arXiv subjects

Miguel Querejeta

Publications and source records attributed to Miguel Querejeta.

122 records · Page 7Linked to original sources

arXiv2016

A Portrait of Cold Gas in Galaxies at 60pc Resolution and a Simple Method to Test Hypotheses That Link Small-Scale ISM Structure to Galaxy-Scale Processes

The cloud-scale density, velocity dispersion, and gravitational boundedness of the interstellar medium (ISM) vary within and among galaxies. In turbulent models, these properties play key roles in the ability of gas to form stars. New high fidelity, high resolution surveys offer the prospect to measure these quantities across galaxies. We present a simple approach to make such measurements and to test hypotheses that link small-scale gas structure to star formation and galactic environment. Our calculations capture the key physics of the Larson scaling relations, and we show good correspondence between our approach and a traditional "cloud properties" treatment. However, we argue that our method is preferable in many cases because of its simple, reproducible characterization of all emission. Using, low-J 12CO data from recent surveys, we characterize the molecular ISM at 60pc resolution in the Antennae, the Large Magellanic Cloud, M31, M33, M51, and M74. We report the distributions of surface density, velocity dispersion, and gravitational boundedness at 60pc scales and show galaxy-to-galaxy and intra-galaxy variations in each. The distribution of flux as a function of surface density appears roughly lognormal with a 1sigma width of ~0.3 dex, though the center of this distribution varies from galaxy to galaxy. The 60pc resolution line width and molecular gas surface density correlate well, which is a fundamental behavior expected for virialized or free-falling gas. Varying the measurement scale for the LMC and M31, we show that the molecular ISM has higher surface densities, lower line widths, and more self-gravity at smaller scales.

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arXiv2016

Creating lenticular galaxies with major mergers

Lenticular galaxies (S0s) represent the majority of early-type galaxies in the local Universe, but their formation channels are still poorly understood. While galaxy mergers are obvious pathways to suppress star formation and increase bulge sizes, the marked parallelism between spiral and lenticular galaxies (e.g. photometric bulge-disc coupling) seemed to rule out a potential merger origin. Here, we summarise our recent work in which we have shown, through N-body numerical simulations, that disc-dominated lenticulars can emerge from major mergers of spiral galaxies, in good agreement with observational photometric scaling relations. Moreover, we show that mergers simultaneously increase the light concentration and reduce the angular momentum relative to their spiral progenitors. This explains the mismatch in angular momentum and concentration between spirals and lenticulars recently revealed by CALIFA observations, which is hard to reconcile with simple fading mechanisms (e.g. ram-pressure stripping).

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arXiv2016

Halpha Kinematics of S4G Spiral Galaxies - III. Inner rotation curves

We present a detailed study of the shape of the innermost part of the rotation curves of a sample of 29 nearby spiral galaxies, based on high angular and spectral resolution kinematic Halpha Fabry-Perot observations. In particular, we quantify the steepness of the rotation curve by measuring its slope dRvc(0). We explore the relationship between the inner slope and several galaxy parameters, such as stellar mass, maximum rotational velocity, central surface brightness (μ0), bar strength and bulge-to-total ratio. Even with our limited dynamical range, we find a trend for low-mass galaxies to exhibit shallower rotation curve inner slopes than high-mass galaxies, whereas steep inner slopes are found exclusively in high-mass galaxies. This trend may arise from the relationship between the total stellar mass and the mass of the bulge, which are correlated among them. We find a correlation between the inner slope of the rotation curve and the morphological T-type, complementary to the scaling relation between dRvc(0) and μ0 previously reported in the literature. Although we find that the inner slope increases with the Fourier amplitude A2 and decreases with the bar torque Qb, this may arise from the presence of the bulge implicit in both A2 and Qb. As previously noted in the literature, the more compact the mass in the central parts of a galaxy (more concretely, the presence of a bulge), the steeper the inner slopes. We conclude that the baryonic matter dominates the dynamics in the central parts of our sample galaxies.

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arXiv2015

Creating S0s with major mergers: a 3D view

A number of simulators have argued that major mergers can sometimes preserve discs (e.g. Springel & Hernquist 2005), but the possibility that they could explain the emergence of lenticular galaxies (S0s) has been generally neglected. In fact, observations of S0s reveal a strong structural coupling between their bulges and discs, which seems difficult to reconcile with the idea that they come from major mergers. However, in Querejeta et al. (2015a) we have used N-body simulations of binary mergers to show that, under favourable conditions, discs are first destroyed but soon regrow out of the leftover debris, matching observational photometric scaling relations (e.g. Laurikainen et al. 2010). Additionally, in Querejeta et al. (2015b) we have shown how the merger scenario agrees with the recent discovery that S0s and most spirals are not compatible in an angular momentum--concentration plane. This important result from CALIFA constitutes a serious objection to the idea that spirals transform into S0s mainly by fading (e.g. via ram-pressure stripping, as that would not explain the observed simultaneous change in $λ_\mathrm{Re}$ and concentration), but our simulations of major mergers do explain that mismatch. From such a 3D comparison we conclude that mergers must be a relevant process in the build-up of the current population of S0s.

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arXiv2015

Interacting galaxies in the nearby Universe: only moderate increase of star formation

We investigate the influence of interactions on the star formation by studying a sample of almost 1500 of the nearest galaxies, all within a distance of ~45 Mpc. We define the massive star formation rate (SFR), as measured from far-IR emission, and the specific star formation rate (SSFR), which is the former quantity normalised by the stellar mass of the galaxy, and explore their distribution with morphological type and with stellar mass. We then calculate the relative enhancement of these quantities for each galaxy by normalising them by the median SFR and SSFR values of individual control populations of similar non-interacting galaxies. We find that both SFR and SSFR are enhanced in interacting galaxies, and more so as the degree of interaction is higher. The increase is, however, moderate, reaching a maximum of a factor of 1.9 for the highest degree of interaction (mergers). The SFR and SSFR are enhanced statistically in the population, but in many individual interacting galaxies they are not enhanced at all. We discuss how those galaxies with the largest SFR and/or SSFR enhancement can be defined as starbursts. This study is based on a representative sample of nearby galaxies, including many low-mass and dwarf/irregular galaxies, and we argue that it should be used to place constraints on studies based on samples of galaxies at larger distances, beyond the local Universe.

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arXiv2015

Formation of S0 galaxies through mergers: Explaining angular momentum and concentration change from spirals to S0s

The CALIFA team has recently found that the stellar angular momentum and concentration of late-type spiral galaxies are incompatible with those of lenticular galaxies (S0s), concluding that fading alone cannot satisfactorily explain the evolution from spirals into S0s. Here we explore whether major mergers can provide an alternative way to transform spirals into S0s by analysing the spiral-spiral major mergers from the GalMer database that lead to realistic, relaxed S0-like galaxies. We find that the change in stellar angular momentum and concentration can explain the differences in the $λ_\mathrm{Re}$--$R_{90}/R_{50}$ plane found by the CALIFA team. Major mergers thus offer a feasible explanation for the transformation of spirals into S0s.

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arXiv2015

The Spitzer Survey of Stellar Structure in Galaxies (S4G): Stellar Masses, Sizes and Radial Profiles for 2352 Nearby Galaxies

The Spitzer Survey of Stellar Structure in Galaxies (S4G) is a volume, magnitude, and size-limited survey of 2352 nearby galaxies with deep imaging at 3.6 and 4.5um. In this paper we describe our surface photometry pipeline and showcase the associated data products that we have released to the community. We also identify the physical mechanisms leading to different levels of central stellar mass concentration for galaxies with the same total stellar mass. Finally, we derive the local stellar mass-size relation at 3.6um for galaxies of different morphologies. Our radial profiles reach stellar mass surface densities below 1 Msun pc-2. Given the negligible impact of dust and the almost constant mass-to-light ratio at these wavelengths, these profiles constitute an accurate inventory of the radial distribution of stellar mass in nearby galaxies. From these profiles we have also derived global properties such as asymptotic magnitudes (and the corresponding stellar masses), isophotal sizes and shapes, and concentration indices. These and other data products from our various pipelines (science-ready mosaics, object masks, 2D image decompositions, and stellar mass maps), can be publicly accessed at IRSA (http://irsa.ipac.caltech.edu/data/SPITZER/S4G/).

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arXiv2015

Short GMC lifetimes: an observational estimate with the PdBI Arcsecond Whirlpool Survey (PAWS)

We describe and execute a novel approach to observationally estimate the lifetimes of giant molecular clouds (GMCs). We focus on the cloud population between the two main spiral arms in M51 (the inter-arm region) where cloud destruction via shear and star formation feedback dominates over formation processes. By monitoring the change in GMC number densities and properties from one side of the inter-arm to the other, we estimate the lifetime as a fraction of the inter-arm travel time. We find that GMC lifetimes in M51's inter-arm are finite and short, 20 to 30 Myr. Such short lifetimes suggest that cloud evolution is influenced by environment, in which processes can disrupt GMCs after a few free-fall times. Over most of the region under investigation shear appears to regulate the lifetime. As the shear timescale increases with galactocentric radius, we expect cloud destruction to switch primarily to star formation feedback at larger radii. We identify a transition from shear- to feedback-dominated disruption through a change in the behavior of the GMC number density. The signature suggests that shear is more efficient at completely dispersing clouds, whereas feedback transforms the population, e.g. by fragmenting high mass clouds into lower mass pieces. Compared to the characteristic timescale for molecular hydrogen in M51, our short lifetimes suggest that gas can remain molecular while clouds disperse and reassemble. We propose that galaxy dynamics regulates the cycling of molecular material from diffuse to bound (and ultimately star-forming) objects, contributing to long observed molecular depletion times in normal disk galaxies. We also speculate that, in more extreme environments such as elliptical galaxies and concentrated galaxy centers, star formation can be suppressed when the shear timescale becomes so short that some clouds can not survive to collapse and form stars.

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arXiv2015

Spitzer Survey of Stellar Structure in Galaxies (S$^4$G). The Pipeline 4: Multi-component decomposition strategies and data release

The Spitzer Survey of Stellar Structure in Galaxies (S$^4$G, Sheth et. al. 2010) is a deep 3.6 and 4.5 $μ$m imaging survey of 2352 nearby ($< 40$ Mpc) galaxies. We describe the S$^4$G data analysis pipeline 4, which is dedicated to 2-dimensional structural surface brightness decompositions of 3.6 $μ$m images, using GALFIT3.0 \citep{peng2010}. Besides automatic 1-component Sérsic fits, and 2-component Sérsic bulge + exponential disk fits, we present human supervised multi-component decompositions, which include, when judged appropriate, a central point source, bulge, disk, and bar components. Comparison of the fitted parameters indicates that multi-component models are needed to obtain reliable estimates for the bulge Sérsic index and bulge-to-total light ratio ($B/T$), confirming earlier results \citep{laurikainen2007, gadotti2008, weinzirl2009}. In this first paper, we describe the preparations of input data done for decompositions, give examples of our decomposition strategy, and describe the data products released via IRSA and via our web page ({\bf \tt www.oulu.fi/astronomy/S4G\_PIPELINE4/MAIN}). These products include all the input data and decomposition files in electronic form, making it easy to extend the decompositions to suit specific science purposes. We also provide our IDL-based visualization tools (GALFIDL) developed for displaying/running GALFIT-decompositions, as well as our mask editing procedure (MASK\_EDIT) used in data preparation. In the second paper we will present a detailed analysis of the bulge, disk, and bar parameter derived from multi-component decompositions.

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arXiv2014

Formation of S0 galaxies through mergers. V. Antitruncated stellar discs resulting from major mergers

Lenticular galaxies (S0s) are more likely to host antitruncated (Type-III) stellar discs than galaxies of later Hubble types. Major mergers are popularly considered too violent mechanisms to form these breaks. We have investigated whether major mergers can result into S0-like remnants with realistic antitruncated stellar discs or not. We have analysed 67 relaxed S0 and E/S0 remnants resulting from dissipative N-body simulations of major mergers from the GalMer database. We have simulated realistic R-band surface brightness profiles of the remnants to identify those with antitruncated stellar discs. Their inner and outer discs and the breaks are quantitatively characterized to compare with real data. Nearly 70% of our S0-like remnants are antitruncated, meaning that major mergers that result in S0s have a high probability of producing Type-III stellar discs. Our remnants lie on top of the extrapolations of the observational trends (towards brighter magnitudes and higher break radii) in several photometric diagrams. In scale-free photometric diagrams, simulations and observations overlap and the remnants reproduce the observational trends, so the physical mechanism after antitruncations is highly scalable. We have found novel photometric scaling relations between the characteristic parameters of the antitruncations in real S0s, which are also reproduced by our simulations. The trends in all the photometric planes can be derived from three basic scaling relations that real and simulated Type-III S0s fulfill: h_i \prop R_brkIII, h_o \prop R_brkIII, and mu_brkIII \prop R_brkIII, where h_i and h_o are the scalelenghts of the inner and outer discs, and mu_brkIII and R_brkIII are the surface brightness and radius of the breaks. Mayor mergers provide a feasible mechanism to form realistic antitruncated S0 galaxies (abridged).

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arXiv2014

Being WISE I: Validating Stellar Population Models and M/L ratios at 3.4 and 4.6 microns

Using data from the WISE mission, we have measured near infra-red (NIR) photometry of a diverse sample of dust-free stellar systems (globular clusters, dwarf and giant early-type galaxies) which have metallicities that span the range -2.2 < [Fe/H] (dex) < 0.3. This dramatically increases the sample size and broadens the metallicity regime over which the 3.4 (W1) and 4.6 micron (W2) photometry of stellar populations have been examined. We find that the W1 - W2 colors of intermediate and old (> 2 Gyr) stellar populations are insensitive to the age of the stellar population, but that the W1 - W2 colors become bluer with increasing metallicity, a trend not well reproduced by most stellar population synthesis (SPS) models. In common with previous studies, we attribute this behavior to the increasing strength of the CO absorption feature located in the 4.6 micron bandpass with metallicity. Having used our sample to validate the efficacy of some of the SPS models, we use these models to derive stellar mass-to-light ratios in the W1 and W2 bands. Utilizing observational data from the SAURON and ATLAS3D surveys, we demonstrate that these bands provide extremely simple, yet robust stellar mass tracers for dust free older stellar populations that are freed from many of the uncertainties common among optical estimators.

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arXiv2014

Evolution induced by dry minor mergers onto fast-rotator S0 galaxies

We analysed collisionless N-body simulations of intermediate and minor dry mergers onto S0s to test whether these mergers can generate S0 galaxies with kinematics intermediate between fast and slow rotators. We find that minor mergers induce a lower decrease of the global rotational support than encounters of lower mass ratios, which results in S0s with properties intermediate between fast and slow rotators. The resulting remnants are intrinsically more triaxial, less flattened, and span the whole range of apparent ellipticities up to $ε_\mathrm{e} \sim 0.8$. They do not show lower apparent ellipticities in random projections than initially; on the contrary, the formation of oval distortions and the disc thickening increase the percentage of projections at $0.4 < ε_\mathrm{e} < 0.7$. In the experiments with S0b progenitor galaxies, minor mergers tend to spin up the bulge and to decrease slightly its intrinsic ellipticity, whereas in the cases of primary S0c galaxies they keep the rotational support of the bulge nearly constant and decrease significantly its intrinsic ellipticity. The remnant bulges remain nearly spherical ($B/A \sim C/A > 0.9$), but exhibit a wide range of triaxialities ($0.20 < T < 1.00$). In the plane of global anisotropy of velocities ($δ$) vs. intrinsic ellipticity ($ε_\mathrm{e,intr}$), some of our models extend the linear trend found in previous major merger simulations towards higher $ε_\mathrm{e,intr}$ values, while others depart from it. This is consistent with the wide dispersion exhibited by real S0s in this diagram compared with ellipticals, which follow the linear trend drawn by major merger simulations. The different trends exhibited by ellipticals and S0 galaxies in the $δ$ - $ε_\mathrm{e}$ diagram may be pointing to the different role played by major mergers in the build-up of each morphological type.

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arXiv2014

Reconstructing the stellar mass distributions of galaxies using S4G IRAC 3.6 and 4.5 micron images: II. The conversion from light to mass

We present a new approach for estimating the 3.6 micron stellar mass-to-light ratio in terms of the [3.6]-[4.5] colors of old stellar populations. Our approach avoids several of the largest sources of uncertainty in existing techniques. By focusing on mid-IR wavelengths, we gain a virtually dust extinction-free tracer of the old stars, avoiding the need to adopt a dust model to correctly interpret optical or optical/NIR colors normally leveraged to assign M/L. By calibrating a new relation between NIR and mid-IR colors of GLIMPSE giant stars we also avoid discrepancies in model predictions for the [3.6]-[4.5] colors of old stellar populations due to uncertainties in molecular line opacities. We find that the [3.6]-[4.5] color, which is driven primarily by metallicity, provides a tight constraint on M/L_3.6, which varies intrinsically less than at optical wavelengths. The uncertainty on M/L_3.6 of ~0.07 dex due to unconstrained age variations marks a significant improvement on existing techniques for estimating the stellar M/L with shorter wavelength data. A single M/L_3.6=0.6 (assuming a Chabrier IMF), independent of [3.6]-[4.5] color, is also feasible as it can be applied simultaneously to old, metal-rich and young, metal-poor populations, and still with comparable (or better) accuracy (~0.1 dex) as alternatives. We expect our M/L_3.6 to be optimal for mapping the stellar mass distributions in S4G galaxies, for which we have developed an Independent Component Analysis technique to first isolate the old stellar light at 3.6 micron from non-stellar emission (e.g. hot dust and the 3.3 PAH feature). Our estimate can also be used to determine the fractional contribution of non-stellar emission to global (rest-frame) 3.6 micron fluxes, e.g. in WISE imaging, and establishes a reliable basis for exploring variations in the stellar IMF.

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arXiv2013

On the Eclipse of Thales, Cycles and Probabilities

According to classical tradition, Thales of Miletus predicted the total solar eclipse that took place on 28 May 585 BCE. Even if some authors have flatly denied the possibility of such a prediction, others have struggled to find cycles which would justify the achievement of the philosopher. Some of the proposed cycles have already been refuted, but two of them, namely those of Willy Hartner and Dirk Couprie, remain unchallenged. This paper presents some important objections to these two possibilities, based on the fact that these authors do not list all the eclipses potentially visible by their criteria. In addition, any explanation based on cycles will need to face the complex problem of visibility (smallest observable eclipse, weather...). The present article also includes a statistical study on the predictability of solar eclipses for a variety of periods, similar to that performed by Willy Hartner for lunar eclipses, resulting in lower probabilities in the solar case (and percentages depend on the chosen magnitude limit). The conclusion is that none of the cycles proposed so far provides a satisfactory explanation of the prediction, and, on statistical grounds, none of the periods studied leads to a significant probability of success with solar eclipse cycles.

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