China-Chile Proposal

China-Chile Joint Research Fund 2016

Exploring the Star Formation Histories of Galaxies in Different Environments from MaNGA Spectra

Abstract

Star formation is one of the fundamental process governing the formation and evolution of galaxies. The star formation history (SFH) of galaxies allow us to investigate when galaxies formed their stars and assembled their mass. We can constrain the SFH with high level of precision from galaxies with resolved stellar populations, since we are able to discriminate between stars of different ages from the spectrum they emit.

MaNGA (Mapping Nearby Galaxies at APO, Bundy et al. 2016) will provide resolved spectroscopy for nearly 10000 galaxies in the local Universe. MaNGA’s key goals are to understand the “life cycle” of present day galaxies from imprinted clues of their birth and assembly through their ongoing growth via star formation and merging, to their death from quenching at late times.

In this research program we aim to use MaNGA data to constrain the SFH of nearby galaxies and to investigate how they formed and evolve. However, the relative importance of secular evolution (nature) over nurture is not yet clear, and separating the effects of interaction-driven evolution in the observed galaxy properties is not trivial.

Our team count with experts on the field of galaxy formation and evolution, galaxy environment, galaxy spectral modelling, and galaxy chemical evolution models, with experience on working together. In this proposal we ask for a 2-year research program that will advance our understanding of the effects of the environment on galaxy formation and evolution by combining theoretical modelling with MaNGA spectra and state-of-art multiwavelength observations.

• Area of Research: galaxies

• Team

Médéric Boquien(1) (PI), Shiyin Shen(2) (PI), Maria Argudo-Fernández(1) (Co-I), Fangting Yuan(2) (Co-I), Jun Yin(2) (Co-I), Ruixiang Chang(2) (Co-I), Lei Hao(2) (Co-I)

- University of Antofagasta, Chile

- Shanghai Astronomical Observatory, Chinese Academy of Sciences, China�

Scientific Justification

Galaxy evolution in the era of the SDSS-IV and MaNGA surveys

The electromagnetic emission of galaxies is our main window into their formation and their evolution. Young stellar populations dominate the UV-optical energy budget of galaxies. As they evolve and die they produce dust and inject vast amounts of metals in the interstellar medium, which have a direct impact on the formation of new stars, shielding and cooling molecular clouds, but also reddening and dimming the UV-to-near-infrared spectrum. In case of intense feedback from young, massive stars, star formation can even be quenched altogether, affecting the shape and appearance of galaxies. At the same time, galaxy can be red and dim simply because they are not forming stars anymore. In other words, the electromagnetic spectrum of galaxies encodes the past and current physical processes that have driven their evolution across cosmic times.

One of the major spectroscopic surveys in existence is the Sloan Digital Sky Survey (SDSS). It has provided us with spectra for a vast number of low redshift galaxies, yielding for each of them information about their star formation history, dust extinction, metallicity, stellar and ionized gas kinematics, etc. However, SDSS spectra are limited to a single spatial position at the galaxy’s center. This means that unfortunately this treasure trove provides us with only very limited information on the local physical processes in galaxies and how galaxies have grown and evolved over cosmic times.

The advent of Integral Field Spectroscopy (IFS) is a giant leap forward. It provides us with a 3-dimensional view of galaxies (one spectral, two spatial) that encodes the processes driving their evolution, allowing us to go far beyond either the broadly used approaches of spatially-resolved photometry and spatially-unresolved spectroscopy. In this endeavor, MaNGA (Mapping Nearby Galaxies at APO, Bundy et al. 2016) is an IFS survey designed to investigate an unprecedented sample of 10,000 nearby galaxies across a range of masses and spatial resolutions. The survey is one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV) that began on 2014 July 1. In previous SDSS programs the observing mode was from a single fiber spectrum. To overcome this limitation and provide spatially resolved spatial maps, MaNGA has adopted a new strategy by bundling fibers together (see Figure 1).

Figure 1: SDSS three-colour images and MaNGA Integral Field Unit (IFU) design for a sample of spiral face-on galaxies.

The MaNGA legacy dataset will be used to address a broad range of open questions in galaxy formation, including: 1. the nature of present-day galaxy growth via merging and gas accretion, 2. the processes responsible for terminating star formation in galaxies, and 3. the broad formation history of galaxy subcomponents, including the disk, bulge, and dark matter halo.

This proposed research program aims at gaining a unique insight into how nearby galaxies form and evolve by comparing their observed properties with results from state-of-the-art chemical evolution models. Using MaNGA 3-D maps, we will measure their physical properties (SFH, mass, metallicity, ionized gas and stellar continuum extinctions, etc.) with unprecedented precision and accuracy combining IFS data from MaNGA with multi-wavelength SED modeling from the ultraviolet to the infrared. We therefore will provide new constraints on the assembly of galaxies across cosmic times. Because processes governing the evolution of galaxies can be strongly intertwined, we propose the Chilean and Chinese communities to collaborate and address these issues putting our own tools and expertise in common, from fundamental spectral modeling to the interpretation of the results of these models to understand the effect of the environment on the evolution of galaxies. While the core of the work will be carried out by the proposers, the project will be fully open to both communities and in particular, we will organize two Chile-China workshops (one each year) of broad interest to researchers outside the core team.

MaNGA to understand galaxy evolution

Modeling of MaNGA galaxies

Spectral and photometric models are outstanding tools for measuring the physical properties of galaxies. Yet, they both have their own strengths and weaknesses. Spectra allow us to detect the presence of an AGN and access to the physical conditions of the ionized gas and of the stellar populations through emission and absorption lines and spectral indices. This yields, for instance, estimates of the extinction of the ionized gas, of the metallicity (both of the ionized gas and the stars), and constraints on the SFR on very short (Halpha, ~10 Myr) and very long (D4000, ~1 Gyr) timescales. However spectra are limited to a relatively narrow range in wavelengths and thus are only sensitive to a fraction of the baryonic components of galaxies. Conversely, photometric observations from the UV to the far-IR allow us to also probe young stellar populations that dominate the UV and the dust that reprocesses short wavelength radiation into the IR. Combined together, such a broad wavelength coverage provides us with the leverage we need to constrain the attenuation curve of the stellar continuum, the UV-based and IR-based SFR, and the stellar mass of galaxies. In other words, spectral and photometric data provide us with complementary and synergistic constraints on the physical properties of galaxies.

Based on the experience of our team working with MaNGA data, modelling SED and spectra, and creating models of chemical evolution, we propose here to carry out a self-consistent spectro-photometric modeling of MaNGA galaxies using the CIGALE code. CIGALE (Code Investigating GALaxy Emission, Noll et al. 2009; Boquien et al., in prep.) is a state-of-the-art self-consistent SED modelling code from the far-ultraviolet to the radio domain, including the contributions from stellar populations of all ages, thermal and non-thermal gas emission, dust (both in absorption and emission; the energy absorbed by the dust in the UV-to-near-IR domain is re-emitted self-consistently in the mid- and far-IR), and optionally an active nucleus. To estimate the physical properties of the modelled targets, it is based on a Bayesian-like analysis method. The Chile-based team is one of the main developers of CIGALE, with an extensive experience on spatially resolved multi-wavelength SED modeling (e.g. Boquien et al. 2012, 2016).

The starting point of any stellar population synthesis (SPS) model is the simple stellar population (SSP), which describes the evolution in time of the SED of a single stellar population (or single age population) at a single metallicity and abundance pattern. An SSP therefore requires three basic inputs: stellar evolution theory in the form of isochrones, stellar spectral libraries, and an IMF, each of which may in principle be a function of metallicity and/or elemental abundance pattern.

The most robust approach to estimating stellar metallicities is to employ spectroscopic features. Optical spectra of galaxies are rich in atomic and molecular absorption features that are readily apparent even at low spectral resolution (R ∼ 1000). For a fixed population age, the strengths of the absorption features will depend not only on the overall metallicity but also on the detailed elemental abundance pattern. In general, this greatly complicates the interpretation of absorption features and spectral indices. However, with the aid of models that allow for a variation in both metallicity and abundance pattern, one can search for combinations of features that are relatively robust against abundance pattern variations.

In this endeavor, two specific improvements will be made to CIGALE: 1) extending CIGALE to fit spectro-photometric data, and 2) including self-consistent chemical evolution.

Spectro-photometric modeling

Even if CIGALE normally only handles broadband data, as a pathfinder work for this project the Chile-based team developed an experimental version that fits spectra, with excellent results (see Figure 2).

Figure 2: Result of the best fit model spectra created by CIGALE (red line) for a randomly selected spectra from MaNGA (black line). The lower panels show a zoom in three different wavelength regions.

To enjoy the synergistic benefits of both approaches, we will develop a new version of CIGALE that will be able to simultaneously fit both broadband and spectra. By combining optical spectra with UV-to-IR broadband observations, this new resource will allow us to get a better handle on the continuum attenuation, since the Balmer decrement is only for the ionised gas and the conversion to a continuum attenuation is uncertain, and break more reliably the age-attenuation degeneracy, among other things. Therefore, used in combination with spatially resolved multi-wavelength data to complement MaNGA observations, this will allow us to obtain better estimates on the SFR, the SFH, and other physical properties of nearby galaxies. As with current versions of CIGALE, this code will be made freely available.

One the modification carried out, we will model the MaNGA galaxies present in our subsample (see in particular Sect. 2.2 and 2.3) and measure their spatially resolved physical properties (SFR over different timescales, metallicities [see also Sect. 2.2.2], dust attenuation, stellar masses, etc.). This will serve as a basis for the investigation of the effect of the environment on the evolution of galaxies.

We have to note that our team is no stranger to the combination of spectral and photometric data. Doing so in a groundwork for this proposal, we have studied the SFH of a galaxy merger observed with MaNGA. Galaxy mergers play an important role in the formation and evolution of galaxies and their dark matter halos. The UV and IR data are used to constrain the star formation rate and dust attenuation for mergers, which are tracers of the evolutionary stage of the systems. However, spatially resolved studies of galaxy mergers are restricted to very close galaxies, especially due to the lack spatial resolution in IR data. In Yuan et al. (in preparation), we show how the use of UV-to-IR broadband SED (see Figure 3), in combination with MaNGA IFS (see Figure 4), provide unique opportunity to study the star formation histories and dust attenuation at the tail and core parts of the merging galaxy Mrk 848.

Figure 3: Fitting results by CIGALE for the regions 1, 2, and 3 of the merger Mrk 848 from GALEX, ugriz SDSS bands, and IRAC data. Black dots are observed band fluxes. Red lines indicate the results of SED fitting assuming a two exponentially decreasing SFH. Yellow lines indicate the results of SED fitting assuming a delayed SFH.

Figure 4: Interacting pair Mrk 848. The left panel shows the SDSS three-colour image of the object. Yellow circles correspond to the regions completely covered by the MaNGA IFU, meanwhile regions with red circles correspond to regions not covered or partially covered. The middle panel shows the MaNGA Hα map of the galaxy merger. Flux units are per spatial pixel. The left panel shows the estimated SFH for each selected region in colour-code according to the legend.

Implementing Chemical evolution model into composite stellar population synthesis model

The second aspect that will be developed is implementing the chemical evolution model into the composite stellar population synthesis model.

Many of the SED fitting models(e.g. CIGALE ) assume mono-metallic stellar populations. Yet, as galaxies form stars that enrich the interstellar medium in metals which can be ejected into intergalactic medium through violent feedback and diluted through the accretion of cold gas, the evolution of stellar metallicity over cosmic times can be drastic and complicated.

Alternatively, the composite stellar population(CSP) synthesis models(e.g. pPXF, STARLIGHT, STECKMAP) allow for combinations of stellar populations with different metallicities. In such models, the metallicity and age are two independent ingredients of the single stellar populations(SSPs). However, in reality, the age and metalicity of the stars in galaxies are intrinsically correlated.A CSP, built by a mathematical combination of SSPs, which may give the best fitting to the observed data, however, may not be a physical solution.

To treat the intrinsic correlation between the metallicties and ages of the stellar populations, the chemical evolution model, which manipulates the cycling of the gas, metals and stars in galaxies, is required. In the past ten years, our group have developed a phenomenological chemical evolution mode, which successfully explains most of the observational properties of the Milk way and nearby spiral galaxies, e.g. M31, UGC8802, NGC5194(Yin et al. 2009, Chang et al. 2012, Kang et al. 2015). In our model, besides the stellar mass and size of galaxies, the star formation history(SFH) is the key ingredient that determines the other observational properties of the galaxies.

On the other hand, the recovering of the SFHs of galaxies from observational data with CSP models is an ill-posed problem. To get a better solution of SFH from CSP model , besides the requirement of the high S/N data, a prior information(or penalized likelihood) on the combination of SSPs is proved to be very useful(e.g. Ruiz-Lara et al. 2015). Therefore, the motivation of this study is to introduce the output of the chemical evolution model as a prior into the CSP model.

We will test this new technique with the isolated spiral galaxies with MaNGA IFS data.

Evolution of Spiral Isolated Galaxies with MaNGA

From models, it is predicted that disks grow from inner to outer parts of galaxies, the so-called inside-out model of galaxy formation. This means that first stars form in the center and later in the outer regions. As a consequence, we expect to observe gradients in stellar age and metallicity as a function of galaxy radii. In practice, it is very difficult to observe this effect since high spatial and spectral resolutions are required. Until recently it was nearly impossible to put precise constraints on the local SFH of anything but the most local galaxies. The new IFS data along with multi-wavelength observations now provide us with sufficient resolution to measure and study the spatially resolved SFH beyond the Local Group.

At the same time, galaxies can evolve secularly (nature) or through the effect of their large-scale environments (nurture). To understand the fundamental physical processes at play within galaxies, it is important to separate nature from nurture. To this effect Isolated galaxies and the power of MaNGA data are the key ingredients to reach this goal.

How exactly do galaxies build their disk when they grow in isolation, unaffected by external influences? We aim of characterizing the star formation history of isolated galaxies, under the assumption of the inside-out scenario, and to compare with chemical evolution models using MaNGA IFU data. The sample of isolated galaxies used in this study is selected from the SDSS-based catalog of Isolated Galaxies (SIG, Argudo-Fernández et al., 2015). Preliminary results are shown in Figure 5. We already studied the effects of the environment in active galactic nuclei (AGN) for these galaxies using one-fiber spectra (Argudo-Fernández et al., 2016). We can also use IFU data to test whether AGN are real or rather if it is more related to star formation. We expect, for the very first time, to find some clues on how spiral galaxies form and evolve combining results from the spectral fitting techniques, with the predictions from CEMs, that are being developed in the Shanghai Astronomical Observatory by experts in the field.

Figure 5: Results for a SIG galaxy with MaNGA data (MaNGA ID galaxy 1-93640). The left upper panel shows the SDSS three-colour image of the galaxy with its IFU plate design. The lower left panel shows the Hα map of the galaxy and the set of radial bins in which we divide the galaxy to study the stellar populations. The left panel shows the results of the radial gradients of the SFH parameters, stellar metallicity, and dust parameters in each radial bin as a function of the effective radius of the galaxy.

Environmental effects on star formation properties in galaxy pairs

Merging is a presumably cause to transfer blue and star-forming galaxies to red and dead galaxies. Simulations show that the interaction of two galaxies in a pair can cause the gas inflow, triggering starbursts in the central regions of galaxies. While numerous observations have found that there is enhancement of star formation in galaxy pairs, the enhancement may depend on the local environment (see e.g. Tonneson et al., 2014). Also, the spatial extent of the enhanced star formation in pairs is still not clear. Although many studies found enhanced star formation in central regions, there are also cases showing star formation widely spread to bridge and tidal tail regions (e.g. Boquien et al. 2007, 2009, 2010, Smith et al. 2010, 2016). A recent work by Moreno et al. (2015) shows that pair interactions can cause the enhancement of star formation in the center, but at the same time suppress the activity at outskirts.

We select the pair sample from the catalog of groups constructed by Yang et al. (2007) based on SDSS-DR7 data. The spectroscopic redshifts for the companion galaxies are collected not only from SDSS-DR7, but also SDSS-DR10, NED, and LAMOST, in order to minimize the fiber collision effect which causes the missing of the spectrum of one member galaxy in a close pair. Especially, the LAMOST data are obtained through the project dedicated to complement the spectroscopic redshifts of SDSS close pairs with LAMOST spectroscopy (Shen et al. 2016). Two galaxies are selected as a pair if they satisfy the following criteria: 1) Both galaxies have spectroscopic redshifts; 2) The projected separation rp is less than 50 kpc; 3) The velocity difference dv is less than 500 km/s. A part of this sample (including approximately 1000 galaxies) is selected for MaNGA observations over the course of the survey.

We focus on exploring how the environment affects the properties in galaxy close pairs. Yang et al. (2007)’s group catalog already provides us with halo mass of each pairs, which can be used as a parameter to indicate the environment. We also use tidal strength parameters that measure the local galaxy density of each pair (Argudo-Fernández et al., 2015) as another approach to describe the environment. Based on this SDSS pair sample, we can statistically explore the environmental dependence of pairs. On the other hand, MaNGA IFU allows us to study the spatially resolved properties of galaxies, and therefore can further help us clarify in which part the SFR/dust is changed, and whether the spatial variation has some dependence on the environment.

Implementation Plan of research

Including project duration (one or two years)

In this proposal we ask for a 2-year research program that will advance our understanding of the effects of the environment on galaxy formation and evolution by combining theoretical modelling with MaNGA spectra and state-of-art multi-wavelength observations.

One of our main activities is the celebration of an annual workshop to discuss and advance in the project. We will celebrate one workshop in Chile and another in China. The workshops will be also open to the participation of other MaNGA team experts members from China, Chile, and the general MaNGA collaboration.

We also plan to use the funding to support the exchange of summer students, both in China and Chile, to work on the research projects.

Part of the budget will be also used to cover the expenses generated by observing runs (travels, masks, etc) for possible follow-up observations of our galaxy targets.

CV of the PI(s), a list of currently supported research programs, and a list of publications relevant to the proposal

CV of the PI(s)

Please, find the CV of the PIs attached to this proposal.

Shiyin Shen: [1]

Supported research programs

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- "Evolution of Spiral Isolated Galaxies with MaNGA" FONDECYT postdoctoral N° 3160304

- "The galaxy pairs in SDSS, NSFC grant 11573050 (Shiyin Shen)

List of publications

Please add publications that you consider

We present here a list of publications where this team has been working together in the context of this proposal or the work of individual team members in related projects.

1. ‘’Milky Way versus Andromeda: a tale of two disks, Yin, J.; Hou, J. L.; Prantzos, N.; Boissier, S.; Chang, R. X.; Shen, S. Y.; Zhang, B., 2009,A&A,505,497
2. ‘’The Growth of the Disk Galaxy UGC8802’’, Chang, R. X.; Shen, S. Y.; Hou, J. L.,2012, ApJL, 753, 10
3. ‘’The evolution of interacting spiral galaxy NGC 5194’’, Kang, Xiaoyu; Chang, Ruixiang; Zhang, Fenghui; Cheng, Liantao; Wang, Lang, 2015, MNRAS,449, 414
4. ‘’Catalogues of isolated galaxies, isolated pairs, and isolated triplets in the local Universe’’ , M. Argudo-Fernández; S. Verley; G. Bergond; S. Duarte Puertas; E. Ramos Carmona; J. Sabater; M. Fernández Lorenzo; D. Espada; J. Sulentic; J. E. Ruiz; S. Leon, A&A, 578A, 110A (2015)
5. ‘’Isolated Galaxies versus Interacting Pairs with MaNGA’’ , M. Argudo- Fernández, F. Yuan, S. Shen, J. Yin, R. Chang, S. Feng, Galaxies, 3, 156F (2015).
6. ‘’A sample of galaxy pairs identi ed from the LAMOST spectral survey and the Sloan Digital Sky Survey’’ , Shiyin Shen, M. Argudo-Fernandez, Li Chen et al., RAA, 16c, 7S (2016).
7. ‘’An Isolated Compact Galaxy Triplet’’, Shuai Feng, Zheng-Yi Shao, Shi-Yin Shen, Maria Argudo-Fernandez, Hong Wu, Man-I Lam, Ming Yang, and Fang-Ting Yuan, RAA, 16e, 3F (2016).
8. ‘’Effects of local and large-scale environments on nuclear activity and star formation’’ , M. Argudo-Fernández; S. Shen; J. Sabater; S. Duarte Puertas; S. Verley, A&A, 592A, 30A (2016).
9. ‘’SDSS-IV MaNGA: Properties of galaxies with kinematically decoupled stellar and gaseous components’’, Jin, Yifei; Chen, Yanmei; Shi, Yong; et al., MNRAS, accepted (2016).
10. ‘’The growth of the central region by acquisition of counter-rotating gas innstar-forming galaxies,’’ YanMei Chen; Yong Shi; C. A. Tremonti et al., submitted to Nature.
11. ‘’Stellar Population Gradients as a Function of Galaxy Mass and Environment’’, D, Goddard; D. Thomas, C. Maraston et al., submitted to MNRAS.
12. ‘’Spatially Resolved Star Formation Histories in Galaxies as a Function of Galaxy Mass and Type’’, D, Goddard; D. Thomas, C. Maraston et al., submitted to MNRAS.
13. ‘’Star Formation History of the Galaxy Merger Mrk848 with SDSS-IV MaNGA’’, F. Yuan; S. Shen; L. Hao; Maria Argudo-Fernandez, in preparation.
14. ‘’Star formation histories of isolated galaxies with MaNGA’’, Maria Argudo-Fernandez; M. Boquien; S. Shen; F. Yuan, J. Yin, R. Chang, K. Bundy, C. Liu, in preparation.