arXiv subjects

Tommy Wiklind

Publications and source records attributed to Tommy Wiklind.

At least 37 records · Page 2Linked to original sources

arXiv2014

Properties of Submillimeter Galaxies in the CANDELS GOODS-S Field

We derive physical properties of 10 submillimeter galaxies located in the CANDELS coverage of the GOODS-S field. The galaxies were first identified as submillimeter sources with the LABOCA bolometer and subsequently targeted for 870um continuum observation with ALMA. The high angular resolution of the ALMA imaging allows secure counterparts to be identified in the CANDELS multiband dataset. The CANDELS data provide deep photometric data from UV through near-infrared wavelengths. Using synthetic spectral energy distributions, we derive photometric redshifts, stellar masses, extinction, ages, and the star formation history. The redshift range is z=1.65-4.76, with two of the galaxies located at z>4. Two SMG counterparts have stellar masses 2-3 orders of magnitude lower than the rest. The remaining SMG counterparts have stellar masses around 1x10^11 Msun. The stellar population in the SMGs is typically older than the expected duration of the submillimeter phase, suggesting that the star formation history of submillimeter galaxies is more complex than a single burst. Non-parametric morphology indices suggest that the SMG counterparts are among the most asymmetric systems compared with galaxies of the same stellar mass and redshift. The HST images shows that 3 of the SMGs are associated with on-going mergers. The remaining counterparts are isolated. Estimating the dust and molecular gas mass from the submm fluxes, and comparing with our stellar masses shows that the molecular gas mass fraction of SMGs is ~28% and that the final stellar mass is likely to be (1-2)x10^11 Msun.

astro-ph.GA↗

arXiv2013

The Progenitors of the Compact Early-Type Galaxies at High-Redshift

We use GOODS and CANDELS images to identify progenitors of massive (log M > 10 Msun) compact "early-type" galaxies (ETGs) at z~1.6. Since merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z~3 based on their mass, SFR and central stellar density and find that these account for a large fraction of, and possibly all, compact ETGs at z~1.6. We find that the average far-UV SED of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration, and consistent with more evolved (aging) star-formation. This is in line with other evidence that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended "halos" surrounding the compact "core", both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas-rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the HST images, with their stellar mass assembling in-situ, and that they have not experienced any major merging until the epoch of observations at z~1.6.

astro-ph.CO↗

arXiv2013

A Critical Assessment of Photometric Redshift Methods: A CANDELS Investigation

We present results from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) photometric redshift methods investigation. In this investigation, the results from eleven participants, each using a different combination of photometric redshift code, template spectral energy distributions (SEDs) and priors, are used to examine the properties of photometric redshifts applied to deep fields with broad-band multi-wavelength coverage. The photometry used includes U-band through mid-infrared filters and was derived using the TFIT method. Comparing the results, we find that there is no particular code or set of template SEDs that results in significantly better photometric redshifts compared to others. However, we find codes producing the lowest scatter and outlier fraction utilize a training sample to optimize photometric redshifts by adding zero-point offsets, template adjusting or adding extra smoothing errors. These results therefore stress the importance of the training procedure. We find a strong dependence of the photometric redshift accuracy on the signal-to-noise ratio of the photometry. On the other hand, we find a weak dependence of the photometric redshift scatter with redshift and galaxy color. We find that most photometric redshift codes quote redshift errors (e.g., 68% confidence intervals) that are too small compared to that expected from the spectroscopic control sample. We find that all codes show a statistically significant bias in the photometric redshifts. However, the bias is in all cases smaller than the scatter, the latter therefore dominates the errors. Finally, we find that combining results from multiple codes significantly decreases the photometric redshift scatter and outlier fraction. We discuss different ways of combining data to produce accurate photometric redshifts and error estimates.

astro-ph.CO↗

arXiv2013

Submillimeter ALMA Observations of the Dense Gas in the Low-Luminosity Type-1 Active Nucleus of NGC 1097

We present the first 100 pc scale view of the dense molecular gas in the central ~ 1.3 kpc region of the type-1 Seyfert NGC 1097 traced by HCN (J=4-3) and HCO+ (J=4-3) lines afforded with ALMA band 7. This galaxy shows significant HCN enhancement with respect to HCO+ and CO in the low-J transitions, which seems to be a common characteristic in AGN environments. Using the ALMA data, we study the characteristics of the dense gas around this AGN and search for the mechanism of HCN enhancement. We find a high HCN (J=4-3) to HCO+ (J=4-3) line ratio in the nucleus. The upper limit of the brightness temperature ratio of HCN (v2=1^{1f}, J=4-3) to HCN (J=4-3) is 0.08, which indicates that IR pumping does not significantly affect the pure rotational population in this nucleus. We also find a higher HCN (J=4-3) to CS (J=7-6) line ratio in NGC 1097 than in starburst galaxies, which is more than 12.7 on the brightness temperature scale. Combined from similar observations from other galaxies, we tentatively suggest that this ratio appears to be higher in AGN-host galaxies than in pure starburst ones similar to the widely used HCN to HCO+ ratio. LTE and non-LTE modeling of the observed HCN and HCO+ lines using J=4-3 and 1-0 data from ALMA, and J=3-2 data from SMA, reveals a high HCN to HCO+ abundance ratio (5 < [HCN]/[HCO+] < 20: non-LTE analysis) in the nucleus, and that the high-J lines (J=4-3 and 3-2) are emitted from dense (10^{4.5} < n_H2 [/cc] < 10^6), hot (70 < Tkin [K] < 550) regions. Finally we propose that the high temperature chemistry is more plausible to explain the observed enhanced HCN emission in NGC 1097 than the pure gas phase PDR/XDR chemistry.

astro-ph.GA↗

arXiv2013

ALMA follows streaming of dense gas down to 40 pc from the supermassive black hole in NGC1097

We present a kinematic analysis of the dense molecular gas in the central 200 parsecs of the nearby galaxy NGC1097, based on Cycle 0 observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA). We use the HCN(4-3) line to trace the densest interstellar molecular gas, and quantify its kinematics, and estimate an inflow rate for the molecular gas. We find a striking similarity between the ALMA kinematic data and the analytic spiral inflow model that we have previously constructed based on ionized gas velocity fields on larger scales. We are able to follow dense gas streaming down to 40 pc distance from the supermassive black hole in this Seyfert 1 galaxy. In order to fulfill marginal stability, we deduce that the dense gas is confined to a very thin disc, and we derive a dense gas inflow rate of 0.09 Msun/yr at 40 pc radius. Combined with previous values from the Ha and CO gas, we calculate a combined molecular and ionized gas inflow rate of 0.2 Msun/yr at 40 pc distance from the central supermassive black hole of NGC1097.

astro-ph.CO↗

arXiv2012

GOODS-Herschel and CANDELS: The Morphologies of Ultraluminous Infrared Galaxies at z~2

Using deep 100-160 micron observations in GOODS-S from the GOODS-H survey, combined with HST/WFC3 NIR imaging from CANDELS, we present the first morphological analysis of a complete, FIR selected sample of 52 ULIRGs at z~2. We also make use of a comparison sample of galaxies without Herschel detections but with the same z and magnitude distribution. Our visual classifications of these two samples indicate that the fraction of objects with disk and spheroid morphologies is roughly the same but that there are significantly more mergers, interactions, and irregular galaxies among the ULIRGs. The combination of disk and irregular/interacting morphologies suggests that early stage interactions and minor mergers could play an important role in ULIRGs at z~2. We compare these fractions with those of a z~1 sample across a wide luminosity range and find that the fraction of disks decreases systematically with L_IR while the fraction of mergers and interactions increases, as has been observed locally. At comparable luminosities, the fraction of ULIRGs with various morphological classifications is similar at z~2 and z~1. We investigate the position of the ULIRGs, along with 70 LIRGs, on the specific star formation rate versus redshift plane, and find 52 systems to be starbursts (lie more than a factor of 3 above the main sequence relation). The morphologies of starbursts are dominated by interacting and merging systems (50%). If irregular disks are included as potential minor mergers, then we find that up to 73% of starbursts are involved in a merger or interaction at some level. Although the final coalescence of a major merger may not be required for the high luminosities of ULIRGs at z~2 as is the case locally, the large fraction of interactions at all stages and potential minor mergers suggest that the high star formation rates of ULIRGs are still largely externally triggered at z~2.

astro-ph.CO↗

arXiv2012

How Do Star-Forming Galaxies at Z>3 Assemble Their Masses?

We investigate how star-forming galaxies typically assemble their masses at high redshift. Using the deep multi-wavelength coverage of the GOODS dataset, we measure stellar mass of a large sample of star-forming galaxies at z~4 and 5, and make a robust determination of stellar mass function (SMF). We report a broad correlation between stellar mass and UV luminosity, such that more UV-luminous galaxies are, on average, more massive. However, the correlation has a substantial intrinsic scatter evidenced by a non-negligible number of UV-faint but massive galaxies. Furthermore, the low-mass end of the SMF does not rise as steeply as the UV luminosity function (alpha_UVLF} -(1.7-1.8), alpha_SMF -(1.3-1.4)) of the same galaxies. In a smooth formation scenario where star formation (SF) is sustained at the observed rates for a long time, these galaxies would have accumulated more mass (by a factor of ~3) than observed and therefore the SMF would mirror more closely that of the UVLF. The relatively shallow slope of the SMF is due to the fact that many of the UV-selected galaxies are not massive enough, and therefore are too faint in their rest-frame optical bands, to be detected in the current observations. Our results favor a episodic formation history in which SF in low-mass galaxies vary significantly over time, a scenario favored by galaxy clustering. Our findings for the UV-faint galaxies are in contrast with those found for more UV-luminous galaxies, which exhibit tighter SFR-M_star correlations. The discrepancy may suggest that galaxies at different luminosities may have different evolutionary paths.

astro-ph.CO↗

arXiv2010

Improving the Estimation of Star formation Rates and Stellar Population Ages of High-redshift Galaxies from Broadband Photometry

We explore methods to improve the estimates of star formation rates and mean stellar population ages from broadband photometry of high redshift star-forming galaxies. We use synthetic spectral templates with a variety of simple parametric star formation histories to fit broadband spectral energy distributions. These parametric models are used to infer ages, star formation rates and stellar masses for a mock data set drawn from a hierarchical semi-analytic model of galaxy evolution. Traditional parametric models generally assume an exponentially declining rate of star-formation after an initial instantaneous rise. Our results show that star formation histories with a much more gradual rise in the star formation rate are likely to be better templates, and are likely to give better overall estimates of the age distribution and star formation rate distribution of Lyman break galaxies. For B- and V-dropouts, we find the best simple parametric model to be one where the star formation rate increases linearly with time. The exponentially-declining model overpredicts the age by 100 % and 120 % for B- and V-dropouts, on average, while for a linearly-increasing model, the age is overpredicted by 9 % and 16 %, respectively. Similarly, the exponential model underpredicts star-formation rates by 56 % and 60 %, while the linearly-increasing model underpredicts by 15 % 22 %, respectively. For U-dropouts, the models where the star-formation rate has a peak (near z ~ 3) provide the best match for age -- overprediction is reduced from 110 % to 26 % -- and star-formation rate -- underprediction is reduced from 58 % to 22 %. We classify different types of star-formation histories in the semi-analytic models and show how the biases behave for the different classes. We also provide two-band calibration formulae for stellar mass and star formation rate estimations.

astro-ph.CO↗

arXiv2008

Biases and Uncertainties in Physical Parameter Estimates of Lyman Break Galaxies from Broad-band Photometry

We investigate the biases and uncertainties in estimates of physical parameters of high-redshift Lyman break galaxies (LBGs), such as stellar mass, mean stellar population age, and star formation rate (SFR), obtained from broad-band photometry. By combining LCDM hierarchical structure formation theory, semi-analytic treatments of baryonic physics, and stellar population synthesis models, we construct model galaxy catalogs from which we select LBGs at redshifts z ~ 3.4, 4.0, and 5.0. The broad-band spectral energy distributions (SEDs) of these model LBGs are then analysed by fitting galaxy template SEDs derived from stellar population synthesis models with smoothly declining SFRs. We compare the statistical properties of LBGs' physical parameters -- such as stellar mass, SFR, and stellar population age -- as derived from the best-fit galaxy templates with the intrinsic values from the semi-analytic model. We find some trends in these distributions: first, when the redshift is known, SED-fitting methods reproduce the input distributions of LBGs' stellar masses relatively well, with a minor tendency to underestimate the masses overall, but with substantial scatter. Second, there are large systematic biases in the distributions of best-fit SFRs and mean ages, in the sense that single-component SED-fitting methods underestimate SFRs and overestimate ages. We attribute these trends to the different star formation histories predicted by the semi-analytic models and assumed in the galaxy templates used in SED-fitting procedure, and to the fact that light from the current generation of star-formation can hide older generations of stars. These biases, which arise from the SED-fitting procedure, can significantly affect inferences about galaxy evolution from broadband photometry.

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arXiv2007

A Comparison between Anomalous 6-cm H$_2$CO Absorption and CO(1-0) Emission in the L1204/S140

We report observations of the dust cloud L1204 with the Onsala 25-m telescope in the 6 cm (1$_{11}-1_{10}$) transition of \htco. The observed region includes the S140 H$α$ arc. This spectral line is seen here in absorption against the cosmic microwave background, indicating the presence of widespread warm molecular gas at intermediate densities. Overall, the distributions of H$_2$CO and CO (taken from the literature) are fairly similar, though significant differences exist at small scales. Most notably, while the CO peak is nearly coincident with the S140 H$α$ arc, the maximum H$_2$CO absorption is clearly separated from it by a full 10$'$ beam ($\sim$ 3 pc). We argue that these differences result from differing abundances and excitation requirements. The CO(1-0) line is more optically thick and more biased towards warm gas than the H$_2$CO 6 cm line. On the other hand, formaldehyde is more easily photodissociated and is, therefore, a poorer tracer of the molecular gas located immediately behind Photon Dominated Regions.

astro-ph↗

arXiv2006

Anomalous H_2CO Absorption Towards the Galactic Anticenter: A Blind Search for Dense Molecular Clouds

We have carried out a blind search in the general direction of the Galactic Anticenter for absorption of the Cosmic Microwave Background (CMB) radiation near 4.83 GHz by molecular clouds containing gaseous ortho-formaldehyde (H_2CO). The observations were done using the 25-m radio telescope at Onsala in Sweden, and covered strips in Galactic latitude -1 < b < +1 at several longitudes in the region 170 < l < 190 . Spectra were obtained in these strips with a grid spacing corresponding to the telescope resolution of 10'. We have detected H_2CO CMB absorption at ~ 10% of the survey pointings. This detection rate is likely to increase with further improvements in sensitivity, and may become comparable to the detection rate expected from a blind CO survey with a corresponding sensitivity limit. We have mapped some of these detections in more detail and compared the H_2CO absorption to existing maps of CO(1-0) emission in the same regions. There appears to be a rough correlation between the velocity-integrated line strength of the CO(1-0) emission and that of the H_2CO absorption. However, the scatter in this correlation is significantly larger than the measurement errors, indicating differences of detail at and below the linear resolution of our observations (~ 4 - 9 pc). Although these two tracers are expected to have similar excitation requirements on the microscopic level characteristic of warm, T_K > 10K, dense, 10^3 < n < 10^5 cm^{-3} condensations in molecular clouds, the CO(1-0) line is expected to be optically thick, whereas the H_2CO line is not. This latter difference is likely to be responsible for a significant part of the scatter in the correlation we have found.

astro-ph↗

arXiv2006

NICMOS Status

We provide an overview of the most important calibration aspects of the NICMOS instrument on board of HST. We describe the performance of the instrument after the installation of the NICMOS Cooling System, and show that the behavior of the instrument has become very stable and predictable. We detail the improvements made to the NICMOS pipeline and outline plans for future developments. The derivation of the absolute photometric zero-point calibration is described in detail. Finally, we describe and quantify a newly discovered count-rate dependent non-linearity in the NICMOS cameras. This new non-linearity is distinctly different from the total count dependent non-linearity that is well known for near-infrared detectors. We show that the non-linearity has a power law behavior, with pixels with high system, or vice versa, pixels with low count rate detecting slightly less than expected. The effect has a wavelength dependence with observations at the shortest wavelengths being the most affected (~0.05-0.1 mag per dex flux change at ~1 micron, 0.03 mag per dex at 1.6 micron).

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arXiv2003

On the colour-colour properties of the Extremely Red Objects

The colours of the galaxy class known as Extremely Red Objects (EROs; R-K>5) are considered to be consistent with two distinct galaxy populations at high redshift: evolved ellipticals or young dusty starbursts. In this paper the properties of EROs, spanned by the five photometric bands RIJHK, are investigated as to the possibility to distinguish between these two galaxy populations using only broad band photometry. The broad band colours of elliptical and starburst galaxies at redshifts up to 5 are computed from synthetic spectra obtained using the spectral evolution synthesis programme PEGASE.2. Two initial mass functions and a range of metallicities and extinctions are used. In order to be extremely red in the redshift range considered, the evolution of the R-K colour sets the requirement that ellipticals have to be less than 7-8 Gyr old, and that the starbursts must have colour excesses of E(B-V)>1, as derived from the nebular emission lines. In investigating the overlap in the different colour-colour planes as a function of redshift, it is found that the planes formed from permutations of the same three filters exhibit very similar overlap characteristics. In colour-colour planes formed within such triplets one of the filters will serve as a "pivot" band against which the two other bands are compared. The configuration where this pivot band lies between the other two bands has the best performance as a discriminator among the three possible configurations. A consistent behaviour cannot be found among the configurations formed by permuting four filters. The minimal filter configuration R-H vs. H-K is found to be the very best discriminator, working as such up to redshift 2.9.

astro-ph↗

arXiv2003

Photometric redshift using the far-infrared SED

A method for using the sub-millimeter band to determine photometric redshifts of luminous high-z dusty galaxies is presented. It is based on the observation that local ultra-luminous IR galaxies show an average spectral energy distribution which has a remarkably small dispersion for wavelengths >50 microns. The shape of the long wavelength part is independent on whether the local ULIRG possesses an AGN or not. A local template is derived from a sample of ULIRG data presented by Klaas et al. (2001). Using this template and the internal dispersion in its shape, it is shown that observations at two fixed submm wavelengths can be used to determine the photometric redshift of galaxies in the redshift range 1<z<5. The uncertainty in the redshift arising from the template SED amounts to Δz/z ~ 0.3.

astro-ph↗

arXiv2002

Gravitational Lensing at Millimeter Wavelengths

With today's millimeter and submillimeter instruments observers use gravitational lensing mostly as a tool to boost the sensitivity when observing distant objects. This is evident through the dominance of gravitationally lensed objects among those detected in CO rotational lines at z>1. It is also evident in the use of lensing magnification by galaxy clusters in order to reach faint submm/mm continuum sources. There are, however, a few cases where millimeter lines have been directly involved in understanding lensing configurations. Future mm/submm instruments, such as the ALMA interferometer, will have both the sensitivity and the angular resolution to allow detailed observations of gravitational lenses. The almost constant sensitivity to dust emission over the redshift range z=1-10 means that the likelihood for strong lensing of dust continuum sources is much higher than for optically selected sources. A large number of new strong lenses are therefore likely to be discovered with ALMA, allowing a direct assessment of cosmological parameters through lens statistics. Combined with an angular resolution <0.1", ALMA will also be efficient for probing the gravitational potential of galaxy clusters, where we will be able to study both the sources and the lenses themselves, free of obscuration and extinction corrections, derive rotation curves for the lenses, their orientation and, thus, greatly constrain lens models.

astro-ph↗

arXiv2001

Environmental effects in galaxies: Molecular Gas, Star Formation, and Activity

In order to study whether there is any correlation between nuclear activities, gas content, and the environment where galaxies reside, we have obtained optical and millimetric spectra for a well-defined sample of intermediate Hubble type spirals in dense environments and in the field. We found that these spirals in dense environments have on average: less molecular gas per blue luminosity, higher atomic gas fraction, lower current star formation rate, and the same star formation efficiency as field galaxies. Although none of these results stand out as a single strong diagnostic, given their statistical significance, taken together they indicate a trend for diminished gas content and star formation activity in galaxies in high density environments. Our results suggest that galaxies in dense environments have either (i) consumed their molecular gas via star formation in the past or (ii) that dense environments leads to an inhibition of molecular gas from atomic phase. The similarities in star formation efficiency of the dense environments and field galaxies suggest that the physical processes controling the formation of stars from the molecular gas are local rather than global. We also found that star formation rate per blue luminosity increases linearly as the total amount of gas increases in LINERs. This result, based on a small sample, suggests that LINERs are powered by star formation rather than an AGN.

astro-ph↗

arXiv2001

Environmental Effects in Galaxies: The Data

We present optical and millimetric data for 47 intermediate Hubble type spiral galaxies located either in dense environments or in the field. We compare correlations between global parameters, such as far-infrared luminosity, blue luminosity, and total molecular gas content, with other samples of galaxies, including normal galaxies, clusters and ultraluminous infrared galaxies. We find that overall our sample is a well defined subset of these other samples of galaxies.

astro-ph↗

arXiv1997

Molecular absorption lines at high redshift

The four known molecular absorption line systems at redshifts z$=$0.25--0.89 are discussed. Two of the systems originate in the galaxy hosting the `background' AGN, while in two cases the absorption occurs in truely intervening galaxies. Indirect evidence for the existence of very small scale structure in the molecular ISM is presented. A new absorption line at z$=$0.89 towards PKS1830--211 opens up interesting possibilities to accurately derive the mass of the lensing galaxy.

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