3C 48: Stellar Populations and the Kinematics of Stars and Gas in the Host Galaxy

We obtained optical spectroscopy of close (< 80 kpc) companion objects of a sample of 12 low redshift quasars (z < 0.3 ) selected from the SDSS Stripe82 area and that are in the subsample of 52 QSOs for which both multicolor host galaxies properties and galaxy environment was recently investigated in detail. We found that for 8 out of 12 sources the companion galaxy is associated to the QSO having a difference of radial velocity that is less than 400 km/s. Many of these associated companions exhibit [OII] $\lambda$3727 \AA~ emission lines suggestive of episodes of (recent) star formation possibly induced by past interactions. The SFR of the companion galaxies as derived from [OII] line luminosity is, however, modest, with a median value of 1.0 +-0.8 M_sun/yr, and the emission lines are barely consistent with expectation from gas ionization by the QSO. The role of the QSO for inducing star formation in close companion galaxies appears meager. For three objects we also detect the starlight spectrum of the QSO host galaxy which is characterized by absorption lines of old stellar population and [OII] emission line.

The thesis focuses on two case studies of the host galaxies of the quasi-stellar objects (QSOs) I Zw 1 and 3C 48. The studies are motivated by the hypothesis that ultra-luminous infrared galaxies (ULIRGs) might represent the early stage of QSO evolution (Sanders et al. 1988). According to this hypothesis, galaxy mergers and interactions trigger gas inflow which is followed by starburst activity and by the formation of an active nucleus. As likely transitionary objects between the ultra-luminous infrared stage and the QSO stage, I Zw 1 and 3C 48 are promising candidates to investigate the active nucleus, the starburst, and the merger properties as the essential links between all stages of the proposed evolutionary sequence. The case study of I Zw 1 has an observational focus and is based on near-infrared (NIR) imaging and spectroscopy, carried out with ISAAC (Infrared Spectrometer and Array Camera) at the Very Large Telescope of the European Southern Observatory (ESO) on Cerro Paranal in Chile. The two-armed spiral host galaxy of I Zw 1 shows two bright knots in the north and in the west of the optical disk region, respectively. Here, the new ISAAC spectra indicate that the northern object is a projected foreground star, as previously assumed but recently challenged. In the surroundings of the western source, the ISAAC J-band image displays tidal features. This gives further support for the scenario that I Zw 1 is presently in a minor merger process with this source. The new spectra and the NIR colors of the western source indicate an old stellar population. The host of I Zw 1 is analyzed in a one-dimensional structural decomposition into bulge, disk, and halo components, which results in mean J-band mass-to-light ratios. The latter indicate a young mean stellar population in the bulge component, while the disk ratios agree with those of normal spiral galaxies. Such a scenario of younger stellar populations in the central region of the I Zw 1 host is also plausible from a two-color analysis. The nuclear ISAAC spectrum of I Zw 1 basically confirms previous results, by showing hydrogen emission lines and indications of extremely blueshifted high-excitation lines. The case study of 3C 48 is the numerical focus of this thesis and based on simulations with multi-particle methods. The aim is to investigate the major merger scenario for 3C 48 by reproducing a 3C 48 model. Such a model is found for a certain projection angle during the inclined merger of two equal-mass spiral galaxies. It inspirits a new idea for the problem of the apparently missing second tidal tail in 3C 48. For the derived projection angle, this tidal tail is located in front of the main body of the modeled host galaxy and is likely to be missed in observations. In the model, both galaxy centers are still separated. This leaves room to interpret the known second luminosity peak, north-east of the QSO in 3C 48, as the center of the galaxy merging with 3C 48. A central gas accumulation and a complex system of gas lanes develop in the simulation with a non-gravitating gas component added. Both results qualitatively agree with the known distribution of molecular gas in 3C 48. The new results about I Zw 1 and 3C 48 complement the existing assumption that both are likely transitionary objects in the evolutionary sequence.

New Galaxy Evolution Explorer (GALEX) observations have detected significant far-UV (FUV; 1530 A) and near-UV (NUV; 2310 A) emission from stellar substructures within the tidal tails of four ongoing galaxy mergers. The UV-bright regions are optically faint and are coincident with H I density enhancements. FUV emission is detected at any location where the H I surface density exceeds ~2 M_☉ pc^-2, and it is often detected in the absence of visible wavelength emission. UV luminosities of the brighter regions of the tidal tails imply masses of 10^6 to ~10^9 M_☉ in young stars in the tails, and H I luminosities imply similar H I masses. UV-optical colors of the tidal tails indicate stellar populations as young as a few megayears, and in all cases ages under 400 Myr. Most of the young stars in the tails formed in single bursts, rather than resulting from continuous star formation, and they formed in situ as the tails evolved. Star formation appears to be older near the parent galaxies and younger at increasing distances from the parent galaxy. This could be because the star formation occurs progressively along the tails, or because the star formation has been inhibited near the galaxy/tail interface. The youngest stellar concentrations, usually near the ends of long tidal tails, have masses comparable to confirmed tidal dwarf galaxies and may be newly forming galaxies undergoing their first burst of star formation.

The host of the short gamma-ray burst (GRB) 050709 is a morphologically disturbed low-luminous galaxy. At a redshift of z = 0.16, it belongs to one of the cosmologically nearest short-GRB hosts identified to date. Consequently, it represents a promising target for sensitive, spatially resolved observational studies. We have used the Multi Unit Spectroscopic Explorer (MUSE) mounted at the Very Large Telescope to characterize the GRB host galaxy. In addition, we performed deep radio-continuum observations of the host with the Australia Telescope Compact Array (ATCA) and with ALMA at 1.3 mm. Moreover, we made use of archival Spitzer Space Telescope 24 μm and Hubble Space Telescope/F814W imaging data of this galaxy. The spatially resolved MUSE data reveal that the entire host is a source of strong line emission, in particular from Hα and [O III] λ 5007, superimposed on a rather weak stellar continuum. Using the Balmer decrement, we map the internal host-galaxy reddening and derive an extinction-corrected star formation rate based on the flux in the Hα line of 0.15 ± 0.02 M⊙ yr−1. The galaxy is detected neither by ALMA nor by Spitzer, excluding a substantial amount of optically obscured star formation activity. Using the O3N2 metallicity indicator, we measure an average 12 + log (O/H) = 8.40 ± 0.05 (corresponding to ∼0.5 solar). Diagnostic emission line diagrams show that a substantial fraction of all MUSE spaxels that cover the GRB 050709 host galaxy lie close to the star-formation demarcation line. Some spaxels even suggest line emission by shocked gas. The ATCA observations reveal faint diffuse radio emission at the eastern part of the host in excess to that expected from pure star formation, possibly further evidence for nonthermal processes. The kinematics of the Hα-emitting gas suggests a rotationally supported host-galaxy system, apparently in contrast to its irregular photometric morphology. A comparison with the field-galaxy population reveals, however, that the kinematics of the gas in the 050709 host fits into the ensemble of merging galaxies well. Finally, we use the ATCA radio data to set deep constraints on any late-time flux from the GRB afterglow or a potentially associated kilonova radio flare ∼10 years after the burst.

The growth of supermassive black holes and their host galaxies are thought to be linked, but the precise nature of this symbiotic relationship is still poorly understood. Both observations and simulations of galaxy formation suggest that the energy input from active galactic nuclei (AGN), as the central supermassive black hole accretes material and grows, heats the interstellar material and suppresses star formation. In this Letter, we show that most host galaxies of moderate-luminosity supermassive black holes in the local universe have intermediate optical colors that imply the host galaxies are transitioning from star formation to quiescence, the first time this has been shown to be true for all AGN independent of obscuration. The intermediate colors suggest that star formation in the host galaxies ceased roughly 100 Myr ago. This result indicates that either the AGN are very long-lived, accreting for more than 1 Gyr beyond the end of star formation, or there is a ~100 Myr time delay between the shutdown of star formation and detectable black hole growth. The first explanation is unlikely given current estimates for AGN lifetimes, so low-lumiosity AGN must shut down star formation before substantial black hole accretion activity is detected. The scarcity of AGN host galaxies in the blue cloud reported here challenges scenarios where significant star formation and black hole growth are coeval. Lastly, these observations also strongly support the `Unified Model' of AGN as the host galaxy colors are independent of obscuration towards the central engine.

We investigate the relation between the characteristics of the circumnuclear stellar population and both the galaxy morphology and the presence of close companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present unambiguous signatures of recent episodes of star formation within ≈300 pc of the nucleus. When we relate this property to the Hubble of the host galaxy, we find that the incidence of recent circumnuclear star formation increases along the Hubble sequence; it seems to be greater than that in non-Seyfert galaxies for the early Hubble types S0 and Sa but similar to that in non-Seyfert galaxies for later Hubble types. In both early-type and late-type Seyfert galaxies, the presence of recent circumnuclear star formation is related to the galaxy morphology in the few kiloparsecs, as observed in Hubble Space Telescope images through the filter F606W by Malkan et al., who have assigned a late inner Hubble type to most Seyfert 2 galaxies with recent circumnuclear star formation. This new classification is due to the presence of dust lanes and spiral structures in the region. The presence of recent star formation around Seyfert 2 nuclei is also related to interactions: among the 13 galaxies of the sample with close companions or in mergers, nine have recent star formation in the nuclear region. These correlations between the presence of companions, the morphology, and the incidence of recent star formation suggest an evolutionary scenario in which the interaction is responsible for sending gas inward, which both feeds the active galactic nucleus and triggers star formation. The starburst then fades with time and the composite Seyfert 2 + starburst nucleus evolves to a pure Seyfert 2 nucleus with an old stellar population. This scenario can reconcile the hypothesis that the active nucleus in Seyfert galaxies is triggered by interactions with the results of previous studies, which find only a small excess of interacting galaxies in Seyfert samples when compared with non-Seyfert samples. The large excess can only be found early after the interaction, in the phase in which a composite (Seyfert+starburst) nucleus is observed.

As early as 10 Gyr ago, galaxies with more than 10^11 Msun in stars already existed. While most of these massive galaxies must have subsequently transformed through on-going star formation and mergers with other galaxies, a small fraction (<0.1%) may have survived untouched till today. Searches for such relic galaxies, useful windows to explore the early Universe, have been inconclusive to date: galaxies with masses and sizes like those observed at high redshift (M*>10^11 Msun; Re<1.5 kpc) have been found in the local Universe, but their stars are far too young for the galaxy to be a relic galaxy. This paper explores the first case of a nearby galaxy, NGC1277 (in the Perseus cluster at a distance of 73 Mpc), which fulfills all the criteria to be considered a relic galaxy. Using deep optical spectroscopy, we derive the star formation history along the structure of the galaxy: the stellar populations are uniformly old (>10 Gyr) with no evidence for more recent star formation episodes. The metallicity of their stars is super-solar ([Fe/H]=0.20+-0.04) and alpha enriched ([alpha/Fe]=0.4+-0.1). This suggests a very short formation time scale for the bulk of stars of this galaxy. This object also rotates very fast (Vrot~300 km/s) and has a large velocity dispersion (sigma>300 km/s). NGC1277 will allow future explorations in full detail of properties such as the structure, internal dynamics, metallicity, dust content and initial mass function at around 10-12 Gyr back in time when the first massive galaxies were built.

Based on Paper I of this series (Mehlert et al. [CITE]), we derive central values and logarithmic gradients for the , Mg and Fe indices of 35 early-type galaxies in the Coma cluster. We find that pure elliptical galaxies have on average slightly higher velocity dispersions, lower , and higher metallic line-strengths than galaxies with disks (S0). The latter form two families, one comparable to the ellipticals and a second one with significantly higher , and weaker metallic lines. Our measured logarithmic gradients within the effective radius are , , and . The gradients strongly correlate with the gradients of σ, but only weakly with the central index values and galaxy velocity dispersion. Using stellar population models with variable element abundance ratios from Thomas et al. ([CITE]) we derive average ages, metallicities and [ ] ratios in the center and at the effective radius. We find that the ratio correlates with velocity dispersion and drives 30% of the Mg–σ relation, the remaining 70% being caused by metallicity variations. We confirm previous findings that part of the lenticular galaxies in the Coma cluster host very young (∼2 Gyr) stellar populations, hence must have experienced relatively recent star formation episodes. Again in accordance with previous work we derive negative metallicity gradients ( dex per decade) that are significantly flatter than what is expected from gaseous monolithic collapse models, pointing to the importance of mergers in the galaxy formation history. Moreover, the metallicity gradients correlate with the velocity dispersion gradients, confirming empirically earlier suggestions that the metallicity gradient in ellipticals is produced by the local potential well. The gradients in age are negligible, implying that no significant residual star formation has occurred either in the center or in the outer parts of the galaxies, and that the stellar populations at different radii must have formed at a common epoch. For the first time we derive the gradients of the ratio and find them very small on average. Hence, enhancement is not restricted to galaxy centers but it is a global phenomenon. Our results imply that the Mg–σ local relation inside a galaxy, unlike the global Mg–σ relation, must be primarily driven by metallicity variations alone. Finally we note that none of the stellar population parameters or their gradients depend on the density profile of the Coma cluster, even though it spans 3 dex in density.

Samples of Extremely Red Galaxies (ERGs) have generally been seen to comprise a mix of actively star-forming galaxies with significant dust reddening and evolved, passive galaxies, at redshifts about z ≈ 1 − 2. Initial results from deep Keck spectroscopy of ERGs (Doherty et al. 2005) revealed dominant old stellar populations in 75% of our spectroscopic sample, but only 28% have spectra with no evidence of recent star formation activity, such as would be expected for a strictly passively-evolving population. This study suggests that the bulk of the ERGs are luminous, spheroidal, evolved galaxies, but undergoing intermittent activity consistent with continued growth.Through a detailed investigation of individual galaxies in our sample we aim to address various outstanding questions. What fraction of their mass is produced in ongoing star formation? Is there a characteristic mass at which star formation is abruptly truncated? What mechanism provokes a secondary burst of star formation in evolved galaxies?We fit Bruzual &amp; Charlot (2003; BC03) simple stellar population models to the broad band SEDs over a wide baseline, using a reduced χ2 minimisation, to investigate ages, stellar masses and star formation histories. The fits for the early types agree well with information in the spectra and return ages of 2–3 Gyr and masses in the range 1011–1012M⊙. The objects with recent star formation episodes are more complex. Some are fit well by continuous star formation models, accounting for the effects of dust. We are now in the process of exploring multi-population fits to investigate the effects of episodic bursts.Previous morphological studies of ERGs have revealed a diverse mix of galaxies – a combination of pure bulges, disks and a small fraction of irregular or interacting systems. We are curious to determine whether a morphological analysis produces results consistent with the spectroscopic properties of our sample. We are investigating a sub-sample of our galaxies which have HST imaging publically available. Initial results from a quantitative analysis using bulge/disk decomposition with GALFIT and GIM2D indicate that most galaxies with Early type spectra are bulge dominated. In contrast, a significant fraction of the galaxies showing spectroscopic signatures of on-going star formation on top of underlying old stellar populations appear to have a well-established classical spiral morphology, wih knots of star formation located in spiral arms around a central bulge. There is tenuous evidence (under further investigation) that at least half of the post-starbursts in our sample are barred spirals, lending support to theories relating post-starbursts to recent mergers.

We have developed an observing program using deep, multiband imaging to probe the chaotic regions of tidal tails in search of an underlying stellar population, using NGC 3256's 400 Myr twin tidal tails as a case study. These tails have different colours of $u - g = 1.05 \pm 0.07$ and $r - i = 0.13 \pm 0.07$ for NGC 3256W, and $u - g = 1.26 \pm 0.07$ and $r - i = 0.26 \pm 0.07$ for NGC 3256E, indicating different stellar populations. These colours correspond to simple stellar population ages of $288^{+11}_{-54}$ Myr and $841^{+125}_{-157}$ Myr for NGC 3256W and NGC 3256E, respectively, suggesting NGC 3256W's diffuse light is dominated by stars formed after the interaction, while light in NGC 3256E is primarily from stars that originated in the host galaxy. Using a mixed stellar population model, we break our diffuse light into two populations: one at 10 Gyr, representing stars pulled from the host galaxies, and a younger component, whose age is determined by fitting the model to the data. We find similar ages for the young populations of both tails, ($195^{-13}_{+0}$ and $170^{-70}_{+44}$ Myr for NGC 3256W and NGC 3256E, respectively), but a larger percentage of mass in the 10 Gyr population for NGC 3256E ($98^{+1}_{-3}\%$ vs $90^{+5}_{-6}\%$). Additionally, we detect 31 star cluster candidates in NGC 3256W and 19 in NGC 2356E, with median ages of 141 Myr and 91 Myr, respectively. NGC 3256E contains several young (< 10 Myr), low mass objects with strong nebular emission, indicating a small, recent burst of star formation.

We present optical spectroscopy of the close companions of 22 low-redshift (z &amp;lt; 0.5) quasars (QSO) selected from a larger sample of QSO in the SDSS Stripe82 region for which both the host galaxy and the large-scale environments have been investigated in our previous work. The new observations extend the number of QSO studied in our previous paper on close companion galaxies of 12 quasars. Our analysis here covers all 34 quasars from both this work and the previously published paper. We find that half of them (15 QSO; ∼44 per cent) have at least one associated galaxy. Many (12 galaxies; ∼67 per cent) of the associated companions exhibit [O ii] 3727 Å emission line as signature of recent star formation. The star formation rate (SFR) of these galaxies is modest (median SFR ∼ 4.3 M⊙ yr−1). For eight QSO, we are also able to detect the starlight of the host galaxy from which three have a typical spectrum of a post-starburst galaxy. Our results suggest that quasars do not have a strong influence on the star formation of their companion galaxies.

We present the results of a photometric and spectroscopic study of seven members of the NGC 5328 group of galaxies, a chain of galaxies spanning over 200 kpc (H 0 = 70 km s -1 Mpc -1 ). We analyse the galaxy structure and study the emission-line properties of the group members looking for signatures of star formation and active galactic nucleus activity. We finally attempt to infer, from the modelling of line-strength indices, the stellar population ages of the early-type members. We investigate also the presence of a dwarf galaxy population associated with the bright members. The group is composed of a large fraction of early-type galaxies including NGC 5328 and 5330, two 'bona fide' ellipticals at the centre of the group. In both galaxies no recent star formation episodes are detected by the Hβ versus MgFe line-strength indices of these galaxies. 2MASX J 13524838-2829584 has extremely boxy isophotes which are believed to be connected to a merging event: line-strength indices suggest that this object probably had a recent star formation episode. A warped disc component emerges from the model subtracted image of 2MASX J13530016-2827061, which is interpreted as a signature of an ongoing interaction with the rest of the group. Ongoing star formation and nuclear activity are present in the projected outskirts of the group. The two early-type galaxies 2MASX J13523852-2830444 and 2MASX J13525393-2831421 show spectral signatures of star formation, while a Seyfert 2 type nuclear activity is detected in MCG-5-33-29.

We present a spectral analysis of a sample of red-sequence galaxies identified by van Dokkum (2005) as dry merger remnants and ongoing dry mergers. Kinematics, stellar population absorption features, and ionisation from emission lines, are derived. We find that approximately half of the sample showing strong tidal features have younger stellar populations than a control sample at a given velocity dispersion. Conversely, galaxies with weak tidal tails and/or ongoing mergers -- with the exception of one galaxy -- do not show this young component. This seems to indicate that the young stellar populations observed in a significant fraction of ellipticals is the consequence of star formation triggered by mergers. This young component is consistent with a light "frosting" of young stars (<2% by mass) superimposed upon a dominant, old (~11 Gyr), stellar population. In terms of stellar populations, these mergers are, in fact, fairly dry. We found, however, that merger remnants with young stellar populations are supported by rotation, contrary to the expectations of a major dry merger. This suggests that the small amount of gas involved has been sufficient to produce a dynamically cold stellar component. Half of the galaxies with strong tidal distortion, however, are slow rotating and have stellar populations compatible with the control sample at a given velocity dispersion. Remarkably, none of the galaxies with velocity dispersions in excess of 250 km/s have a young stellar component, independent of the merger stage.

In this thesis I study the gas distribution and dynamics, as well as the stellar populations, in the circumnuclear star forming rings in the nearby barred galaxies NGC5248 and NGC6951. Circumnuclear star forming rings are regions of high gas and star formation density in the centers of disk galaxies. These star formation rings can have a strong influence on the secular evolution of their host galaxies, by halting gas inflow towards the nucleus. The gas masses that build up in these rings are high and this consequently leads to intense star formation. This makes circumnuclear star forming rings also an important tool to build up the stellar (pseudo-)bulge. These two galaxies were selected because they have very similar global properties (mass, size, metallicity), but very different circumnuclear regions, both of which containing circumnuclear star forming rings. Detailed investigation of both, and a comparison between them, thus provides new detailed insight on the interplay between circumnuclear star forming rings and the circumnuclear region as a whole. Three questions are addressed in this work. First of all, I investigate the true effectiveness of the circumnuclear rings as a gas barrier in both galaxies. I study star formation progression in these rings and its relation to the gas flow onto the ring. The age and life span of the circumnuclear rings is derived from the analysis of their stellar populations. In both galaxies I find that while the circumnuclear rings are effective barriers, they are not absolute ones. The interplay between the large scale stellar bar, circumnuclear ring and other dynamical components in the circumnuclear region allows some gas to flow towards the very center past the ring. I find that the location of the youngest stars in the ring is close to the inflow position of the gas onto the ring in both galaxies, consistent with star formation proceeding in a ’pearls-on-a-string’ mechanism with the youngest ’pearls’ close to the gas inflow location. Finally, I derive ages of the rings around 1.5 Gyr, which implies that circumnuclear star forming rings have a significant amount of time to shape the circumnuclear regions of their host galaxies.

We study the feedback from an AGN on stellar formation within its host\ngalaxy, mainly using one high resolution numerical simulation of the jet\npropagation within the interstellar medium of an early-type galaxy. In\nparticular, we show that in a realistic simulation where the jet propagates\ninto a two-phase ISM, star formation can initially be slightly enhanced and\nthen, on timescales of few million years, rapidly quenched, as a consequence\nboth of the high temperatures attained and of the reduction of cloud mass\n(mainly due to Kelvin-Helmholtz instabilities). We then introduce a model of\n(prevalently) {\\em negative} AGN feedback, where an exponentially declining\nstar formation is quenched, on a very short time scale, at a time t_AGN, due to\nAGN feedback. Using the Bruzual &amp; Charlot (2003) population synthesis model and\nour star formation history, we predict galaxy colours from this model and match\nthem to a sample of nearby early-type galaxies showing signs of recent episodes\nof star formation (Kaviraj et al. 2007). We find that the quantity t_gal -\nt_AGN, where t_gal is the galaxy age, is an excellent indicator of the presence\nof feedback processes, and peaks significantly around t_gal - t_AGN \\approx\n0.85 Gyr for our sample, consistent with feedback from recent energy injection\nby AGNs in relatively bright (M_{B} \\lsim -19) and massive nearby early-type\ngalaxies. Galaxies that have experienced this recent feedback show an\nenhancement of 3 magnitudes in NUV(GALEX)-g, with respect to the unperturbed,\nno-feedback evolution. Hence they can be easily identified in large combined\nnear UV-optical surveys.\n