6 research outputs found
Composite Bulges -- IV. Detecting Signatures of Gas Inflows in the IFU data: The MUSE View of Ionized Gas Kinematics in NGC 1097
Using VLT/MUSE integral-field spectroscopic data for the barred spiral galaxy
NGC 1097, we explore techniques that can be used to search for extended
coherent shocks that can drive gas inflows in centres of galaxies. Such shocks
should appear as coherent velocity jumps in gas kinematic maps, but this
appearance can be distorted by inaccurate extraction of the velocity values and
dominated by the global rotational flow and local perturbations like stellar
outflows. We include multiple components in the emission-line fits, which
corrects the extracted velocity values and reveals emission associated with AGN
outflows. We show that removal of the global rotational flow by subtracting the
circular velocity of a fitted flat disk can produce artefacts that obscure
signatures of the shocks in the residual velocities if the inner part of the
disk is warped or if gas is moving around the centre on elongated
(non-circular) trajectories. As an alternative, we propose a model-independent
method which examines differences in the LOSVD moments of H and [N
II]6583. This new method successfully reveals the presence of
continuous shocks in the regions inward from the nuclear ring of NGC 1097, in
agreement with nuclear spiral models.Comment: MNRAS accepted (19 pages, 18 figures, 1 table
Disc galaxies are still settling: The discovery of the smallest nuclear discs and their young stellar bars
When galactic discs settle and become massive enough, they are able to form
stellar bars. These non-axisymmetric structures induce shocks in the gas,
causing it to flow to the centre where nuclear structures, such as nuclear
discs and rings, are formed. Previous theoretical and observational studies
have hinted at the co-evolution of bars and nuclear discs, suggesting that
nuclear discs grow "inside-out", thereby proposing that smaller discs live in
younger bars. Nevertheless, it remains unclear how the bar and the nuclear
structures form and evolve with time. The smallest nuclear discs discovered to
date tend to be larger than , even though some theoretical
studies find that when nuclear discs form they can be much smaller. Using MUSE
archival data, we report for the first time two extragalactic nuclear discs
with radius sizes below . Additionally, our estimations reveal the
youngest bars found to date. We estimate that the bars in these galaxies formed
and
ago, for NGC\,289
and NGC\,1566, respectively. This suggests that at least some disc galaxies in
the Local Universe may still be dynamically settling. By adding these results
to previous findings in the literature, we retrieve a stronger correlation
between nuclear disc size and bar length and we derive a tentative exponential
growth scenario for nuclear discs.Comment: Accepted in A&A (in press), 13 pages, 5 figures, 2 tabel
Composite Bulges -- III. A Study of Nuclear Star Clusters in Nearby Spiral Galaxies
We present photometric and morphological analyses of nuclear star clusters
(NSCs) -- very dense, massive star clusters present in the central regions of
most galaxies -- in a sample of 33 massive disk galaxies within 20 Mpc, part of
the "Composite Bulges Survey." We use data from the Hubble Space Telescope
including optical (F475W and F814W) and near-IR (F160W) images from the Wide
Field Camera 3. We fit the images in 2D to take into account the full
complexity of the inner regions of these galaxies (including the contributions
of nuclear disks and bars), isolating the nuclear star cluster and bulge
components. We derive NSC radii and magnitudes in all 3 bands, which we then
use to estimate NSC masses. Our sample significantly expands the sample of
massive late-type galaxies with measured NSC properties. We clearly identify
nuclear star clusters in nearly 80% of our galaxies, putting a lower limit on
the nucleation fraction in these galaxies that is higher than previous
estimates. We find that the NSCs in our massive disk galaxies are consistent
with previous NSC mass-NSC radius and Galaxy Mass-NSC Mass relations. However,
we also find a large spread in NSC masses, with a handful of galaxies hosting
very low-mass, compact clusters. Our NSCs are aligned in PA with their host
galaxy disks but are less flattened. They show no correlations with bar or
bulge properties. Finally, we find the ratio of NSC to BH mass in our massive
disk galaxy sample spans a factor of 300.Comment: Accepted to The Astrophysical Journa
Disc galaxies are still settling
When galactic discs settle and become massive enough, they are able to form stellar bars. These non-axisymmetric structures induce shocks in the gas, causing it to flow to the centre where nuclear structures, such as nuclear discs and rings, are formed. Previous theoretical and observational studies have hinted at the co-evolution of bars and nuclear discs, suggesting that nuclear discs grow ‘inside-out’ and thereby proposing that smaller discs reside in younger bars. Nevertheless, it remains unclear how the bar and the nuclear structures form and evolve over time. The smallest nuclear discs discovered to date tend to be larger than ∼200 pc, even though some theoretical studies have reported that when nuclear discs form, they can be much smaller. Using MUSE archival data, we report, for the first time, two extragalactic nuclear discs with radius measurements below 100 pc. Additionally, our estimations reveal the youngest bars found to date. We estimate that the bars in these galaxies formed and ago, for NGC 289 and NGC 1566, respectively. This suggests that at least some disc galaxies in the Local Universe may still be dynamically settling. By adding these results to previous findings in the literature, we are able to retrieve a stronger correlation between nuclear disc size and bar length. We also derive a tentative exponential growth scenario for nuclear discs
Composite Bulges -- III. A Study of Nuclear Star Clusters in Nearby Spiral Galaxies
We present photometric and morphological analyses of nuclear star clusters (NSCs) -- very dense, massive star clusters present in the central regions of most galaxies -- in a sample of 33 massive disk galaxies within 20 Mpc, part of the "Composite Bulges Survey." We use data from the Hubble Space Telescope including optical (F475W and F814W) and near-IR (F160W) images from the Wide Field Camera 3. We fit the images in 2D to take into account the full complexity of the inner regions of these galaxies (including the contributions of nuclear disks and bars), isolating the nuclear star cluster and bulge components. We derive NSC radii and magnitudes in all 3 bands, which we then use to estimate NSC masses. Our sample significantly expands the sample of massive late-type galaxies with measured NSC properties. We clearly identify nuclear star clusters in nearly 80% of our galaxies, putting a lower limit on the nucleation fraction in these galaxies that is higher than previous estimates. We find that the NSCs in our massive disk galaxies are consistent with previous NSC mass-NSC radius and Galaxy Mass-NSC Mass relations. However, we also find a large spread in NSC masses, with a handful of galaxies hosting very low-mass, compact clusters. Our NSCs are aligned in PA with their host galaxy disks but are less flattened. They show no correlations with bar or bulge properties. Finally, we find the ratio of NSC to BH mass in our massive disk galaxy sample spans a factor of ∼300