray column densities, if we assume that all the sources not detected in X-rays are heavily absorbed. IR power-law galaxies may account for between 20% and 50% of the predicted number density of mid-IR detected obscured AGN. The remaining obscured AGN probably have rest-frame SEDs dominated by stellar emission.

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arXiv2006

Integral field spectroscopy of the Luminous Infrared Galaxy Arp299 (IC694+NGC3690)

The luminous infrared galaxy Arp299 (IC694+NGC3690) is studied using optical integral field spectroscopy obtained with the INTEGRAL system, together with archival Hubble Space Telescope WFPC2 and NICMOS images. The stellar and ionized gas morphology shows lambda-dependent variations due to the combined effects of the dust internal extinction, and the nature and spatial distribution of the different ionizing sources. The two-dimensional ionization maps have revealed an off-nuclear conical structure of about 4 kpc in length, characterized by high excitation conditions and a radial gradient in the gas electron density. The apex of this structure coincides with B1 region of NGC3690 which, in turn, presents Seyfert-like ionization, high extinction and a high velocity dispersion. These results strongly support the hypothesis that B1 is the true nucleus of NGC3690, where an AGN is located. In the circumnuclear regions HII-like ionization dominates, while LINER-like ionization is found elsewhere. The Halpha emitting sources with ages from 3.3 to 7.2x10^6 years, have masses of between 6 and 680x10^6 Msun and contribute (extinction corrected) about 45% to the bolometric luminosity. The ionized (Halpha) and neutral (NaD) gas velocity fields show similar structure on scales of several hundred to about 1 kpc, indicating that these gas components are kinematically coupled. The kinematic structure is complex and on scales of about 0.2 kpc does not appear to be dominated by the presence of ordered, rotational motions. The large velocity dispersion measured in NGC3690 indicates that this galaxy is the most massive of the system. The low velocity amplitude and dispersion of the interface suggest that the ionized gas is slowly rotating or in a close to quiescent phase.

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arXiv2005

Spitzer view on the evolution of star-forming galaxies from z=0 to z~3

We use a 24 micron selected sample containing more than 8,000 sources to study the evolution of star-forming galaxies in the redshift range from z=0 to z~3. We obtain photometric redshifts for most of the sources in our survey using a method based on empirically-built templates spanning from ultraviolet to mid-infrared wavelengths. The accuracy of these redshifts is better than 10% for 80% of the sample. The derived redshift distribution of the sources detected by our survey peaks at around z=0.6-1.0 (the location of the peak being affected by cosmic variance), and decays monotonically from z~1 to z~3. We have fitted infrared luminosity functions in several redshift bins in the range 0 1.e11 L_sun) to the total SFR density increases steadily from z~0 up to z~2.5, forming at least half of the newly-born stars by z~1.5. Ultraluminous infrared galaxies (L_TIR>1.e12 L_sun) play a rapidly increasing role for z>~1.3.

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arXiv2005

Metal Abundances of KISS Galaxies. IV. Galaxian Luminosity-Metallicity Relations in the Optical and Near-IR

We explore the galaxian luminosity-metallicity (L-Z) relationship in both the optical and the near-IR using a combination of optical photometric and spectroscopic observations from the KPNO International Spectroscopic Survey (KISS) and near-infrared photometry from the Two-micron All Sky Survey (2MASS). We supplement the 2MASS data with our own NIR photometry for a small number of lower-luminosity ELGs that are under-represented in the 2MASS database. Our B-band L-Z relationship includes 765 star-forming KISS galaxies with coarse abundance estimates from our follow-up spectra, while the correlation with KISS and 2MASS yields a total of 420 galaxies in our J-band L-Z relationship. We explore the effect that changing the correlation between the strong-line abundance diagnostic R_23 and metallicity has on the derived L-Z relation. We find that the slope of the L-Z relationship decreases as the wavelength of the luminosity bandpass increases. We interpret this as being, at least in part, an effect of internal absorption in the host galaxy. Furthermore, the dispersion in the L-Z relation decreases for the NIR bands, suggesting that variations in internal absorption contribute significantly to the observed scatter. We propose that our NIR L-Z relations are more fundamental than the B-band relation, since they are largely free of absorption effects and the NIR luminosities are more directly related to the stellar mass of the galaxy than are the optical luminosities.

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arXiv2004

Near-infrared K-band Spectroscopic Investigation of Seyfert 2 Nuclei in the CfA and 12 Micron Samples

We present near-infrared K-band slit spectra of the nuclei of 25 Seyfert 2 galaxies in the CfA and 12 micron samples. The strength of the CO absorption features at 2.3-2.4 micron produced by stars is measured in terms of a spectroscopic CO index. A clear anti-correlation between the observed CO index and the nuclear K-L color is present, suggesting that a featureless hot dust continuum heated by an AGN contributes significantly to the observed K-band fluxes in the nuclei of Seyfert 2 galaxies. After correction for this AGN contribution, we estimate nuclear stellar K-band luminosities for all sources, and CO indices for sources with modestly large observed CO indices. The corrected CO indices for 10 (=40%) Seyfert 2 nuclei are found to be as high as those observed in star-forming or elliptical (=spheroidal) galaxies. We combine the K-band data with measurements of the L-band 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission feature, another powerful indicator for star-formation, and find that the 3.3 micron PAH to K-band stellar luminosity ratios are substantially smaller than those of starburst galaxies. Our results suggest that the 3.3 micron PAH emission originates in the putative nuclear starbursts in the dusty tori surrounding the AGNs, because of its high surface brightness, whereas the K-band CO absorption features detected at the nuclei are dominated by old bulge (=spheroid) stars, and thus may not be a powerful indicator for the nuclear starbursts. We see no clear difference in the strength of the CO absorption and PAH emission features between the CfA and 12 micron Seyfert 2s.

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arXiv2004

Obscured star formation in the central region of the dwarf galaxy NGC5253

We present HST/NICMOS observations (1.1-2.2micron) and 1.9-4.1micron spectroscopy of the central region of the dwarf galaxy NGC5253. The HST/NICMOS observations reveal the presence of a nuclear double star cluster separated by 0.3-0.4arcsec or 6-8pc (for a distance d=4.1Mpc). The double star cluster, also a bright double source of Pa-alpha emission, appears to be coincident with the double radio nebula detected at 1.3cm. The eastern near-infrared star cluster (C1) is identified with the youngest optical cluster, whereas the western star cluster (C2), although it is almost completely obscured in the optical, becomes the brightest star cluster in the central region of NGC 5253 at wavelengths longer than 2micron. Both clusters are extremely young with ages of approximately 3.5 million years old. C2 is more massive than C1 by a factor of 6 to 20 (M(C2)= 7.7 x 10^5 - 2.6 x 10^6Msun, for a Salpeter IMF in the mass range 0.1-100Msun). Analysis of the circumnuclear spectrum excluding C1 and C2, as well as of a number of other near-infrared selected clusters with a range of (young) ages, suggests that the star formation was triggered across the central regions of the galaxy. We have also modelled the nuclear UV to mid-infrared spectral energy distribution (SED) of NGC5253 and found that the infrared part is well modelled with a highly obscured (A_V= 17mag) young starburst with a stellar mass consistent with our photometric estimates for C1 and C2. The SED model predicts a moderately bright polycyclic aromatic hydrocarbon (PAH) feature at 3.3micron that is not detected in our nuclear L-band spectrum. NGC5253's low metallicity and a top-heavy IMF likely combine to suppress the 3.3micron PAH emission that is commonly seen in more massive starburst systems.

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arXiv2003

A Medium-Resolution Near-Infrared Spectral Library of Late Type Stars: I

We present an empirical infrared spectral library of medium resolution (R~2000-3000) H (1.6 micron) and K (2.2 micron) band spectra of 218 red stars, spanning a range of [Fe/H] from ~-2.2 to ~+0.3. The sample includes Galactic disk stars, bulge stars from Baade's window, and red giants from Galactic globular clusters. We report the values of 19 indices covering 12 spectral features measured from the spectra in the library. Finally, we derive calibrations to estimate the effective temperature, and diagnostic relationships to determine the luminosity classes of individual stars from near-infrared spectra. This paper is part of a larger effort aimed at building a near-IR spectral library to be incorporated in population synthesis models, as well as, at testing synthetic stellar spectra.

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arXiv2003

Spectral Energy Distributions of Seyfert Nuclei

We present nuclear spectral energy distributions (SEDs) in the range 0.4-16micron for an expanded CfA sample of Seyfert galaxies. The spectral indices from 1-16micron range from alpha_IR 0.9 to 3.8. The shapes of the spectra are correlated with Seyfert type in the sense that steeper nuclear SEDs (nu*f_nu increasing with increasing wavelength) tend to be found in Seyfert 2s and flatter SEDs (nu *f_nu constant) in Seyfert 1-1.5s. The galaxies optically classified as Seyferts 1.8s and 1.9s display values of alpha_IR as in type 1 objects, or values intermediate between those of Seyfert 1s and Seyfert 2s. The intermediate SEDs of many Seyfert 1.8-1.9s may be consistent with the presence of a pure Seyfert 1 viewed through a moderate amount (A_V <5mag) of foreground galaxy extinction. We find, however, that between 10 and 20% of galaxies with broad optical line components have steep infrared SEDs. Torus models usually adopt high equatorial opacities to reproduce the infrared properties of Seyfert 1s and 2s, resulting in a dichotomy of infrared SEDs (flat for type 1s, and steep for type 2s). Such a dichotomy, however, is not observed in our sample. The wide range of spectral indices observed in the type 2 objects, the lack of extremely steep SEDs, and the large numbers of objects with intermediate spectral indices cannot be reconciled with predictions from existing optically thick torus models. We discuss possible modifications to improve torus models, including low optical depth tori, clumpy dusty tori, and high-optical-depth tori with an extended optically thin component.

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arXiv2002

The [FeII]1.644micron emission in M82 and NGC253: Is it a measure of the supernova rate?

We present HST/NICMOS [FeII]1.644micron, Pa-alpha (1.87micron) and continuum images of the starburst galaxies M82 and NGC253 at an unprecedented spatial resolution. In both galaxies we detect [FeII] compact sources superimposed on a diffuse background in the disk of the galaxies together with a component above and below the plane of the galaxy. The radio and [FeII] emissions perpendicular to the disk of M82 show a remarkable similarity to each other, suggesting that both emissions originate in shocks from supernova explosions. We find a spatial correspondence between bright compact [FeII] emitting regions and the location of radio supernova remnants (SNR) for approximately 30-50% of radio SNRs in M82 and NGC253. This lack of a one-to-one correspondence, more than being indicative of a different origin for the radio and [FeII] emission in starbursts, suggests two populations of SNRs: an older population(<10^4yr) traced by the [FeII] emission and a younger population (a few hundred years old) traced by the radio SNRs. We therefore conclude that the [FeII] emission in starburst galaxies provides a good estimate of the supernova activity. Using our newly determined [FeII] luminosities (corrected for extinction) of M82 and NGC253, we reevaluate the calibration of the supernova rate in terms of the [FeII] luminosity for starburst galaxies.

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arXiv2002

An Infrared View of Local Universe AGN

We summarize recent results on the infrared (IR) nuclear properties of Seyfert galaxies. 100% of all Seyfert 1-1.9$ and ~50% of all Seyfert 2s in the CfA and RSA samples observed with HST/NICMOS show nuclear point sources at 1.6micron. We find that the unresolved emission is variable in 9 out of the 14 Seyferts with duplicate observations, indicating a non-stellar origin. We have also put together non-stellar 0.4-16micron spectral energy distributions (SEDs) for the entire CfA sample of Seyfert galaxies. The shape of the SEDs for a complete sample of AGN can be used to constrain torus models.We report a good correlation between the IR and hard X-ray luminosities of local universe AGN suggesting that their mid-IR emission is approximately an isotropic property, and thus can be used as an indicator of the AGN luminosity. Finally we show that the hard X-ray and mid-IR luminosities appear to be related to the black hole mass in AGN.

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